CA2265534C - Color sensor - Google Patents

Abstract

A sensor (10) for measuring reflective, transmissive, or self-luminous samples, comprises a plurality of light sources (18), where each of the light sources emit light of a substantially different wavelength band spaced in the visible spectrum; a reference channel photodetector (20); a sample channel photodectector (22); an optical cap (26) adapted to direct a first portion of the light emitted by each of the light sources to the reference channel photodetector (20); a reflector cone for directing a second portion of the light emitted by each of the light sources to the sample; and a receptor piece (36) for directing the diffuse portion of the light reflected from the sample to the sample channel photodetector (22). The sensor (10) is preferably incorporated into a hand-held "mouse" device, which includes an area on its top surface for seating an index finger of the human hand.

Description

?CA 02265534 2002-01-10W0 98/1 1410 PCT/U S97! 16009COLOR SENSORBACKGROUNDThe present invention relates generally to devices for measuring reflective,transmissive, or self-luminous samples and reporting their spectrophotometric,spectroradiometric, densitometric, or other colorimetric appearance attributes.Users of desktop color systems have a need to accurately measure color.Color systems end-users expect accurate color matching between their source (scanneror monitor) and the color hardcopy produced by their color digital printer. In order toachieve WYSIWYG (“What You See is What You Get") color, color imaging devicesmust be characterized and calibrated transparently to the user, and work seamlesslywith popular software applications. Support for import and export of image datadescribed in a device independent format such as CIE is increasingly a requirement.The inputting of colors represented by physical samples into an electronicdesign for display and printing is currently a tedious process. The end-user has severaloptions. The sample may be scanned, but the color reproduction will be poor for thereasons discussed above. The sample may be visually matched by specifying RGB orCMYK amounts for the application, but the match will be device dependent and highlyvariable. A color specification system may be employed by visually matching thesample to one of the specified colors then entering the color specification to theapplication, but the match will be device dependent, highly variable, and rely uponapplication compatibility with the color specification system.One application where WYSIWYG is particularly important is color desktoppublishing. in many desktop and workstation environments, color end-users (e.g.publishing, prepress, design, graphics, etc.) have a wide selection of input and output?CA 02265534 l999-03- 10W0 98/1 1410 PCT/US97/ 16009devices (e.g. scanners, monitors, printers, imagesetters, etc.) and color creationapplications. Since this environment is generally open-architecture, the colorimetriccharacteristics of the various devices and applications are not, and cannot be, well-matched due to the multi-vendor nature of the market. As a result, the quality of colorreproduction between input and output is highly variable and generally poor. In orderto achieve consistent color matching in this highly disparate environment, severalcompanies have introduced software-based color management system (CMS)technology based on device independent color. The principle of operation of suchsystems is to reference all devices (device dependent) to a common CIE color space(device independent). A simple work ?ow for such a CMS might be comprised of thefollowing steps: (1) the image scan (Scanner RGB) is referenced to CIE; (2) the imageis converted to Display RGB for editing; (3) the displayed image is referenced back toCIE; and (4) the image is converted to printer CMYK for output.Generally, the CMS vendor provides several key elements: a library ofdevice characterization profiles, a color matching method, and color transformationsoftware. Since the expertise and equipment required for the creation of profiles isexpensive, device characterization is performed by the CMS vendor for an averagedevice for average viewing conditions. Unfortunately, characterization accomplishedin this manner is done under factory conditions and not for the end-user conditions(device, light source, media, viewing, etc.). Since an end-user's device will be quitedifferent from the device that was profiled at the factory, some vendors offerhardware/software calibration. Calibration only partially compensates for thedifferences between factory conditions and end-user conditions but does not satisfy theneed for end-user device characterization compatible with open architecture CMS.However, there are no end-user characterizors for ambient, display, or hardcopycharacterization due to the high cost of such equipment and lack of suitable end-usersoftware. Currently, CMS products are generally proprietary and must be purchasedfrom the CMS vendor. As open architecture CMS technology will be included as partof the operating system, device manufacturers, third parties, and end-users will requirecustom CMMs or profiles.The combination of CMS technology, factory characterization, and end-?CA 02265534 l999-03- 10W0 98/11410 PCT/US97/16009user calibration improves the average color reproduction on the desktop somewhat, butis expensive, complex, and does not meet the end-user requirements for quality, cost,speed, and compatibility. End-users need a fast, low-cost, simple hardware/softwarecharacterizor enabling them to easily characterize (not just calibrate) their specificscanners, monitors, and printers to their speci?c viewing conditions in a manner that iscompatible with open architecture CMS technology.United States Patent No. 5,137,364 discloses a color sensor that employsa plurality of LEDs (light emitting diodes) and an array of photodetectors. The McCarthysensor utilizes individually addressable, customized LEDs as its illumination sourcesand a photodetector array, where each photodetector measures re?ectance of thesample at a different visible wavelength, and where each element's spectral sensitivitymust be individually optimized. The McCarthy sensor also utilizes a single beamre?ectance measurement scheme. Thus, the photodetectors must be made as stableas possible by maintaining at a constant temperature and being protected fromhumidity, etc. Furthermore, the McCarthy device requires expensive and stablecomponents since the measured energy collected from the color sample must beconstant. Such expensive and customized components nevertheless may alsoexperience long term drift and may be highly sensitive to noise.United States Patent No. 5,377,000 to Berends discloses a color sensorthat utilizes a single illumination source positioned directly above the sample, 21 samplephotodetectors arranged circumferentially around the illumination source and positionedat 45° angles with respect to the sample, and 2 reference photodetectors positioned toreceive light directed from the single illumination source. Berends utilizes a "pseudo—dual beam" re?ectance measurement scheme in an attempt to eliminate the need of anadditional 19 reference photodetectors that would be required in a classic dual-beamreflectance measurement scheme. This is performed by configuring the 2 referencephotodetectors to sample the illumination source at opposite extremes of the visiblelight spectrum, and by applying a "least squares fit" calibration calculation to simulatethe required 21 reference channel readings.?CA 02265534 l999-03- 10W0 98/1 1410 PCT/US97ll6009SUMMARYThe present invention provides an extremely accurate color sensor,designed to utilize a classic dual-beam re?ectance measurement scheme withinexpensive and commercially available components.In accordance with one aspect of the present invention, a device formeasuring reflective, transmissive, or self-luminous samples, comprises: a plurality oflight sources, which are preferably light emitting diodes (LEDs), where each of the lightsources emit light of a substantially different wavelength band spaced in the visiblespectrum; a reference channel photodetector; a sample channel photodetector; anoptical cap adapted to direct a first portion of the light emitted by each of the lightsources to the reference channel photodetector; a re?ector cone for directing a secondportion of the light emitted by each of the light sources to the sample; and a receptorpiece for directing the diffuse portion of the light re?ected from the sample to the samplechannel photodetector.In a preferred embodiment of the invention, the reference channel andsample channel photodetectors are identical devices and are mounted back-to-back toshare environmental characteristics, and in turn, minimize the variation between theirrespective responses. The output signals from the photodetectors are processed toprovide at least one appearance value, e.g., spectrophotometric, spectroradiometric,densitometric, and other colorimetric appearance attributes for the samples.Preferably, the optical cap has a non-absorbing interior integrating surfaceand is mounted over the LEDs and the reference photodetector such that the referenceportion of light generated by the emitters re?ects off of the integrating surface to thereference photodetector. The LEDs are each preferably mounted such that a portionof each LED extends into a cylindrical cavity pointing downwards towards the sample,the cavities acting to collimate the sample portion of light emitted by the LEDs. There?ector cone preferably has a conical re?ecting surface, positioned in alignment withthe cavities, and angled 22.5° with respect to the cavities‘ axes, such that the sampleportion of light directed downward towards the conical re?ecting surface will first re?ectoff of the conical re?ecting surface and then be directed towards the sample at an angle4?CA 02265534 l999-03- 10W0 98/1 1410 PCTlUS97l16009of 45°. Therefore, the sample photodetector, positioned directly over the sample andbehind an aperture in the receptor piece, will receive the diffuse component of the lightre?ected from the sample.The device may further comprise means for supplying current to the LEDwherein the LED current provided by the current supplying means is programmed suchthat assumed incident light which is proportional to the LED current is a knownmeasurement.The invention also provides a method of measuring color samples usinga color measurement device, the method comprising: (a) activating at least one lightsource of a certain wavelength band, (b) directing a first portion of the light emitted bythe light source to a reference photodetector, (c) directing a second portion of the lightemitted by the light source to the sample surface, (d) directing light re?ected from thesample surface to a sample photodetector, (e) calculating a re?ectance for the lightsource based upon output readings of the sample photodetector in step (d) and outputreadings of the reference photodetector in step (b), and (f) repeating steps (a) through(e) for several light sources, each emitting light of substantially different wavelengthbands.The method, in a preferred embodiment, also includes the steps of (f)obtaining a reading from the sample photodetector when none of the light sources areilluminated and (g) obtaining a reading from the reference photodetector when none ofthe light sources are illuminated, where step (e) involves the step of calculating are?ectance for the light source based upon a ratio of the difference of output readingsof the sample photodetector in step (d) and output readings of the samplephotodetector in step (f) versus a difference of output readings of the referencephotodetector in step (b) and output readings of the reference photodetector in step (g).And the method, in the preferred embodiment further includes the steps of (h) activatingthe light source of step (a), (i) directing a ?rst portion of light emitted by the light sourceto a reference photodetector, (j) directing a second portion of the light emitted by thelight source to a substantially non-re?ecting calibration surface, (k) calculating a “black”calibration re?ectance reading forthe light source based upon the sample photodetector5?CA 02265534 l999-03- 10W0 98/1 1410 PCT/US97ll6009reading from step (j) and the reference photodetector reading from step (i), (I) againactivating the light source of step (a), (m) directing a first portion of light emitted fromthe light source to the reference photodetector, (n) directing a second portion of lightemitted from the light source to a substantially white calibration surface, (0) calculatinga “white" calibration re?ectance reading for the light source based upon the samplephotodetector reading from step (n) and the reference photodetector reading from step(m), and (p) calculating a normalized and bias corrected re?ectance for the light sourceusing the re?ectance calculated in step (e), the “black" calibration re?ectance calculatedin step (k) and the “white" calibration re?ectance calculated in step (0).in another embodiment, step (e) includes the step of accounting for thesensitivity of the reference photodetector and the spectral power distribution of the lightsource. In yet another embodiment, the method further comprises the step of profilingthe characteristics of the reference photodetector and the light source by performingsteps (a) through (f) on at least two samples having known reflectances. And in yetanother embodiment, the method further comprises the step of transforming there?ectance calculated in step (e) into a CIE XYZ tristimulus value for the sample, wherethe transformation accounts for the sensitivity of the human visual system.Accordingly, a preferred embodiment of the present invention provides acolor sensor which utilizes the "dual beam" re?ectance sensing scheme byincorporating a reference photodetector. Because both the sample photodetector andthe reference photodetector share common optical, electrical, environmental andmechanical characteristics, the present invention can utilize low price componentswhich may tend to exhibit drift. Further, because the sample and reference channelsare ratioed in calculating the re?ectance, this drift will be canceled out, thereby yieldingultra-high performance at very low cost. The present invention has also been designedto, and physically con?gured to, allow the measurement of re?ected light from an areaof 3 millimeters in diameter using illumination with 45° incidence and 0° detectionangle. The sensor is highly rugged, having no moving parts, and through theapplication of the dual optical paths, is extremely reliable and inexpensive. Off-the-shelfphotodetectors with integrated amplifiers reduce dark errors, noise, cost and allow for6?CA 02265534 l999-03- 10W0 98/1 1410 PCT/US97ll6009accurate gain tracking over their operational temperature range. The design alsoaccommodates an interchangeable selection of LEDs, allowing the latest, most ef?cientand least costly commercially available LEDs to be used.The present invention is also designed to be easily assembled and canbe integrated into a main circuit board to further reduce assembly costs. Furthermore,optical and beam-directing components of the present invention can also be integrateddirectly as part of an outer hand-held enclosure (i.e., hand-held mouse), furtherincreasing the system integration, and therefore, further reducing the end user price.Because temperature stabilization and compensation is no longer required, LEDreliability is enhanced. Since the heat producing LEDs are now isolated from thethermally sensitive photodiodes, there is also a reduced opportunity for adverse thermalshock. Furthermore, the present invention reduces the consumed power and reducesmeasurement overhead. Thus, the invention is specially designed to be inexpensivelymass-produced, without sacrificing accuracy or reliability.As indicated above, one aspect of the present invention incorporates thecolor sensor into a hand-held “mouse” device. The mouse device includes an area onits top surface for seating an index ?nger of the human hand. Positioned within thisarea is a pressure-activated switch that is operatively coupled to the circuitry forperforming the colorimetric and re?ectance readings. Preferably, the sensor is mountedinto the mouse device such that the focal aperture of the downward pointing re?ectorcone is in axial alignment with the pressure-activated switch. Accordingly, a user willbe able to use the mouse to "point" with his or her index ?nger to an area of the samplesurface, and will then simply press the switch using the same index finger.Other foreseeable applications for the color sensor of the presentinvention include high-speed color—inspection/control in a manufacturing or productionenvironment, analogous to the uses described in U.S. Pat. No. 5,021,645 to Satula etal.; or color matching of cosmetics or clothing accessories to a skin-color or foundationmake-up color of a customer, analogous to the uses described in U.S. Pat. No.5,537,211 to Dial.?CA 02265534 l999-03- 10W0 98/1 1410 PCT/US97/ 16009BRIEF DESCRIPTION OF THE DRAWINGSFig. 1 is a cross-sectional, elevational view of an embodiment of thecolor sensor of the present invention;Fig. 2 is a top view of the annular collar component of the color sensor;Fig. 3 is a side view of the annular collar component of the colorsensor;Fig. 4 is a top view of the color sensor of an embodiment of the presentinvention as incorporated into a circuit board;Fig. 5 is a cross-sectional, elevational view of the embodiment of Fig.4;Fig. 6 is a cross-sectional, elevational, and exploded view of anembodiment of the color sensor and circuit board of the present invention;Fig. 7 is a schematic block diagram representation of circuitry for usewith the present invention;Fig. 8 is a schematic block diagram representation of alternate circuitryfor use with the present invention;Fig. 9 is a perspective view of a hand-held “mouse” deviceincorporating the present invention;Fig. 10 is a top view of the hand-held “mouse” device of Fig. 9;Fig. 11 is a cross-sectional, elevational view of the hand-held “mouse"device incorporating the present invention;Figs. 12a-c respectively show a top view, one cross-sectionalelevational view, and another cross-sectional elevational view of an alternateembodiment of the annular collar component of the present invention;Fig. 13 is a cross-sectional, elevational view of an alternateembodiment of the color sensor of the present invention;Fig. 14 is a cross-sectional, elevational view of another alternateembodiment of the present invention;Fig. 15 is a cross-sectional, elevational view of another alternateembodiment of the present invention;?CA 02265534 l999-03- 10W0 98/1 1410 PCT/US97/16009Fig. 16 is a cross-sectional, elevational view of another alternateembodiment of the collar and re?ector cone components of the present invention;andFig. 17 is a cross-sectional, elevational view of another alternateembodiment of the collar and re?ector cone components of the present invention.DETAILED DESCRIPTIONAs shown in Fig. 1, the color sensor 10 for sampling the color of a samplesurface 12 comprises a printed circuit board 13; a plurality of light sources, such as lightemitting diodes (LEDs) 18 mounted in apertures 19 extending through the circuit board13, each of the LEDs emitting light of a substantially different wavelength band spacedin the visible spectrum; a reference photodetector 20 mounted to the top surface 14 ofthe printed circuit board; a sample photodetector 22 mounted to the bottom surface 15of the printed circuit board, substantially back-to-back with the reference photodetector20; an annular collar 24 mounted to the bottom surface 15 of the printed circuit board;an optical cap 26 mounted to the top surface 14 of the printed circuit board; and are?ector cone 28 mounted onto the collar 24.As shown in Figs. 1, 2, and 3, the collar 24 is an annular componenthaving a plurality of emitter apertures 30 bored through the collar in an axial direction,and circumferentially spaced around a receiver aperture 32 that is axially bored througha frustoconical receptor piece 36. The receptor piece 36 extends and points downwardfrom the bottom surface 38 of the collar 24 along axis A. A rectangular cavity 40 forhousing the sample photodetector 22 extends into the collar 24 from the top surface 42of the collar, substantially at axis A, and is in direct optical communication with thereceiver aperture 32, such that light waves re?ected into the receiver aperture 32 of thereceptor piece 36 can contact the sample photodetector 22.Those skilled in the art will appreciate that the same cavity 40 for housingthe sample photodetector 22 can also be used to mount an optional optical filter 41constructed of ?at glass or plastic. Such a ?lter 41, mounted at the bottom of the cavity40, at the opening of the receiver aperture 32, and aligned with axis A, could serve the9?CA 02265534 l999-03- 10WO 98/11410 PCT/US97/16009purpose of excluding any unwanted light, e.g., infrared light in the case of a sensoroptimized for the visible spectrum. Accordingly, the ?lter 41 could be an infrared ?lteror any other band-pass filter as necessitated by the envisioned use of the sensor 10.The ?lter 41 will also protect the face of the sample photodetector. It is also within thescope of the invention to provide a similar optical ?lter over the reference photodetector20 to band-pass filter the light re?ected thereto.As shown in Fig. 1, the plurality of LEDs 18 are preferably mounted intoapertures 19 of the printed circuit board 13 such that a light emitting portion of eachLED extends into a corresponding one of the emitter apertures 30. The optical cap ismounted to the printed circuit board 13 such that the cap forms an enclosed cavity 44over the entire array of LEDs 18. Each of the plurality of LEDs are also mounted on theprinted circuit board 13 such that a portion of the light emitted by the LED is emitted orreflected backwards (upwardly) into the cavity 44 formed by the optical cap 26. Thereference photodetector 20 is centrally mounted with respect to the array of LEDs 18to the top surface 14 of the printed circuit board, within the cavity 44. The inner surface46 of the optical cap is preferably coated with an opaque, substantially non-absorbing,integrating coating such as a ?at white paint. Therefore, light waves emitted from theLEDs through the top surface 14 of the printed circuit board, and into the cavity 44, aretransmitted to the integrating surface 46 and are directly or indirectly re?ected from theintegrating surface to the reference photodetector 20.The reflector cone 28 is attached to the bottom 38 of the collar and hasan aperture 48 through the tip 50 of the reflector cone. The aperture 48 is in axialalignment with the axis A of the collar 24, and in turn, is in alignment with the receiveraperture 32 of the collar. The re?ector cone 28 has a substantially frustoconical innersurface which forms a cavity 53 between the collar 24 and the re?ector cone 28. Thereflector cone 28 also has an conical re?ector surface 52, preferably coated with achrome plating (or any other suitable re?ective coating), in alignment with each of theaxes B of the emitter apertures 30, and which is angled, with respect to the axes B,inwardly towards the axis A, at an angle that is approximately 225°. Therefore, thelight emitted through the emitter apertures 30 and re?ected from the conical re?ective10?CA 02265534 l999-03- 10W0 98/1 1410 PCT/US97/16009surface 52 will contact the aperture 48 of the reflector cone, and in turn the samplesurface 12, substantially at an angle of 45° with respect to the ?at tip 50 of the re?ectorcone (or with respect to the sample surface 12, if applied to a sample). Thus, thediffuse component of the re?ected light waves will be transmitted upwardly from thesample along axis A, through the receiver aperture 32 to the sample photodetector 22.Accordingly, when one of the LEDs is activated, and when the ?at tip 50of the re?ector cone is abutting a sample surface 12, the reference photodetector 20will sample light waves emitted from the portion of the LED 18 extending through thetop surface 14 of the printed circuit board and reflected from the optical cap's innersurface 46, and the sample photodetector 22 will sample the diffuse component of thelight waves emitted from the portion of the same LED 18 extending through the bottomsurface 15 of the printed circuit board and re?ected off the sample surface 12. The useof the 45°/0° geometry of the present invention corresponds more closely with thevisual viewing of samples and excludes the specular component of re?ectance. Thespectral reflectance of the color sample can thus be calculated for this particular LED(or combination of LEDs) from the readings of the reference photodetector 20 and thesample photodetector 22 using a dual beam method as is described in detail below.When using the 45°/0° geometry, the operable range for the angle of theconical re?ector surface is 20.5° to 24.5°, while the preferred range is 22.4° to 22.6°.Thus, those skilled in the art will appreciate that it is within the scope of the presentinvention that the angle of the conical re?ector surface be within the above ranges forthe 45°/0° geometry. It will also be appreciated to those of ordinary skill in the art thatangle A can be altered to obtain optical geometries that are better suited for particularuses. For example, when using the present invention to detect characteristics of apatient’s skin (such as skin color), a 20°/0° geometry as shown in Fig. 16 or modified-diffuse/0° geometry as shown in Fig. 17 may be preferred. These alternateembodiments are better suited for remote color sensing of a sample surface and will bediscussed in greater detail below.Referring again to the embodiment shown in Fig. 1, each emitter aperture30 is preferably formed with an upper cylindrical channel 54 and a lower cylindrical11?CA 02265534 l999-03- 10W0 98/1 1410 PCT/US97/16009channel 56. The lower cylindrical channel is concentrically aligned with the uppercylindrical channel 54 along the axis B of the emitter aperture and has a smallerdiameter than the upper cylindrical channel 54. The lower cylindrical channel thus actsto collimate the light waves emitted through the emitter aperture 30. to the conicalre?ective surface 52, and onto the sample surface 12. Additionally, although shown ashaving the same size, the emitter apertures 30 can vary in size to allow for maximum?exibility in LED selection.As shown in Fig. 6, in an alternate embodiment of the invention, and tosimplify mass-production of the device, the LEDs 18' can be mounted to a separatedaughterboard 162 that, in turn, is mounted to the top surface 14' of the main circuitboard 13' via mating connectors 164, 166. The main circuit board 13' will contain theelectronics (described below) that drive the LEDs 18', and the electronic signals fordriving the LEDs, produced by the main circuit board 13', will be passed to thedaughterboard through the mating connectors 164, 166. When the daughterboard ismated to the main circuit board, the LEDs 18' will extend into the apertures 19'extending through the main circuit board 13'. An optical cap 26' designed to be fittedover the daughterboard 162 will be mounted to the top surface 14' of the main circuitboard 13' and will act to re?ect light emitted from the LEDs 18' to the referencephotodetector 20 as described above. Likewise, the collar 24 and re?ector cone 28 willbe mounted to the bottom surface 15' of the main circuit board 13' and will act to directlight emitted from the LEDs 18' to the sample surface at approximately 45°, and todirect the diffuse component of the light re?ected from the sample to the samplephotodetector 22 as described above.Those skilled in the art will appreciate that the light sources can bemounted or positioned in other ways; so long as a portion of the light emitted from thelight source is transmitted or re?ected through the emitter aperture 30 and anotherportion of the light emitted from the light source is emitted or re?ected into the cavity 44within the optical cap 26. For example, it is within the scope of the invention to mounteach LED to the bottom surface 15 of the printed circuit board, extending completelywithin the emitter aperture 30, and to bore a hole through the printed circuit board 1312?CA 02265534 l999-03- 10W0 98/1 1410 PCT/US97/16009above the LED 18 such that the light emitted from the LED 18 will be transmittedthrough the hole and into the cavity 44 and will also be transmitted through the lowercylindrical channel 56 and into the cavity 53 as described above. In another example,as shown in Fig. 15, the LEDs can be clustered at any light-shielded point of circuitboard; and when activated, an acrylic "light-pipe" 180 can be used similar to a fiberoptic element, to direct a ?rst fraction 182 of light waves to the sample surface 12 sothat it is incident at 45°. Of course, the sample photodetector 22 will be in position todetect the diffuse component of the light reflected from the sample surface. Becausethe light-pipe 180 tends to bleed light waves radially therefrom, the referencephotodetector 20 can be in any shielded position to receive a fraction 184 of lightbleeding from the light-pipe 180.It will also be apparent to those skilled in the art, that photodetectors foruse with present invention can include photoconductive cells, photodiodes,photoresistors, photoswitches, phototransistors, phototubes, photovoltaic cells, light-to-frequency converters, or any other type of photosensor capable of converting light intoan electrical signal. Such photodetectors can include integrated conversion of light tovoltage with electronic ampli?cation components; integrated conversion of light to digitalfrequency components; or integrated analog to digital conversion components.As shown in Figs. 4 and 5, the color sensor 10 is preferably mounted tothe distal or pointed end 59 of a pointed printed circuit board 13. The circuitry forcontrolling the LEDs and photodetectors, and for transferring the measured signals toa host computer are also mounted to the printed circuit board 13. The referencephotodetector 20 and the sample photodetector 22 are shown in Fig. 5 as off-the-shelfchip devices mounted to the printed circuit board 13 via pins 60. The LEDs are also off-the-shelf devices mounted to the printed circuit board 13 via leads 61. Suitablephotodetectors for use with the present invention are OPT209PJ from Burr-Brown orTSL230A from Texas Instruments; and suitable LEDs for use with the present inventionare NLPB-300A from Nichia, NSPB-300A from Nichia, NSPG-300A from Nichia, E166from Gilway, E104 from Gilway, E198 from Gilway, E102 from Gilway, E472 fromGilway, BL-B4331E from America Bright, or HLMP-K640 from Hewlett Packard.13?CA 02265534 l999-03- 10WO 98/11410 PCT/US97l16009Preferably, the LEDs are encapsulated so as to provide a lens integral with the LED.Optionally, a non-encapsulated light source can be utilized by incorporating a lenswithin the emitter aperture 30.As shown in Figs. 9-11, a shell 62, consisting of an upper shell piece 64and a lower shell piece 66 is mounted around the printed circuit board 13 and colorsensor 10 and is ergonomically shaped for gripping and manipulation by an operator'shand. The shell contains a pressure—activated switch 68 that is operatively coupled tothe circuitry contained on the printed circuit board, such that the operator will be ableto initiate a color measurement process by activating the switch. Preferably, the opticalcap 26 is an integral part of the upper shell piece 64 and the collar 24 is an integral partof the lower shell piece 66. The re?ector cone 28 is preferably an independent piecewhich is mounted to the shell 62 after mounting the upper shell piece 64 and lower shellpiece 66 to the printed circuit board 13. The separate sensor parts, the upper shellpiece 64, the lower shell piece 66 and the re?ector cone 28 are all preferably made froman opaque black ABS plastic.As shown in Figs. 10 and 11, upper shell piece 64 has an area 168 forseating an index ?nger 170 of a human hand. Positioned in this area 168 is the switch68, operatively coupled to the circuitry of the sensor such that pressure activation of theswitch will activate the circuitry, and will thus initiate a colorimetric reading of the samplesurface 12. The switch 68 is in axial alignment with the aperture 48 and with axis A ofthe collar 24, and in turn, is in alignment with the receiver aperture 32 of the collar.Accordingly, in the preferred embodiment, the user will be able to use the presentinvention to “point” with his or her index ?nger 170 to an area of the sample surface 12that he or she wishes to take a colorimetric reading of the sample surface, and will thensimply press the switch 68 using the same index finger.Referring back to Figs. 4 and 5, the measured signals received by thesensor 10 and preprocessed by the circuitry 13 are transmitted back to a host computerthrough a serial interface modular connector 58. The serial interface connector 58 canbe made to.support the RS-232 protocol, or can be configured to support the AppleDesktop Bus (ADB) protocol. Preferably, the serial interface connector 58 also supplies14?CA 02265534 l999-03- 10W0 98/11410 PCT/US97/16009power to the sensor circuitry, as provided by the host computer.The reference photodetector and the sample photodetector 20,22 arepreferably identical devices and are preferably mounted back to back on the printedcircuit board so that, not only will the photodetectors be substantially identical, they willshare environmental characteristics, such as temperature, humidity, electrical noise,etc. Therefore, the photodetectors 20,22 will be substantially thermally matched so thattemperature stabilization of the photodetectors is not required. This is because thetemperature variances will cancel each other out in the re?ectance calculation asdescribed below. The collar 24 also provides thermal isolation between the LEDs 18and the sample photodetector 22, thus preventing heat generated by the LEDs frominterfering with the thermal matching between the reference and samplephotodetectors. Furthermore, no gratings or filters are required because the use ofLEDs of different wavelengths eliminates the need for such gratings or ?lters.As shown in Fig. 6, each pin 60’ of the reference photodetector 20 isoptionally coupled to a corresponding pin 60" of the sample photodetector by athermally conductive material 167. This material 167 enhances the matching of thermalcharacteristics between the reference photodetector 20 and the sample photodetector22. If the corresponding pins 60’, 60" are coupled to the same circuit connection, i.e.,Vcc or ground, the material 167 may also be electrically conductive so as to facilitatethe matching of electrical characteristics.Those skilled in the art will appreciate that the sensor 10 need not bedesigned as mounted to a printed circuit board. A mounting plate or base can beprovided in place of the printed circuit board; or the collar 24 or another component canbe designed to house the LEDs and photodetectors within the sensor 10 in thearrangement required by the present invention.As shown in Fig. 7, circuitry for controlling the sensor 10 andpreprocessing the measured signals taken from the sensor comprises a microcontroller80, an analog to digital converter 82, a series of LED switches 84, and a programmableLED power source 86. The microcontroller 80 is operatively coupled to the switch 68.The microcontroller communicates with the serial interface modular connector 5815?CA 02265534 l999-03- 10W0 98/ I 1410 PCTIU S97/ 16009through a serial communications bus 88. The microcontroller 80 can control which ofthe LEDs 18 are activated through an LED select line 90 and can control the intensityof the activated LEDs through an LED level line 92 sent to the programmable LEDpower source 86. The programmable LED power source provides the LED powerthrough an LED power line 96, and also includes an LED current sense input line 94from a current detector housed within the sensor 10 as feedback to the programmableLED power source calculation. However, since the sensor 10 is by nature a dual-beamdevice, and emitter ?uctuations are thereby cancelled during the process of measuringcolor, the LED current control circuit is not critical to the operation of the device, but canbe used to optimize LED signal to desired levels.A sample channel output line 97 is coupled between a multiplexer device98 and the sample photodetector 22 housed within the sensor 10, and a referencechannel output line 99 is coupled between the multiplexer device 98 and the referencephotodetector 20 housed within the sensor 10. The microcontroller 80 is able to controlwhich of the sample outlet channel line 97 or the reference channel output line 99 issent to the analog to digital converter device 82 through a multiplexer control line 100,and the microcontroller 80 controls the operations of the analog to digital converter 82through an A/D control line 102. The digitized measurement from the samplephotodetector 22 or the reference photodetector 20 output by the analog to digitalconverter 82 is sent back to the microcontroller 80 via a digitized value line 104. As willbe apparent to one of ordinary skill in the art, the LED current sense line 94 can alsobe used to further correct the reference channel response for each individual LEDagainst some expected response.The color sensor 10 is operated as follows. To obtain reflectancereadings of a sample surface, the re?ector cone aperture 48 is first placed on thesample surface 12 and the switch 68 is activated. Upon activation of the switch 68, themicrocontroller will take the digitized “dark” readings of the sample output channel 97(lsd) and of the reference output channel 99 (lrd) through the analog to digital converterdevice 82 (note that none of the LEDs 18 are activated). Next, the LED switches 84 areactivated in sequence by the microcontroller 80, thus activating the LEDs 18 of the16?CA 02265534 l999-03- 10W0 98/11410 PCT/US97/16009respective wavelengths in sequence. For each LED activated, the microcontrollertakes the digitized readings of the sample output channel 97 (lsy) and of the referenceoutput channel 99 (lry), where "A" represents the peak wavelength of the particular LEDactivated. The raw, unscaled device re?ectance Ruy for each given LED of peakwavelength A is then calculated using the following equation:(Equ. 1)Generally, the microcontroller 80 measures is, and lrd right after eachother, but just before the '3)‘ and lry measurements are made. This is an attempt toreduce the effect of photodetector offset drifts, which can be seen by the darkmeasurement. Similarly, |S)\ and Ir, are measured right after each other for a given LED,which is obviously an attempt to reduce photodetector gain drifts. Although it logicallyfollows that for each LED, a measurement of lsd and Ir, should be made immediatelyprior to or following the particular IS)‘ and lry measurement for that LED, a “shortcut”measurement cycle has been developed and verified through experimentation thatrequires only one Is, and lrd measurement to be taken for all LEDs during themeasurement cycle. The shortcut measurement cycle is as follows:Measurement Cycle ~ls,,, lrd. ISM, lrM, ism, |r,2 ism, lrmwhere N is the number of LEDs illuminated during the cycle.Additionally, it has also been found that a slight gain in precision can beobtained using the following measurement cycle:Measurement Cycle —-lsdo, lrdo, ls“, Ir“, ism, lrm ism, lrm, ism, lrd,From this the resulting value of Is,“ and lrdy for each given LED of peak wavelength Aused in the dark correction scheme are obtained by interpolating from the measured17?CA 02265534 l999-03- 10W0 98/ 1 1410 PCT/US97/16009values lsdo, lrdo, lsd,, and lrd, as follows:Isa)‘ = «N |Sdo +lSd1)+(i'1)(ISd1'|Sd0))/(N T 1) (EqU- 2)for i = 1 Nand similarlyIrd)\ : “N "do +|rd1)+(i'1)([rd1'[rd0))/(N +1) (EqU- 3)where Al corresponds to measurements made for the ith LED in the measurement cycle.Preferably, at some point prior to measuring the re?ectance of a coloredsample surface, the color sensor should first be used to “calibrate” the photodetectorsby activating the color sensor 10 over a black or non-reflecting calibration surface toobtain a “black" re?ectance reading Rmack, and then activating the color sensor 10 overa re?ecting diffuse white surface to obtain a “white" reflectance reading Rwme. Thereflector cone aperture 48 is first placed on the ‘‘black’’ non-reflecting calibrationsurface, the switch 68 is pressed, and reflectance readings are made of the non-reflecting calibration surface. Then, the re?ector cone aperture 48 is placed on the“white” re?ecting calibration surface, the switch 68 is pressed, and re?ectance readingsare made of the re?ecting calibration surface. These stored black and white calibrationmeasurements Rmack and Rwme are then used to scale subsequent re?ectancemeasurements Ruy, described above, as follows:RA = (RUA ‘ Rblack) / (Rwhite - Rblack» x(Sw ‘ Sb) + Sb (Eql-L 4)where R, is the normalized and bias corrected reflectance measurement for a givenLED of peak wavelength A; SW is a “white” scaling factor obtained from a standardreference device, such as a Gretag SPM100 laboratory grade spectrophotometer; and8,, is a “black” scaling factor obtained from a standard reference device. It also followsthat:18?CA 02265534 l999-03- 10WO 98111410 PCTIUS97/ 16009R’ = “R”, ' R,black) / (R’white— Riblack» “(SW ' Sb) + Sb (EqU- 5)where R’ is a vector of size N representing the normalized and bias correctedreflectance color value of the sample just measured; Ru’ is a vector of size Nrepresenting the raw, unscaled device re?ectances of the sample just measured, R’b,aCkis a vector of size N representing the raw, unscaled device reflectances of a pre-measured “black” calibration tile; and R’wh,,e is a vector of size N representing the raw,unscaled device reflectances of a pre—measured white calibration tile.In the above process, it is also within the scope of the invention to activatemore than one LED at a time, and to determine the reflectance for the combinedwavelengths of the illuminated LEDS. For example, to provide better CIE colormatching functions and to quicken the sampling process, a combination of LEDs canbe illuminated to provide the combined wavelengths for an entire RGB color wavelengthband; thus requiring only three samples to be taken and calculated -- i.e., one for allLEDs making up the RED band, one for all LEDS making up the GREEN band, and onefor all LEDs making up the BLUE band. Additionally, multiple LEDs of the samewavelength may be activated simultaneously to increase the apparent brightnessassociated with a particular wavelength band.As discussed above, it is within the scope of the invention to utilize a light-to-frequency converter device, such as a TSL230 from Texas Instruments, as aphotodetector. The light-to-frequency converter device emits a frequency signal thatcorresponds to the brightness of the light detected by the device. Accordingly, use ofa light-to-frequency converter device may eliminate the need for analog-to-digitalconverter devices in the present invention, because the host computer ormicrocontroller will be able to calculate the re?ectance based upon the frequency of thesignal transmitted by the light-to-frequency device.An on-board programmable memory may optionally be included in thecircuitry to contain hardware setup and calibration information; and accommodation ofa liquid crystal display may also be included to provide a user read-out. Otherenhancements can include an internal battery for wireless operation.19?CA 02265534 l999-03- 10W0 98l1l410 PCT/US97/16009The present embodiment for colorimetric operation utilizes ten LEDs ofdifferent wavelengths. Preferably, the peak wavelengths of the ten LEDs utilized are430, 450, 470, 525, 558, 565, 585, 594, 610, and 635 nanometers respectively. It isnoted that although the peak wavelengths are indicated, each LED transmitswavelengths of a particular bandwidth, which typically ranges from ten to one-hundrednanometers wide.It should be apparent to one of ordinary skill in the art that additional orfewer LEDs may be utilized in the present invention, depending upon the accuracy andrepeatability requirements of the sensor. For example, a sensor utilizing three or sixLEDs will have lesser accuracy or repeatability than in the ten LED embodiment and asensor utilizing sixteen LEDs will have greater accuracy and repeatability than the tenLED embodiment. Preferably, if a three LED embodiment is utilized, the peakwavelengths of the three LEDs are 450, 555, and 610 nanometers respectively; if a sixLED embodiment is utilized, the peak wavelengths of the six LEDs are 450, 470, 512,555, 580, and 610 nanometers respectively; and if a sixteen LED embodiment isutilized, the peak wavelengths of the sixteen LEDs are 430, 450, 470, 489, 512, 525,558, 565, 574, 585, 594, 605, 610, 620, 635, and 660 nanometers respectively.Although it is within the scope of the invention to utilize the three LEDembodiment (i.e., 450, 555, and 610 nanometers respectively), there are severalpractical advantages for using more than three spectrally unique LEDs or channelsfor collecting data. First, using more than three spectral shapes makes it easier tospan the space defined by the CIE color matching functions, since each additionalchannel provides an additional degree of freedom with which to compute tristimulusvalues.Second, in practice, it is difficult to cover the space defined by the colormatching functions for one particular illuminant with a combination of only threeLEDs. This is due to the fact that there is a limited number of LED spectral shapescommercially and physically available, and the LED power spectral distributions varysignificantly between LEDs from a single lot. Therefore, by using more than threeLEDs the instrument will not be as sensitive to known variations in the LEDs (due to20?CA 02265534 l999-03- 10W0 98/11410 PCT/US97/16009redundancy). In addition, combining LEDs will provide effective shapes that aredifferent than those provided by single LEDs. This can facilitate in matching themulti-peak CIE color matching functions.Finally, using more than three channels will allow the device to providecolorimetric information for the sample under measure for multiple illuminants withoutobtaining the entire spectral re?ectance of the sample. This capability results in aninstrument with signi?cant ?exibility over a three channel device.Once the corrected, re?ectance ratio vector R’ of the sample is determinedas described above, this ratio vector R’ is then converted into a usable colormeasurement value t (for example, the color measurement value t can be the CIE XYZtristimulus value for the sample) through a linear or a non-linear operation. To executethis conversion, a mathematical profile of the sensor 10, which obtained themeasurement, must first be determined. This is because the conversion from there?ectance ratio vector R’ to the color measurement value t depends upon the uniquespectral characteristics of thesensor components used to obtain the re?ectance ratiovector R’.The mathematical profile of the sensor 10 is based upon the followingmatrix/vector equation:R’ = STDr + b (Equ. 6)where (assuming M is the number of LEDs 18 and N is the number of wavelengths tobe sampled in the visible spectrum) R’ is an Mxl vector of the recorded reflectanceratios; ST is an MxN matrix representing the spectral power distributions of the Mspectrally unique LEDs 18; D is an NxN diagonal matrix representing the spectralsensitivity of the sample photodetector; b is an Mx1 bias vector; and r is an Nx1 vectorrepresenting the actual spectral re?ectance of the sample. The mathematical pro?le ofthe device is determined by finding ST, D, and b. These values are found by takingmeasurements of samples having known spectral re?ectances r, using the sensor 10.In simple terms, the determination of the mathematical sensor profile is based on the21?CA 02265534 l999-03- 10W0 98l1l4l0 PCT/US97/16009spectral sensitivity of the sample photodetector, the spectral power distributions of theLEDs 18, and the re?ectance of several known samples.The spectral sensitivity D of the sample photodetector is available fromthe photodetector’s manufacturer. Provided a quality photodetector is used, thesensitivity should be the same, within an acceptable range, over all detectors. At thetime of the present invention, LED manufacturers are not able to ensure the exactspectral shape and peak wavelength across a particular batch of LEDs. For thisreason, it is necessary to measure the spectral power distribution ST of each LED witha spectraradiometer. But because, the spectraradiometer provides only the relativeshape of the distribution, the absolute power of the LEDs and the bias vector b aredetermined by performing a measurement with the sensor 10 on each of two spectrallyknown samples having known reflectances r, and r2. Typically, black and whitesamples are used. These measurements provide M sets of 2 equations to solve for thetwo unknowns (ST and b), where we already have each row of S7 to within a constant:R1’ = S‘Dr, + b (Equ. 7a)R2’ = S'Dr, + b (Equ. 7b)Once the sensor is pro?led, i.e., once ST, D and b are found, an accurateestimate of the actual spectral reflectance r of the sample can be obtained. The CIEXYZ tristimulus value t for the sample under illuminant L can be denoted by thevector/matrix equation:t = ATLr (Equ. 8)where L is an NxN diagonal matrix containing the illuminant spectral power distributionand the columns of the Nx3 matrix A contain the CIE XYZ color matching functions.The determination or approximation oft according to Equ. 8 will be apparent to one ofordinary skill in the art. Mathematically the problem of transforming from the ratios to22?CA 02265534 l999-03- 10W0 98/ 1 1410 PCT/U S97! 16009a descriptor like t can be described as:min E{lll=(t) - F(G(c))||} (Equ. 9)Gwhere E{} is the expectation operation over the system noise and reflectance spectraof interest, F is a function which transforms from the color space containing t to aperceptually uniform color space (i.e., accounts for the sensitivity of the human visualsystem), and G is the function approximating t (G(c) z t). Accordingly, G is the functionto be found. Depending upon the application, it may be desirable to select G as eithera linear or non-linear function.For applications in which a spectrally widely varying set of samples ismeasured, a linear transformation will be more robust than non-linear transformations.Typically, the function F will be of a form which makes an analytical solution to theabove optimization problem difficult. In this case, the transformation G can be foundby a two step process. In the ?rst step, an initial estimate for G is obtained analyticallywhich minimizes the error in the CIE XYZ space (i.e., the function F is ignored). Thissolution is easily computed using simple matrix algebra. in the second step, thisestimate is used as a starting point for a numerical optimization algorithm whichminimizes the above non-linear problem. Any standard non-linear optimizationalgorithm will be sufficient for this task.The expectation operator in the above optimization problem is taken overa set (or ensemble) of re?ectance spectra. The above approach requires somerepresentative re?ectance samples which the sensor may be used to measure. Fromthese samples, a re?ectance correlation matrix is constructed and used in the firstanalytical step (when the function F is ignored). Specifically, the analytical solution isgiven by:G = ATK,S[STK,S]" (Equ. 10)23?CA 02265534 l999-03- 10W0 98/1 1410 PCT/U S97! 16009where K, = E{rrT} is the reflectance correlation matrix which is estimated by:K, = 1/NR §r.r.* (Equ. 11)where R is some ensemble of NR re?ectance spectra. The numerical step is thenperformed over each sample in R. The conditioning of the transformation matrix canbe set as an optimization constraint. The amount of conditioning or regularizationshould be a function of the system noise thereby producing a transformation whichgives good repeatability.Alternatively the function F can be locally linearized for each sample in theensemble R. This linearization provides a means to obtain analytically a solution whichmay be perceptually acceptable, and allow the incorporation of the system noise forgood repeatability.Finally, for the spectral case, the optimization problem becomes:min E{||r - G(c)ll} (Equ. 12)Gfor which the analytical solution is given by:G = K,S[STK,S]“‘ (Equ. 13)Depending upon the system noise and the LEDs it may be necessary toperform a pseudo-inverse operation for computing the above inverse. The pseudo-inverse operation may also be needed in the colorimetric case. The pseudo-inversecan be computed by dropping numerically insigni?cant singular values in the matrixSTK,S.As shown in Fig. 14, the present invention, by installing a photodetector160 in place of an LED, can also be used as a gloss meter. The photodetector 160would have to replace a ?rst LED which is positioned 180° away from a second LED24?CA 02265534 l999-03- 10W0 98/ 11410 PCT/US97Il600918" with respect to axis A. The photodetector would thus be able to detect the specularcomponent of the light waves transmitted from the second LED and reflected off thesample.it should also be apparent to one of ordinary skill in the art that LEDs 18having wavelengths in the non—visible spectrum can be utilized for various purposes.For example, utilizing LEDs in the infrared spectrum will allow the sensor to measurethe infrared re?ectivity of a sample surface. In a related application, the infrared LEDscan also be used to transmit data to a host computer through the re?ector cone'saperture 48; thus eliminating the need for the serial interface connector 58.The present invention can also be used to measure the radiance of agiven sample that is self luminous, i.e., a CRT display screen. This application does notneed to utilize the LEDs.As shown in Fig. 8, an alternate circuit for the present invention utilizesa successive approximation technique to perform measurement of spectral re?ectancesfrom the dual beam sensor 10. The circuitry for performing the successiveapproximation measurement comprises a microcontroller 100, a digital to analogconverter 102, a D latch 104, another D latch 106, a sample channel voltagecomparator device 108, a reference channel voltage comparator device 110, and an ORgate 112. Themicrocontroller 100 communicates with the serial interface modular connector 58The microcontroller is operatively coupled to the switch 68.through a communications bus 114. The microcontroller can control which of the LEDs18 are activated through an LED select line 116, which indicates which of the LEDswitches 118 are to be activated or inactivated. The approximation circuit also utilizesa voltage reference 120, which is a ?xed stable voltage reference used by the digital toanalog converter 102 and the reference channel voltage comparator device 110. Theapproximation circuit also utilizes an LED variable control device 122, which providesvariable resistance used to gate the amount of current allowed to ?ow from the LED'scurrent drain. This device is fed by an LED gate signal 124 sent from a delay rampcircuit 126 that generates a voltage ramp beginning at a low voltage level, and overtime, becomes higher in a linear fashion.25?CA 02265534 l999-03- 10WO 98/11410 PCT/US97/16009The approximation circuit is initialized as follows: initially, upon activatedby the switch 68, the microcontroller activates a clear line 128 which clears both Dlatches 104,106; the microcontroller transmits to the digital to analog converter 102 a0.5 full scale code through a D/A control line 130; and the internal LED select variablei(which indicates the particular LED switch to be activated) is initialized to zero (no LEDselected).The re?ectivity measurement process for a given LED iis conducted asfollows: The microcontrolier 100 will first select LED i through the LED select line 116sent to the LED switches 118. LED power 132 is applied to the LED i, but the LED idoes not yet activate because the LED variable control device 122 is "off." Next, themicrocontroller 100 activates a "start" line 134, which is coupled to the clock input 136of the D latch 106. Accordingly, the Q output 138 of the D latch 106 ("LED on")becomes activated as the "start" signal ripples through the D latch 106. This "LED on"signal 138 is sent to the delay ramp circuit 126, which in turn begins applying thelinearly increasing "LED gate" signal 124 to the LED variable control device 122. TheLED variable control device 122, now activated, allows current to ?ow from the LEDpower source 132 through the previously selected LED switch 118 through thecorresponding LED 18 and LED cathode 140 and into an "LED current drain" 142. Ofcourse, since current is ?owing through the LED 18, the LED now emits light at itspredetermined wavelength; however, the brightness depends upon the linearlyincreasing "LED gate" signal 124.The reference photodetector 20, as discussed above, receives light fromthe activated LED 18 and transmits a signal indicative of the measured light to thevoltage comparator 108 over a reference channel output line 144. Likewise, the samplephotodetector 22 receives light re?ected from the color sample, as discussed above,and transmits a signal indicative of the detection of this light to the voltage comparatorcircuit 108 over a sample channel output line 146. As the LED gate signal 124increases, intensity of light from the LED 18 increases until either the sample outputvoltage comparator 108 or the reference output voltage comparator 110 is activated.The sample channel voltage comparator 106 will be activated if the voltage on the26?CA 02265534 l999-03- 10W0 98/1 1410 PCT/U S97/ 16009sample channel output line 146 is greater than the analog output 152 from the digitalto analog comparator 102, and the reference channel voltage comparator 104 will beactivated if the reference channel output line 144 voltage is larger than the voltagereference 120.If the sample output voltage comparator 108 is activated, it activates the "X>Y"line 148, which is coupled to the clock input 149 of the D latch 104. Therefore, uponthe "X>Y" 148 signal being activated by the sample channel voltage comparator, theD latch 104 will correspondingly notify the microcontroller 100 over the "X>Y (held)" line154. At this time, the microcontroller 100 will determine if the granularity (resolution) ofthe D/A setting has been reached and, if so, this measurement is reported to the hostcomputer through the RS232 serial link 58, and the measurement is complete. Ifthemicrocontroller 100 determines that the granularity of D/A setting has not been reached,the digital to analog signal is increased over the digital to analog control line 130, themicrocontroller sets the "clear line" 128 clearing both D latches 104,106 and restarts theabove measurement process beginning with activating the "start" line 134.Alternatively, if the reference channel voltage comparator 1 10 is activated,this indicates that the reference channel output 144 voltage is larger than the voltagereference 120. Activation of the reference channel comparator 108 thus causes the Dlatch 106 to be cleared. The microcontroller 100 will sense this, through a "fault" signal,and will decrease the D/A control signal 130 to the digital to analog converter 102, andwill again proceed with the above measurement process starting by activating the "startline" 134.The above successive approximation circuit can be used to measure thesample channel output 146 with respect to the reference channel output 144. Thismethod uses the digital to analog (D/A) converter 102 to set a threshold level 152against which the sample channel output 146 is compared. if the threshold level is toolow, it is increased; if it is too high, it is decreased. The threshold value being soughtis that which matches the sample channel output 146 at the closest possible instant towhen the reference channel output 144 matches the stable voltage reference 120. Thismethod of "hunting" for the appropriate threshold is iterative, and when certain27?CA 02265534 1999-03-10W0 98/ 1 1410 PCT/U S97/ 16009optimizations, such as a binary tree search algorithm, are employed, it can be relativelyquick. When the ?nal threshold is discerned, the digital representation correspondingto the sample channel is known. Since the stable voltage reference 120 is both usedto generate the output voltage 152 of the D/A 102, as well as in the comparison of thereference photodiode channel output 144, the act of comparing the sample photodiodechannel output 146 to the D/A output voltage 152 is effectively the same as the directcomparison of the sample photodiode channel output 146 with the referencephotodiode channel. Furthermore, since the output voltage 152 of the D/A 102 isdefined by the following relationship:D/A output = voltage reference x (digital setting/full scale digitalvalue)and since at the moment when the comparison is valid, the reference photodiodechannel output 144 is nearly equivalent to the voltage reference 120 while the samplephotodiode channel output 146 is nearly equivalent to the D/A output, the above relationbecomes:(sample chan./ref. chan.) = (digital setting/full scale digital value)Therefore, the act of ?nding the digital setting corresponding to the appropriatethreshold is the same as ?nding the value of sample channel divided by the referencechanneiOf course, the above circuit is equally valid when a means to scale thesample and/or reference channels by known quantities is used. in addition, assuminglinearity of the output of the combination of the "delay (ramp)" 126 and "LED variablecontrol" 122 circuits, the binary tree search algorithm can even be further improved bynoting the time discrepancy between the triggering of the sample channel comparisonand the triggering of the reference channel comparison, and scaling next resulting"expected" threshold setting accordingly. Finally, it is also valid that the referencechannel output 144 be used as the reference voltage by which the D/A output voltage28?CA 02265534 l999-03- 10W0 98/1 1410 PCT/US97/ 16009152 is generated; this is the most direct means of calculating the desired value ofsample channel output divided by reference channel output.As shown in Figs. 12a-12c, an alternate embodiment of the annular collar24' has a plurality of emitter apertures 72 angled at a 45° angle towards the axis A,eliminating the need for the 22.5° conical re?ective surface within a re?ector cone, andeliminates the need for the re?ector cone altogether. It is noted that the different sizesdepicted for the emitter apertures 72 corresponds to different sizes of LEDs used.As shown in Fig. 13, an alternate embodiment of the invention includesan optical lens 76 in place of the re?ective surfaces of the re?ector cone. The opticallens acts to bend the lightwaves emitted from the emitter apertures 30 to a 45° angletowards the axis A.It is also within the scope of certain aspects of the present invention toprovide a “|ow-end” color sensor that does not utilize the reference photodetector 20,yet may utilize any or all of the novel elements described above for use with a hand-held, single-beam color sensor. Referring to Fig. 7, an important element in such asing|e—beam color sensor would be the LED current sense input line 94. It would alsobe bene?cial in such an embodiment to inject the current sense signals into the samplephotodetector 22 and an associated amplification circuit (whether internal or externalto the sample photodetector) to provide advanced gain and offset correction.As mentioned above, alternate embodiments of the invention as shownin Figs. 16 and 17 may be better suited for remote color sensing of a sample surface,such as a patient’s skin. As shown in Fig. 16, a re?ector cone 28" includes a conicalre?ector surface 52 ” that is angled with respect to the axes B of the emitter apertures30" at a 10° angle. Light emitted through the emitter apertures 30" and reflected fromthe conical re?ective surface 52" will contact the sample surface 12 at an angle of 20°.Therefore the embodiment as shown in Fig. 16 will have a 20°/0° geometry. As shownin Fig. 17, a collar 24"’ includes a radially outwardly extending, and substantially conicalre?ective surface 190, which intersects axes B such that light emitted through theemitter apertures 30"’ will be re?ected towards a curved, inner re?ective surface 192of a re?ector cone 28"’. The light reflected from the inner re?ective surface 192 towards29?CA 02265534 l999-03- 10W0 98/ 1 1410 PCTfUS97/ 16009the sample surface 12 will thus be substantially diffuse. Therefore, the embodiment asshown in Fig. 17 will have a modified-diffuse/0° geometry.Having described the invention in detail and by reference to preferredembodiments thereof, it will be apparent that modifications and variations are possiblewithout departing from the scope of the invention defined in the appended claims.What is claimed is:30

Claims (55)

WHAT IS CLAIMED IS:

1. A hand-manipulatable device for gathering reflective, densitometric, spectrophotometric, colorimetric, self-luminous or radiometric readings from a sample surface, comprising:
a housing having a substantially flat bottom surface and a top surface contoured to fit comfortably in the fingers and palm of the human hand;

a sensor mounted to said housing, including a focal aperture and including circuitry and optics for performing a reflective, densitometric, spectrophotometric or colorimetric reading from a portion of said sample surface aligned with said focal aperture;
a switch operatively coupled to said sensor for activating said sensor to perform said reading; and a data link adapted to be operatively coupled between said sensor and a computer for relaying data from said sensor to said computer;
said sensor including (a) a printed circuit board mounted within said housing, (b) a plurality of light sources, mounted within said housing, respectively emitting light of a substantially different wavelength band, (c) a sample photodetector mounted to said printed circuit board, (d) a first optical element adapted to direct a first portion of light emitted by each of said light sources to the sample surface, and (e) a second optical element adapted to direct a portion of light reflected from the sample to said sample photodetector.

2. The hand-manipulatable device of claim 1, wherein said focal aperture is in substantial vertical alignment with an area in said top surface of said housing for seating a tip of an index finger.

3. The hand-manipulatable device of claim 2, wherein said switch is a pressure activated switch mounted to said housing substantially in said area for seating a tip of an index finger.

4. A hand-manipulatable device for gathering reflective, densitometric, spectrophotometric, colorimetric, self-luminous or radiometric readings from a sample surface, comprising:
a housing having a substantially flat bottom surface and a top surface contoured to fit comfortably in the fingers and palm of the human hand;
a sensor mounted to said housing, including a focal aperture and including circuitry and optics for performing a reflective, densitometric, spectrophotometric or colorimetric reading from a portion of said sample surface aligned with said focal aperture;
a switch operatively coupled to said sensor for activating said sensor to perform said reading; and a data link adapted to be operatively coupled between said sensor and a computer for relaying data from said sensor to said computer;
said sensor including (a) a printed circuit board mounted within said housing, (b) a plurality of light sources, mounted within said housing, respectively emitting light of a substantially different wavelength band, (c) a sample photodetector mounted to said printed circuit board, (d) a first optical element adapted to direct a first portion of light emitted by each of said light sources to the sample surface, (e) a second optical element adapted to direct a portion of light reflected from the sample to said sample photodetector, (f) a reference photodetector mounted to said printed circuit board, and (g) a third optical element adapted to direct a second portion of light emitted by each of said light sources to said reference photodetector.

5. The hand manipulatable device of claim 4, wherein said third optical element is an optical cap having a substantially non-absorbing interior integrating surface, mounted over said light sources and said reference photodetector, said optical cap forming a reference chamber, whereby, said second portion of light emitted by said light sources reflects off of said interior integrating surface of said optical cap and said reference photodetector receives a substantial amount of said second portion of light reflected off of said interior integrating surface.

6. The hand manipulatable device of claim 5, wherein said optical cap is integral with said housing.

7. The hand manipulatable device of claim 4, wherein said reference photodetector and said sample photodetector are mounted substantially back-to-back so as to share environmental characteristics.

8. The hand manipulatable device of claim 1, wherein:
said printed circuit board includes a plurality of apertures extending completely therethrough;
at least one aperture is provided for a corresponding one of said light sources; and each of said sources are positioned substantially adjacent to said corresponding aperture.

9. A hand-manipulatable device for gathering reflective, densitometric, spectrophotometric, colorimetric, self-luminous or radiometric readings from a sample surface, comprising:
a housing having a substantially flat bottom surface and a top surface contoured to fit comfortably in the fingers and palm of the human hand;
a sensor mounted to said housing, including a focal aperture and including circuitry and optics for performing a reflective, densitometric, spectrophotometric or colorimetric reading from a portion of said sample surface aligned with said focal aperture;
a switch operatively coupled to said sensor for activating said sensor to perform said reading; and a data link adapted to be operatively coupled between said sensor and a computer for relaying data from said sensor to said computer;
said sensor including (a) a printed circuit board mounted within said housing, (b) a plurality of light sources, mounted within said housing, respectively emitting light of a substantially different wavelength band, (c) a sample photodetector mounted to said printed circuit board, (d) a first optical element adapted to direct a first portion of light emitted by each of said light sources to the sample surface, and (e) a second optical element adapted to direct a portion of light reflected from the sample to said sample photodetector;
said printed circuit board including a plurality of apertures extending completely therethrough, at least one aperture being provided for a corresponding one of said light sources, and each of said sources being positioned substantially adjacent to said corresponding aperture;
said light sources being arranged about said sample photodetector;
said second optical element being mounted over said sample photodetector and including a receiving aperture in optical alignment with said sample photodetector;
said first optical element including a reflector cone, said reflector cone including hollow cavity, a tip., and said focal aperture in said tip, said focal aperture providing optical communication into said hollow cavity;

said reflector cone being mounted over said apertures and said second optical element to form a sample chamber, said focal aperture being positioned in optical alignment with said receiving aperture;
said reflector cone including a frustoconically shaped reflective inner surface, axially aligned with said focal aperture, and positioned to intersect with said first portion of light emitted by said light sources; and said reflective inner surface of said hollow cavity being angled inwardly toward said focal aperture at an angle so as to direct said first portion of light emitted by said light emitting toward sample at substantially a 45° angle, such that said first portion of light reflects from the sample at substantially a 45° angle and said sample photodetector receives a diffuse component of said light reflected from the sample.

10. The hand manipulatable device of claim 8, wherein said light sources are light emitting diodes which are mounted to a daughterboard, the daughterboard being coupled to said printed circuit board.

11. A hand-manipulatable device for gathering reflective, densitometric, spectrophotometric, colorimetric, self-luminous or radiometric readings from a sample surface, comprising:
a housing having a substantially flat bottom surface and a top surface contoured to fit comfortably in the fingers and palm of the human hand;
a sensor mounted to said housing, including a focal aperture and including circuitry and optics for performing a reflective, densitometric, spectrophotometric or colorimetric reading from a portion of said sample surface aligned with said focal aperture;
a switch operatively coupled to said sensor for activating said sensor to perform said reading; and a data link adapted to be operatively coupled between said sensor and a computer for relaying data from said sensor to said computer;

said sensor including (a) a plurality of light emitting diodes, each of said diodes respectively emitting light of a substantially different wavelength band, mounted within said housing, (b) a light-pipe for directing light emitted from said diodes to said optics, said optics being adapted to transmit light directed from said light pipe to said focal aperture at an angle of approximately 45°, (c) a reference photodetector for receiving light bled from said light-pipe, and (d) a sample photodetector for receiving a diffuse component of said light transmitted to said focal aperture by said optics and reflected from a sample surface.

12. A sensor for sensing reflective, spectrophotometric, densitometric, colorimetric, self Luminous or radiometric characteristics of a sample, comprising:
a base having a lower surface;
a plurality of fight sources respectively emitting light of a substantially different wavelength band, mounted to said base such that a first portion of fight emitted by said light sources transmits below said lower surface of said base;
a sample photodetector; and a first optical assembly, mounted to said lower surface of said base, said first optical assembly being configured to direct said first portion of the light to the sample while excluding direct transmission of said first portion of light to said sample photodetector and being further configured to direct a portion of light reflected from the sample to said sample photodetector;
said first optical assembly includes an annular collar mounted to said lower surface of said base and positioned over said plurality of light sources and over said sample photodetector, said collar including a plurality of emitter apertures extending axially therethrough, each emitter aperture being provided for a corresponding one of said light sources, and said collar includes a receiver aperture extending axially through said collar so as to provide optical communication between the sample and said sample photodetector.

13. The sensor of claim 12, wherein:
said emitter apertures are arranged about said sample photodetector;
said first optical assembly further includes a reflector cone, said reflector cone including a central axis, a hollow cavity, a tip, and a focal aperture in said tip, said focal aperture providing optical communication into said hollow cavity;
said reflector cone extends from a lower surface of said collar;
said focal aperture and said receiver aperture are coaxially aligned on said central axis;
each of said emitter apertures include an emitter axis;
said reflector cone includes a frustoconically shaped reflective inner surface, coaxially aligned with said central axis and positioned to intersect with each of said emitter axes;
said reflective inner surface of said hollow cavity is angled inwardly toward said central axis at an angle so as to direct said first portion of light emitted by said light sources towards said focal aperture at substantially a 45°
angle with respect to said central axis;
whereby, when acid focal aperture is placed on a sample surface said first portion of light emitted by said light sources is reflects off of said inner surface of said hollow cavity, and is directed toward the sample surface at substantially a 45° angle, and reflects off of the sample surface at substantially a 45° angle, and said sample photodetector receives a diffuse component of said light reflected off of the sample surface.

14. The sensor of claim 12, wherein said emitter apertures include a cylindrical channel coaxially aligned with said corresponding emitter axis so as to collimate said first portion of light emitted by said light sources.

15. The sensor of claim l2, further comprising:
a reference photodetector; and a second optical assembly configured to direct a second portion of light emitted by said light sources to said reference photodetector.

16. The sensor of claim 15, wherein:
said reference photodetector includes a reference output channel;
said sample photodetector includes a sample output channel; and the sensor further comprises a processing circuit coupled to said reference output channel and to said sample output channel, said processing circuit being configured to generate reflective, spectrophotometric, densitometric or colorimetric data pertaining said sample according, at least in part, to a ratio of data received from said reference channel versus data received from said sample channel.

17. The sensor of claim 16, wherein said processing circuit includes an adjustable current source: for providing controlled power to each of said light sources.

18. The sensor of claim 17, wherein said processing circuit includes a current detector for monitoring current to said light sources, said current detector generating a current signal used by said processing circuit in controlling said adjustable current source.

19. The sensor of claim 18, wherein said current signal is used by said processing circuit to correct an output response from said reference output channel against an expected output response.

20. The sensor of claim 16, wherein said processing circuit includes a switch component, coupled to said plurality of light sources, allowing said processing circuit to activate each of said fight sources separately, or in combination, in a predetermined sequence.

21. The sensor of claim 16, wherein said processing circuit is a successive approximation circuit.

22. The sensor of claim 12, further comprising a second photodetector, positioned to receive a specular component of said light reflected from the sample, whereby said sensor can be used to measure surface gloss of the sample.

23. The sensor of claim 12, wherein said light sources are light emitting diodes, and said light emitting diodes are encapsulated so as to provide a lens integral with said light emitting diodes.

24. The sensor of claim 12, wherein said collar is configured to thermally isolate said light sources from said sample photodetector.

25. The sensor of claim 12, wherein:
said sample photodetector includes a sample output channel;
the sensor further comprises a processing circuit coupled to said sample output channel, said processing circuit being configured to generate reflective, densitometric, spectrophotometric or colorimetric, self-luminous or radiometric data of said sample based upon data received from said sample channel; and said processing circuit includes a current detector for monitoring current to said light sources, said current detector generating a current signal used by said processing circuit to correct said reflective, densitometric, spectrophotometric or colorimetric data.

26. The sensor of claim 12, further comprising an optical filter mounted within said receiver aperture for excluding a predetermined component of light entering said receiver aperture.

27. The sensor of claim 12, wherein said base is a printed circuit board, said light sources acre light emitting diodes, and said light emitting diodes and said sample photodetector are mounted to said printed circuit board.

28. A sensor for detecting colorimetric, reflective, densitometric or spectrophotometric, self-luminous or radiometric characteristics of a sample, comprising:
a plurality of light emitting diodes for emitting light of substantially different wavelength bands spaced in a predefined spectrum;
a base hauling top surface and a bottom surface, a plurality of emitter apertures extending completely therethrough from said top surface to said bottom surface;
an optical cap having a substantially non-absorbing interior integrating surface mounted over said top surface of said base;
a reference photodetector mounted to said top surface of said base;
a sample photodetector mounted to said bottom surface of said base;
said light emitting diodes being mounted within said cap and adjacent to or in said emitter apertures such a first portion of light emitted by said light emitting diodes is transmitted through said emitter apertures to a sample and reflected from said sample to said sample photodetector, and such that a second portion of light emitted by said light emitting diodes is transmitted to said integrating surface and directly or indirectly reflected from said integrating surface to said reference photodetectors.

29. The sensor of claim 28, wherein said light emitting diodes are encapsulated so as to provide a lens integral with said light emitting diodes.

30. The sensor of claim 28, wherein said reference photodetector and said sample photodetector are mounted in close proximity to one another so as to share environmental characteristics.

31. The sensor of claim 30, further comprising a thermally conductive material mounted in said base, between said sample photodetector and said reference photodetector.

32. The sensor of claim 31, wherein said thermally conductive material is also electrically conductive.

33. The sensor of claim 28, further comprising a reflecting surface extending below said bottom surface of said base and in alignment with said apertures, said reflector surface being angled so as to reflect said second portion of light emitted by said light emitting diodes to said sample at substantially a 45°
angle.

34. The sensor of claim 28, wherein said emitter apertures include a cylindrical channel axially aligned with said corresponding emitter axis so as to collimate said portion of light emitted by said light emitting diodes.

35. The sensor of claim 28, further comprising a programmable light emitting diode power source for activating said light emitting diodes or combinations of said light emitting diodes in a predetermined sequence.

36. The sensor of claim 28, wherein said predefined spectrum is a spectrum of visible light.

37. The sensor of claim 28, wherein said predefined spectrum is a spectrum of infrared light.

38. A sensor for sensing colorimetric, reflective, densitometric or spectrophotometric characteristics of a sample, comprising:
a printed circuit board;
a plurality of light sources, each of said light sources respectively emitting light of a substantially different wavelength band;
a reference photodetector surface mounted to a surface of said printed circuit board;
a sample photodetector surface mounted to an opposite surface of said printed circuit board;
a first optical element adapted to direct a first portion of the light emitted by each of said light sources to said reference photodetector;
a second optical element adapted to direct a second portion of the fight emitted by each of said fight sources to the sample; and a third optical element adapted to direct a portion of light reflected off of the sample to said sample photodetector.

39. A method for measuring color of a sample surface, comprising the steps of:
(a) activating at least one light source, said light source emitting light of a wavelength band;
(b) directing a first portion of said fight emitted by said light source to a reference photodetector;

(c) directing a second portion of said light emitted by said light source to the sample surface;
(d) directing light reflected from the sample surface to a sample photodetector;
(e) calculating a reflectance of the emitted light based upon output readings of the sample photodetector in step (d) and output readings of the reference photodetector in step (b); and (f) repeating steps (a) through (e) for at least three light sources, each emitting light of substantially different wavelength bands.

40. The method of claim 39, wherein a combination of light sources, each emitting light of different wavelength bands, are activated simultaneously in step (a).

41. The method of claim 39, further comprising the steps of:
(f) obtaining a reading from the sample photodetector when none of the light sources are illuminated; and (g) obtaining a reading from the reference photodetector when none of the light sources are illuminated;
wherein step (e) involves the step of calculating a reflectance for said light source based upon a ratio of a difference of output readings of the sample photodetector in step (d) and output readings of the sample photodetector in step (f) verses a difference of output readings of the reference photodetector in step (b) and output readings of the reference photodetector in step (g).

42. The method of claim 41, further comprising the steps of:
(h) activating the light source of step (a);
(i) directing a first portion of light emitted by the light source to the reference photodetector;

(j) directing a second portion of light emitted by the light source to a substantially non-reflective calibration surface;

(k) calculating a black calibration reflectance for the light source based upon output readings of the sample photodetector in step j) and output readings of the reference photodetector in step (i);

(l) activating the light source of step (a);

(m) directing a portion of light emitted by the light source to the reference photodetector;

(n) directing a second portion of light emitted by the light source to a substantially white calibration surface;

(o) calculating a white calibration reflectance for the light source based upon output readings of the sample photodetector in step (n) and the output readings of the reference photodetector in step (m); and (p) calculating a normalized and bias corrected reflectance for the light source using the reflectance calculated in step (e), the black calibration reflectance calculated in step (k) and the white calibration reflectance calculated in step (o).

43. The method of claim 39, wherein said wavelength bands are within a visible spectrum.

44. The method of claim 39, wherein said reference photodetector and said sample photodetector are positioned within close proximity of one another so as to share environmental characteristics.

45. The method of claim 44, wherein said reference photodetector and said sample photodetector are thermally insulated from said light sources.

46. The method of claim 39, wherein said steps (a) through (e} are performed with a hand-held mouse device, which includes:
said light sources;
said reference photodetector;
said sample photodetector;
a first optical element adapted to direct a first portion of the light emitted by each of said light sources to said reference photodetector;
a second optical element adapted to direct a second portion of the light emitted by each of said light sources to the sample; and a third optical element adapted to direct a portion of light reflected off of the sample to said sample photodetector.

47. The method of claim 39, wherein step (e) includes the step of accounting for the sensitivity of the reference photodetector and the spectral power distribution of the light source.

48. The method of claim 39, further comprising the step of profiling the characteristics of the reference photodetector and the light source by performing steps (a) through (f) on at least two samples having known reflectances.

49. The method of claim 39, further comprising the step of transforming the reflectance into a CIE XYZ tristimulus value for the sample, wherein the transformation accounts for the sensitivity of the human visual system.

50. A hand-manipulatable device for gathering reflective, densitometric, spectrophotometric, colorimetric, self-luminous or radiometric readings from a sample surface, comprising:
a housing having a substantially flat bottom surface and a top surface contoured to fit comfortably in the fingers and palm of the human hand;
a sensor mounted to said housing, including a focal aperture and including circuitry and optics for performing a reflective, densitometric, spectrophotometric or colorimetric reading from a portion of said sample surface aligned with said focal aperture;
a switch operatively coupled to said sensor for activating said sensor to perform said reading; and a data link adapted to be operatively coupled between said sensor and a computer for relaying data from said sensor to said computer;
said sensor including (a) a plurality of light sources, mounted within said housing, respectively emitting light of a substantially different wavelength band, (b) a sample photodetector mounted within said housing, (c) a first optical element adapted to direct a first portion of light emitted by each of said light sources to the sample surface, and (d) a second optical element adapted to direct a portion of light reflected from the sample to said sample photodetector.

51. The hand-manipulatable device of claim 50, wherein said sensor further includes (e) a reference photodetector mounted within said housing, and (f) a third optical element adapted to direct a second portion of light emitted by each of said light sources to said reference photodetector.

52. The hand-manipulatable device of claim 51, wherein said sample photodetector and said reference photodetector are mounted in close proximity to one another so as to share environmental characteristics.

53. The hand-manipulatable device of claim 50, wherein said focal aperture is in substantial vertical alignment with an area in said top surface of said housing for seating a tip of an index finger.

54. A hand-manipulatable device for gathering reflective, densitometric, spectrophotometric, colorimetric, self-luminous or radiometric readings from a sample surface, comprising:
a housing having a substantially flat bottom surface and a top surface contoured to fit comfortably in the fingers and palm of the human hand;
a sensor mounted to said housing, including a focal aperture and including circuitry and optics for performing a reflective, densitometric, spectrophotometric or colorimetric reading from a portion of said sample surface aligned with said focal aperture;
a switch operatively coupled to said sensor for activating said sensor to perform said reading; and a data link adapted to be operatively coupled between said sensor and a computer for relaying data from said sensor to said computer;
said sensor including (a) a plurality of light sources, mounted within said housing, respectively emitting light of a substantially different wavelength band, (b) a sample photodetector mounted within said housing, (c) means for directing a first portion of light emitted by each of said light sources to the sample surface, and (d) means for directing a portion of light reflected from the sample to said sample photodetector.

55. A sensor for detecting colorimetric, reflective, densitometric or spectrophotometric, self-luminous or radiometric characteristics of a sample, comprising:
a plurality of light sources for emitting light of substantially different wavelength bands spaced in a predefined spectrum;
a base having top surface and a bottom surface, a plurality of emitter apertures extending completely therethrough from said top surface to said bottom surface;

a substantially non-absorbing interior integrating surface positioned over said top surface of said base;
a reference photodetector mounted to said top surface of said base;
a sample photodetector mounted to said bottom surface of said base;
said light sources being positioned adjacent to or in said emitter apertures such that a first portion of light emitted by said light sources is transmitted to a sample and reflected from said sample to said sample photodetector, and such that a second portion of light emitted by said light sources is transmitted to said integrating surface and directly or indirectly reflected from said integrating surface to said reference photodetector.