US2710889A - Color reproduction - Google Patents

Description

June 14, 1955 P. E. ToBlAs coLoR REPRODUCTION 5 Sheets-Sheet l Filed Jan. 21, 1953 MN. x

MVS,

P. E. TOBIAS COLOR REPRODUCTION June 14, 1955 5 Sheets-Sheet 2 Filed Jan. 2l, 1953 m, BY @m ATTORNEYS.

June 14, 1955 P. E. ToBlAs 2,710,839

coLoR vREPRODUCTION Filed Jan. 2l, 1953 5 Sheets-Sheet 5 TToRNEY;

June 14, 1955 Filed Jan. 21, 1953 P. E. TOBIAS COLOR REPRODUCTION 5 Sheets-Sheet 4 June 14, 1955 P. E. 'roBlAs COLOR REPRODUCTION 5 Sheets-Sl'lseil 5 Filed Jan. 2l, 1953 ATTORNEYS,

United States Patent Oh ice 2,710,889 Patented June 14, 1955 COLOR REPRODUCTION Philip E. Tobias, Upper Darby, Pa., assignor to Edward Stern & Company, Incorporated, Philadelphia, Pa., a corporation of Pennsylvania Application January 21, 1953, Serial No. 332,258

26 Claims. (Cl. 178-5.2)

The subject matter of any patent granted on this application may be used by or for the United States Government for governmental purposes without the payment of any royalty to the inventor or his assignee.

The present invention relates to color reproduction, especially by means of printing plates. t

A purpose of the invention is to obtain greater fidelity in color reproduction by printing plates.

A further purpose is to reduce the cost of making high quality color reproductions, and particuluarly to minimize or wholly eliminate the cost of making corrections on color saparation negatives, positives or plates.

A further purpose is to predetermine a printing density for each printing ink involved in reproducing colored copy having local areas of a particular chromaticity, the printing density being predetermined by an optical density of a mask as it transmits a beam of incident light.

A further purpose is to automatically adjust the color reproduction by a set of color separation plates in accordance with the luminance (brightness) of the colored copy at each point thereon.

A further purpose is to match the colored copy in hue and saturation with binary mixtures of colored printing inks and to duplicate the brightness of the colored copy by adding black or a combination of colored inks which produce black.

A further purpose is to scan colored copy, measure the chromaticity of the copy at the point scanned, desirably by measuring the ratio of reectance of one color such asv red to the total reflectance, and by measuring the ratio of the reflectance of another color such as green to the total reflectance, to control the position of an electron beam suitably in a kinescope according to the rectilinear coordinates of the chromaticity of the copy at the different points scanned and thereby generating a light beam from the impingement of the electron beam on a phosphor on the face plate at a point corresponding to the chromaticity, to pass the light beam through a transparent mask which modulates in a predetermined manner the intensity of the light beam passing through the mask in accordance with the position of impingement of the light beam on the mask, and to expose photosensitive material to a light beam to produce a printing density which varies with the quantity of light passing through the mask.

A further purpose is to vary the printing density on the photosensitive material by generating on electric current responsive to the light passing through the masks, scanning the photosensitive material by a light beam in synchronism with the scanning of the copy and modulating the latter light beam in accordance with the magnitude of the electric current to vary the exposure of the photosensitive material.

A further purpose is to form the mask for the color printing plates with varying optical densities in dilerent active areas, with the optical density in any active area corresponding to the chromaticities given by the rectilinear co-ordinates on the mask and with isooptical-density lines on the mask corresponding to a particular amount of the ink to be printed by the plate in binary combination with other colored inks having adjoining chromaticities in all possible combinations.

A further purpose is to determine the printing density of a black printing plate by a mask which varies in optical density over its active area, and which has for each point, determined by the chromaticity co-ordinates an optical density which corresponds to the luminance of the binary combination of printing inks which produced that chromaticity.

A further purpose is to compare the actual luminance of a particular point on the copy with the luminance of the binary mixture of inks which gave that chromaticity in determining the ultimate printing density of the black printing plate.

A further purpose is to vprovide for automatic printing of chromaticities which are a predetermined function of the chromaticities on the copy, or of any given chromaticity on the copy, thus permitting overall modication 0f the color of the copy, or of selected colors on the COPY- A further purpose is to modify the gain on one of the input channels from a color selective optical pickup, or on all of such channels, so that the chromaticities indicated are artificially constrained to fall within the gamut of the printing inks.

A further purpose is to provide arbitrarily assigned printing densities on color separation plates for colors on the copy which are beyond the chromatcity gamut of the printing inks used.

A further purpose is to accomplish the Variation in optical densities of the mask within the gamut of chromaticity capabilities of the particular printing inks, and also desirably to assign arbitrary or preselected optical densities for each printing ink color for chromaticities outside the range of capabilities of the particular colored printing inks.

A further purpose is to employ a mask which has opaque areas between areas allowing light transmission, so as to selectively exclude from reproduction all but certain chosen chromaticities.

Further purposes appear in the specication and in the claims.

In the drawings I have chosen to illustrate a few only of the numerous embodiments in which my invention may appear, `selecting the forms shown from the standpoints of convenience in illustration, satisfactory operation and clear demonstration of the principles involved.

Figure l is a conventional diagram of a color reproduction device in accordance with the present invention, arranged to accomplish color separation and preparation of color printing plates.

Figure 2 is a conventional diagram of a device similar to Figure 1 showing a variation, in that the light from the kinescope passes directly to the photosensitive material.

Figure 3 is a fragmentary view corresponding to Figure l, showing a variation in the gain of one of the pickup channels.

Figure 4 is a diagrammatic illustration of mechanism in accordance with the invention for producing a black printing plate.

Figure 5 is a diagram of a chromaticity duplicating mask for one of the color plates, in this case the magenta printer.

Figure 6 is a diagram of a mask for the black printer.

Figure 7 is a diagram of a chromaticity duplicating mask which selects colors.

Describing in illustration but not in limitation and referring to the drawings:

One may define the attributes of a color in terms of its red, green and blue reectances.

lf the spectral response of each of the receptors used to measure these reflectances is matched to the eyc response of a standard observer, these red, green and blue reflectances will uniquely define the appearance of a color to this standard observer. A match is obtained between two colors when the red, green and blue reflectances (usually referred to as tristimulus values) of the two colors are respectively equal.

From the tristimulus values it is possible to obtain three other co-ordinates which are significant. The ratio of the red reflectance to the total (red-l-gren-l-blueL'and that of the green 'reflectance to the total, which are designated x and y respectively, establish two variables which define the chromaticity co-ordinates of a color and are capable of being plotted on rectilinear co-ordinates. The chromaticity co-or'dinates are functions o'f the hue and saturation of the color.

There remains a third independent variable, since the color has an attributable of luminance, commonly called brightness, which can be 'imparted by adding black to a pigment which produces the color or by altering the level of illumination. The luminance may be measured for example by the absolute value of the green reflectance.

Thus it is seen that the establishment of the `chromaticity co-ordinates `and the luminance completely specify a color. This coincides with thcm'annery in which an observer instinctively arranges colors and also vwith the results of process colorprint'ing. i

ln color printing using three or four process colors (with permissible addition of other colors, in order to obtain the purity and improve the reproductibility of a certain 'important color of the work), it is common practice to make color separation negatives which are records of the red reflectance, the green reflectance and the blue reflectance respectively. l The negatives approximately define the amount of the'printing inks (cyan, magenta and yellow respectively) which nominally reflect wave lengths except the particular' color used in making the separation (red, green and blue respectively) and therefore reflect the red, green and blue respectively in approximation to theoriginal copy. These processes using complementary printing inks are well known as subtractive printing processes.

When the dots on a particular location of the three printing plates are printed inthe ultimate reproduction, they apply at that point a predeterminedamount of the cyan, magenta and yellow inks which absorb from white light the colors not desired, and reflect a colored light which approximates the original. The inks themselves Vin Vthis process are approximately transparent so that colors of the same approximate order of appearance are obtained from dots of different inks which are superimposed as from dots which are side by Aside.

lf no additional processing of the plates were accomplished, however, the colors of the reproduction would only slightly resemble the original colored copy. These deficiencies result 'partly from the defects in the color separation filters, from the fact that the inks deviate from ideality in their rcectances, from the fact that the behavior of superimposed dots in printing is different from that of dots arranged side by side, from the fact that in the printing process distortions are obtained in the 'tonal scale, from the light absorption-into the paper on which the reproduction is made and because of the color-wise contribution of the paper on which the reproduction `is printed. It has accordingly become standard practice to 'correct for these deviations from idealityto make the reproduction look like the original colored copy.

lt will be evident that the method of correction will depend upon the particular printing process, and that the correction will have the effect of changing the printing density on a particular color plate in a particular area. In those processes in which printing density is established by the size of the dots in a particular area it will be eviiit) Lit)

dent that correction must take the form of varying dot sizes.

The most generally used method of color correction is by a color correction artist who relies upon his expcriencc or refers to a color chart which covers all printed tone combinations of the four process colors, magenta, yellow, cyan and black, showing the necessary printing densities of each plate to establish the particular color. The separations, whether as plates or as separation negatives or positives, are corrected either by dyein'7 or etching to thelcrrcct Voptical density or printing `density as thc case may be to give a printed result which will reproduce the original. This is a very laborious and expensive process, as it requires much time `by a highly skilled opcrator and may require making several provings. ln the end the result commonly leaves much to be desired from the standpoint of fidelity. ln common practice as much as 80% of the cost of making color plates is spent on the color correction of the plates.

A second method of accomplishing correction consists in masking, that is, the addition or removal of density to one separation negative as a function of the densities of the other two at a particular point. This may be accomplishcd by a number of different processes, but the result is often lacking in high fidelity, requiring further hand correction to produce accurate results, and these corrective processes are themselves difficult to use.

A third method of correction employs a scanning systern and then corrects by the masking effect as previously mentioned. This is subject to the same difficulties and inaccuracies as the second `rhethod mentioned.

A fourth method, which is still in the developmental stage, uses an electronic scanningdevice and predicts the printing operation by application of Neugebauer equations, which express the tristimulus result in terrns of dot areas of magenta, yellow, cyan and black printing plates, 'and in terms of the tristimulus values of the individual colored inks and their solid combinations. The procedure is to scan the original to obtain a linear transformation of the tristimulus values at a particular point on the copy, and then by a feedback arrangement, determine the 'printing densities of the colored printing plates and the black printing plate which would give identical rcsults. This system is complicated by errors resulting from a theoretical prediction which cannot define all of the complex variables which affect the physical process of printing. Thus errors through distortion in the tonal scale, dot spread, penetration of light into the paper, the interelfects of surprinting half tone dots of different colors and the break-up of the structure by the paper cannot properly be predicted. ln addition, due to the feed-back type. of circuit, difficulty is had under conditions where the density of the original colored copy exceeds 'that obtainable from the printing inks or where a color in thc original is beyond the gamut of the printing inks.

In accordance with the present invention, the difficulties ofthese prior art processes are largely eliminated. A faithful reproduction of the color ofthecopy is obtained, andthe necessity of subsequent color correction is largely eliminated. Thus a major factorinthe expense of making color reproductions is no longer present, and any deviation inquality which has heretofore b ecn experienced due to the skill of thc operator is entirely avoided.

ln thepresent process the chromaticity and luminance of a particular point on the colored copy is matched with an ,actual printed result which is obtained by printing mixtures ofthe process color inks. For exampleany deviations in accuracy `of reproduction due to departures from ideality of standard printing inksor of'the paper, any effects due to surprintin'g of dots, effects due to tone distortion at the ends of the scale, necessary variations in the color of the process inks actually used as compared to the standard, and the like, are simply eliminated and do not affect. the process. The results obtained are as high in quality as can be secured by the nrost capable color correction artist, with the advantage of eliminating cost of hand color correction and producinga photographic quality which is not possible in hand correction of intricate subject matter.

Likewise the invention is applicable to give high quality results where speed is a factor, as in reproduction of maps and other similar subject matter.

At any particular point on the colored copy, the ratio of the red light reflected to the total light reflected and the ratio of the green light reilected to the total rcected' establishes the trichromatic coetlicients generally referred to as chromaticity. These trichromatic coeflicients arel determined by filter-photocell combinations which are equivalent to the distribution functions of the C. I. E. standard observer. The chromaticity defines the dominant wavelength and degree of saturation of the color, but does not establish its luminance or brightness.

In a three color printing ink system, of the character of magenta, yellow and cyan, all proportions of the printing inks taken two at a time such as magenta and yellow, yellow and cyan and magenta and cyan, define the entire chromaticity gamut of the system. The addition of the third ink to a binary mixture causes graying, and does not alter the chromaticity gamut available from the binary mixtures.

It will thus be evident that graying or variation in luminance of the pigment can be obtained either by adding black to the binary mixture, or by adding the equivalent of black, that is, magenta, yellow and cyan in the required proportions to produce black. As a corollary it will be evident that the binary mixtures which produce different chromaticities vary in their luminance. It will of course be evident that where a black prin-ting plate is used, the black should be as devoid of hue as possible.

If now we assume a point on colored copy which is to be matched on the reproduction, it is obvious that such a match can be accomplished as to chromaticity by a binary mixture of colored printing inks, bu-t that in order to duplicate luminance or brightness it is necessary to add black or an equivalent of black in order to reduce ,luminance to the desired extent. t

The trichromatic coeicients of the point of the copy plotted according to rectilinear coordinates dene the dominant wavelength and the saturation of the copy, and likewise the similar trichromatic coeliicients of the reproduction define the dominant wavelength and saturation obtained from the printing inks. A perfect match of chromaticity results when the x and y values of the binary mixture of printing inks which print the reproduction are the same as the x and y values which uniquely identify. the chromaticity of the point on the copy.

If therefore we make a press .print on the nature to be used in the printing process, using all possible tonal combinations of magenta and yellow, yellow and cyan and magenta and cyan, each one of thesetonal combinations will have x and y values measuring chromaticity which uniquely identities the particular mixture. Likewise a given value of x and y on the copy will be matchedvby these emprically determined densities of magenta, cyan and yellow in binary mixtures. It will thus be seen that at any one point in the chromaticity coordinate system, 1

the density of at least one colored ink must equal zero. The amount of black required to gray the reproduction `to give it a luminance corresponding to the copy will then complete the color match.

It should be noted that when only three colored printing inks are involved,-each one alects an area on the mask adjoining both others and the binary combinations include all combinations taken two at a time. Where, however, one introduces a fourth, fth, etc. color of ink to increase the gamut of chromaticities available from the printing inks, one will, in general, only make binary combinations of each ink with those which on the chromaticity chart or the mask adjoin it on either side, and not with the color or colors which are remote.

In accordance With the present invention, the empi'- rically determined relationships between the x and y values and the printing densities of the color separation plates are utilized. The colored copy is scanned so that at any instant a given point only is subject to analysis. The chromaticity and the luminance of this point is measured, and these chromaticity and luminance values thus determined are used to establish the particular printing densities of the binary mixture of color printing inks which gives that chromaticity, and also to establish the printing density of black to accomplish the required graying. s

Considering now the embodiment of the invention shown in Figure l, colored copy 20 such as a photograph, painting, map, advertisement or the like is subject to the view of an analysis scanner 21 which in the preferred embodiment scans in synchronism with the scanning by a synthesizing scanner 22 projecting a light beam to photosensitive material 23. Depending on the character of the process, the material 23 may become a color separation negative or positive, or an actual photo-sensitive color printing plate. It will of course be understood that depending on whether or not the particular process contemplates granulation or breaking up of the surface into dots, the photosensitive reproduction 23 may be screened or unscreened. Of course where the photosensitive material 23 is an intermediate step in accomplishing reproduction, screening may take place at a later stage before the ultimate printing plate is produced.

The analysis scanner may be of any conventional type, whether electronic or mechanical, and no detail of the same is considered to be critical in the present invention. For example, electronic scanners used in facsimile type transmission are suitable, as well as iiying spot scanners and rotating mirror scanners. Likewise the scanner may be strictly of mechanical type, for example of the well known character in which a drum carries both the copy 20 and the reproduction 23 and manipulates them through identical paths in reference to similarly placed light beams aswell known in the art.

While the invention is considered to iind its widest application in connection with the reproduction Vof opaque colored copy, and reference has been made herein to reflectance of the copy, it will be understood that the invention is also applicable to reproduction of transparencies,

in which case the light given off by the copy may be trans-l mitted light, and it will be understood that when reference is made herein to reiiectance from the copy it is intended to include the possibility that the copy may transmit light in addition to reectance or instead of reilectance.

At a position at which light given off by the copy can be received, a group of photosensitive pickups 24, 25 and 26 are located, each of which is selective in its response to a particular one of the analysis light colors. Normally these will be phototubes provided with lters or dichroic mirrors, the lter-phototube combination giving the desired spectral sensitivity. Permissibly a combination of dichroic mirrors and iilters may be employed, using one pickup for one light color and another pickup for another. The particular pickups 24, 2S and 26 should have spectral responses as close as possible to the tristimulus distribution functions or linear transformations thereof, for example 24 responding to red (x), 25 to green (y) and 26 to blue (z). Suitable tristimulus colors are set forth in Arthur C. Hardy, Handbook of Colorimetry (Technical Press 1936, Massachusetts Institute of Technology), 8. As a convenient way of positioning, the pickups as shown are disposed apart around the crossed mirrors which accomplish the beam splitting.

A kinescope tube or cathode ray oscilloscope 27 is employed, having the usual electron gun with a detiection system 28 which causes the electron beam to position on a face plate 30 having a suitable phosphor. The phosphot will preferably be of the short persistence type,

where a high scanning rate is to be used t the analysis scanner. The deflection sysiternZS is subjected to control on the horizontal and vertical coordinates by a 'computer 31 which receives an electric voltage responsive 'to Vthe pickup of one tristimulus color from pho'topiekup 24 through channel 32, receives `an electric Voltage responsive to another tristimulus color from photopickup 25 through channel 33 and receives an electric Voltage 4responsive to the other tristimulus color through `chi'a'; 'el 34. The computer produces 'an output lvoltage controlling either the horizontal or vertical coordinates o f `the electron beam through channel 35, and produces 'another output voltage controlling the other coordinate' of the electron beam through channelv36.

lt is important that tlie voltage through 'channel 35 corresponds to the chromaticity `coeicient x, that i's, it should coliform to the expression:

x K V where Vr is the voltage in channel 32, Vg is the voltage in channel 33 and Vb is the voltage in channel 34. Likewise the voltage in channel 36 varies in accordance with the chromaticity cocincient y accordingto the expression:

7 :WLQM J V.+V+Vi The relationship between the ratios indicated by it land y and the deection of the beam may be one of identity, proportionality o1' other function as desired. 1

The computor 3'1 may be a computer as well known in the art which directly takes in three variables and produces responses according to the ratio of each of two variables to the sum of three variables. In a lpreferred embodiment, however, the computer k31 will`be a "logarithmic amplifier well known in the art. I

In this case it is proposed to use the sum of log Vr, log Vg and log Vb vectorially disposed 'at 120 to each other. Here the resultant is a function of thechromaticities and is independent of the absolute values of Vr, Vg and Vb. The horizontal component of the esiiltant proposed is:

and 'the vertical component is:

lt will be understood that any one of the available commercial logarithmic ampliers can be used as a computer for the present purposes. y A It will thus be seen that the analysis scanner analyzes the copy by scanning different points and the pickup system responds to the tristimlus values of any lpoint on the colored copy, and the computer then causes the electron beam to reach a point on the face plate which indicates the chromaticity of the copy at thispoint. The electron beam striking the phosphor creates alight beam 37 which passes from the kinescope, and is employed to optically determine the printing density required at the corresponding point on the reproduction. It will be understood that the light beam 37 as it leaves the face plate is invariable in intensity, but that its position corresponds to the chromaticity of thatrpoint on the copy.

In the preferred embodiment the density determining light beam 37 passes through a projector lens 38p, then through a chromaticity duplicating viriask 40,* and next througha condenser`lens`41 to rphotopi'cku'p 42, suitably a phototube. The photopickup 42 is connected throgh channel 43 to the input of amplier'44 -which through channel 45 modulates the intensity of the vlight beam emanating from synthesizing scanner 22. The device'is, however, applicable to direct photographic exposieof the photo sensitive surface 23 by the light Beam 37. 'In

this case ltlrie inaskL 40' (of Icourse with its control areas die ently located to suit the diierent beam position) `is jxtaposed in front "of the `face plate, the beam 37' passing through condenser lens '41 directly to the photo sensitive material 23, which in this case is manipulated to agree with ythe manipulation 'of the colored copy 20, as by a mechanical scanning device.

The lenses 38 and 41 are 4vdesirably of the type ernployed in 'ying spot television scanners.

The density of Athe mask 40 v'aries over the active area of theftnask as later Aexplained in more detail, in order to "modulate the light 'beam 37 in accordance with the optical 'density lo'futhe mask, so :that `the light reaching the 'photopiolcu'p 42 from the beam 37 is determined by t'h'exand ychroinaticity coetlicients ofthe colored copy at the gii/'en point. This density of the mask therefore directly controls the `intensity Aof the exposure of the photo sensitive material 23, and the printing density of the reproduction.

vIt will be understood that a separate binary chromatict'y duplicating mask will be used for each of the coloredprinting inks whichis to be used in reproduction, but `that the saine 'pickup from the copy will be employed in makin'geach ofthes'ecolor separations. r

A separate mechanism is employed in producing the black 'printing plate, 'in aceordance with Figure 4.

The form of Figure `4 may conveniently correspond to 'that of Figiire 1 Afor making lthe color 'plates except for some modification as 'explained below. It will be understood that the mechanism of Figure 4 can simply be 'niodiledfby suitable switching to ina'ke it conform to that of Figure 1 and vice versa, or separate Vkines`copes can be used connected respectively in parallel to the channels -V and 3`6 of Figure l.

The fiinctionofthetmechanisna of Figure 4 is to make a comparison between the reflectance of the copy at a given p'oint 'and `the`r"etiectan`ce of the binary mixture of printing inks which produce Vthe corresponding chromaticity. For Vthis purpose a lead 45 connected to the outputof pho'topi'c'kiip 2S (conveniently the green pick-up is chosen) is fed into a comparator 46, and the output 43" of the photopicknp '42 is fed into the same comparator in opposition to the output through lead 45. In this casca binaryliitninanccrecord mask corresponding 'to the green 'reflectance of the binary mixture of printing inks 'which "produced the chromatiity is used to modulate the light from the kinescope.

The comparator is of any suitable character, as well known in the art, which rmakes a computation in which the'output equals:

The colori-mask"ofttheitypejeinployed at 40 in making the colored platesy'still@A suitably be of the general type shown lin Fi'gnre 5. The x and y coordinates shown conform to the ohi'omaticity coordinates, although obviously if the4 l'eris'revetses position they will be obverse of the nomal position. As shown tthere are areas of theeoordinates Vasat 47 which have no physicalembodiment, and all thephysicalf'ei'nbodiments of the magenta printingplate as shown in Figure 5 will lie Within the limits of the chromaticity curve48'of the spectrally pure colors land the purple curve 50.

Howey'en the printing inks available do 'not provide a glarniit oovering theientire chromaticity range, and all ot the chromaticities 'which can be actually'produced by thc particolar typical ypir'intin'g inks lie inthe polygon cyan (c), cy'n plus yellow (cy), yellow (y), Ymagenta plus yellow "(iny), magenta (rn), cyan plus magenta (cm) and cyan (c).

lf additional colored inks'ar'e introduced such as that indicated by points G or R, the area of the gamut of the inks can be increased. Binary mixtures then will be dened as m and RR and y, y and G, G and c and c and m. Thus, chromaticity dupiicating masks can be used for reproducing any printing color if all of the inks are combined in binary mixtures as explained. If the number of colors is more than three, the binary mixtures of each color include the two colors which have adjacent chromaticities. For example, use of green in general would result in mixtures of green and cyan and green and yellow, but not with mixtures of green and magenta. Each point within the polygon has a characteristic optical density, that is, the proportion of the light beam 37 which is transmitted by the mask, and there is a characteristic relationship of each x and y coordinate position within this range which determines the chromaticity. The mask for printing a particular color, in this case magenta, will include a number and preferably an infinite number of isooptical-density lines which respectively correspond to dilerent quantities of the particular ink being printed (in this case magenta) with all other binary combinations of the other inks. Thus, the line aber is an isooptical-density line for magenta and the line aIV bIV 111V is an isooptical-density line for 100% magenta, while the intervening similar lines are isooptical-density lines for intermediate quantities of magenta, with varying quantities of cyan or yellow as the case may be.

The manner of making the maskvfor color plate printing will vary with the particular application to be made of the process. Without limitation to any particular manufacturing technique, one way of making the mask is to scan printed copy which initially has 100% magenta and all possible binary combinations with yellow and cyan, thus obtaining a trace on the kinescope which corresponds to the isooptical-density line for 100% magenta,

and to expose optically sensitive material for the mask to the beams from this trace, thus obtaining the outline of the trace on the mask. The same repeated for each other percentage of magenta with each other percentage of the other inks will give the trace for the isooptical-density line for each other content of magenta. The isooptical-density lines can then be dyed to the desired density, either by hand application of dye, or by accomplishing the photography through neutral density filters.

Note that the line of isooptical-density for 0% of the particular colored ink being printed will go through C which represents white light and through the points corresponding to the two other pure inks being printed. Thus in the case of the yellow printing mask this line will follow the course cCm for 0% yellow whereas cyymy corresponds to 100% yellow. Likewise for the cyan printing plate, the 0% cyan line will follow the course yCm for 0% and cyccm for 100% cyan.

Figure 6 shows the mask for the black printer which has the polygon as already described. Included are isooptical-density lines e, e1, etc. which are determined experimentally. Each spot has an optical density corresponding to the luminance of the corresponding binary mixture of printing inks which gives that chromaticity. This mask is produced by individually determining the required optical densities and producing the mask photographically by means of varying exposures or by dyeing the masks to the required densities.

If desired the invention can be employed to create artificially a diierence between the color of the copy at a particular point and the color reproduced. This can be done arbitrarily to redden, green or blue the reproduction by providing densities within the range of the polygon on Figure which are deliberately displaced with respect to those called for by the x and y coordinates. In this case there will in some instances be a slight departure either in all colors or in certain colors from the x and y coordinate positions, whereas in other instances one or more individual colors may be markedly changed to alter the color in the reproduction for special elects. Thus all reds on the colored copy can be rendered as purple on 10 the reproduction by simply modifying the optical densities of the chromaticity duplicating masks to the required extent at the desired regions.

This principle is applicable also to the reproduction of chromaticities which fall outside of the polygon in Figure 5. Such colors are beyond the gamut of the standard printing inks and cannot be reproduced accurately. This condition occurs for example when pure color G is to be matched, which is in a range of saturation beyond the limits of mixtures of the inks C and Y. There is then a choice presented of matching the hue but not the luminance, of matching the luminance but not the hue or of compromising to any desired degree on both luminance and hue. Depending upon the nature of the copy it will be preferable to adopt one or the other of these expedients. This is readily accomplished by providing at the desired point outside of the polygon on each of the chromaticity duplicating masks and also on the binary luminance record mask, densities which control the particular chromaticity and luminance to be obtained in reproducing that point on the copy.

The flexibility with which such compromise can be applied in scanning the copy at points outside the chromaticity gamut of the printing inks is one of the great advantages of this invention, as obviously various masks making this compromise in varying ways can be ernployed for different requirements of reproduction.

In` some instances it is desirable to obtain a uniform modification of chromaticity coordinates of the copy to points within the polygon. This is accomplished as shown in Figure 3 by modifying the gain in one or more of the channels 32, 33 and 34 as by inserting any desired voltage control 51 in one or more of such channels. It will of course be evident that the voltage may be modied by series or parallel impedance, regulation of control grid voltage, or any other well known method, and 51 may be considered by way of example only as a series resistance.

Another procedure for which the invention can be utilized is to discriminate among colors which are quite similar. Thus, in map reproduction, lines may be drawn of various colors which it may be practically impossible to separate photographically. Even similar colors will have different chromaticity. In this case it is merely necessary to employ a chromaticity duplicating mask as in Figure 7, which has all of the area 51 opaqued except those areas at 52 corresponding to x and v values which it is desired to reproduce, and at the desired points the mask will conform to the practice explained previously. No color which does not have a chromaticity falling within one of the areas of transparency 53 will be reproduced. Thus it is possible to obtain a separation among the various lines in linear colored copy.

The invention is also applicable to two-color pseudoprocess, in which case, of course, chromaticity duplicating masks will be used only for the colored printing inks which are being employed.

The invention is also applicable to reproduction of textile design colors, wall paper colors and other color matching or reproduction which does not necessarily correspond to usual three-color process work. l

In view of my invention and disclosure, variations and modifications to meet individual whim or particular need will doubtless become evident to others skilled in the art, to obtain all or part of the benets of my invention without copying the process and apparatus shown, and I, therefore, claim all such insofar as they fall within the reasonable spirit and scope of myclaims.

Having thus described my invention, what I claim as new and desire to secure by Letters Patent is:

1. In mechanism for color reproduction, means for scanning colored copy by a light beam, means for measuring the chromaticity of the copy at individual points of scanning, a kinescope, means for deecting the position of the electron beam in the knescope in response to the chromaticity coordinates of `the cpyat l'the point of scanning and thereby generating a light beam 'having a position determined by a chromaticity of the point scanned, a mask varying in optical density over ritsarea in a predetermined manner, zmeans for passing the light beam from the kinescope through the mask at a particular area corresponding to the chromaticity and means for exposing photosensitive material to light at scanning positions Corresponding to the scanning "of 'the `copy Ein accordance withl the magnitude "o'f 'the 'light transmitted through the mask. y

2. 'In mechanism fr c'olor reproductlnM'mleans for scanning coloredcopy 'by a light beam, means 'for measuring the chromaticities of the copy at individual :points of scanning, a kinescope, means for Vdetlecting"the -position of :the electron beam ihfihe kinescope in rs'pnsejto the chromaticity coordinats'of the copy at Athe 'point of scanning and thereby generating alight beam having a'pos'ition determined by'theclir'omaticityof thepointsscanxied, a mask varying in optical density over 'its area Vin a prdetermined manner, means for passing the light beam from the kinescopeithr'ough the 'mask at a'particfiilar area corresponding to th'e chromaticity, 'ph'telectric means for producing an electric currentmsponsive to the 4beam of light passing through the ',mask, phot'sen'sitive 'material, means for'scannirig'the photoseris'itiveimaterial by Va light beam in synchronism uiiththe scaiii'iiigjof the copy, and means for modulating Ithe latter light beam lin accordance with the magnitude "of the current to Vvary the exposure of the photosensitive A`rnaterial in accordance withthe chromaticity.

3. In mechanism for color reproduction, `mens for scanning colored copy 'by a light beam, means "for'measring the ratios of reflectances of at leasttu/o "tristimulus colors of th'e copytototal reecta'rice at :individual points of scanning, a kinescope,"r1eans `foi' Adeflecting the position of theelectron Abeam in 'the kinescope inr'csponse to the relationship between the functions of the ratios and thereby generating a light beam having a position determined by the relationship of the functions at Aa particular point scanned, a mask varying in optical 'density over its 'area in a predetermined'manner, means for `passing the light beam from the kinescope through the mask at a particular area corresponding lto the chromaticity, photoelectric means for producing an electric current responsive to the beam of light passing through the mask, photosensitive material, means for scanning the photosensitive material by a light Vbeam in synchronism with the scanning of the copy and means for modulating the latter light beam in accordance withthe magnitude of the current to vary the eitposurefof vthe'photosensitive matcrial in accordance with :the chromaticity.

4. In mechanism `for photo-reproduction, means for scanning colored co'py by a light beam, means for measuringthe chromaticity of the 'copy at individualpoints of t scanning, akinescope, means for defiecting thev position of the electron beam in the kinescope in response'to the chromaticity coordinates of the -copyfa't the point of scanning and thereby generating a light beam having a -position determined by the chromaticity of lthe`poiut scanned, a mask varying in optical densityov'erits area in a p'redetermined manner, `th'e optical densities at .particular active areas on the mask `corresponding to rectilinear chromaticity'coordinates and the mask having isooptical density lines each corresponding 'to aparticular `content of one printing inkin binary combination with all other printing'inks of 'adjoining chrcmaticities in a set in all possible quantities, meansfor'passing "theligh't beam from the kinescope through the mask at a particular areahregulated by the chromaticity andmeans for exposing `photosensitivematerialto produce printing densities 'responsive to the light transmitted'throug'h themask.

5. In mechanism for photreproduction, means for scanning colored copy by a light beam, means for measuring the chromaticity of the copy at individual points 'of scanning, a kinescope, means for deilecting the position yof the-electron beam in the kinescope in response to =the :chromaticity coordinates of the copy at the point of scanning and thereby generating a light beam having a position determined by the chromaticity of the point scanned, a mask varying in optical density over its area in a predetermined manner, the optical densities at particularly active vareas on the mask corresponding to rectilinear chromaticity coordinates and the mask having isooptical density lines each corresponding to a particular content of one printing ink in binary combination with all other printing Ainks of adjoining chromaticities' in a set lin all possible quantities, 'means for passing the light beam 'from the 'kinescope through the mask at a particular area regulated by the chromaticity, photoelectric means for V"producing `electric current responsive to the beam of light passing through the mask, photosensitive material, means Afor scanning the phtoserisitive material by a light beam `'in synehronisr'n with the scanning of the copy and mearis for modulating the Alatter light beam in accordance with the magnitude of the current to vary the exposure 'f the iphotosensitive material in accordance with the chromaticity.

6. In mechanism ffor color reproduction, means for scanning colored 'copy by a light beam, means for measuring fthe `chromaticity of the `copy at individual points "of scanning, a kinescope, means for detlecting the iposition "'f the electron beam in the kinescope in response `to the `chromaticity coordinates of the copy at the V1point of 'scanning and thereby generating a light beam fhaving a position determined by the chromaticity of the `point scanned, a set of masks each corresponding to a printing ink color other than black, each varying in optical densityover its area in a predetermined manner individual to the printing densities required for the particular color of printing ink, means for passing the light beams -from the kinescope through the masks at 4afparticular area corresponding to the chromaticity, means for scanning photosensitive materials by light in synchronism With the scanning of the copy while the light 'beam from the kinescope is passing through the different masks, and means for modulating the light which contacts the Vphotosensitive material in accordance With the `Iigl'it beam transmitted through the different masks.

7. In mechanism for color reproduction, means for scanning colored copy by `a light beam, means for measuring the chromaticity of the copy at individual points of scanning, a kinescope, means for deflecting the position of the 'electron beam in the kinescope in response to the 'chromaticity coordinates of the copy at the point of :scanning 'and thereby generating a light beam having aposition determined by the chromaticity of 'the point scanned, 'a 'set of masks each corresponding to -a.printing `ink color other than black, veach varying in loptical density over its area in a predetermined manner individual to the printing densities required for thefparticula'r col'orof printing ink, means for passing the light beams lfrom the kinescope through the masks at a particular area corresponding to the chromaticity, photoelectric means `for producing electric current responsive to the beam of light `passing 'through the particular mask, a set of photosensitive materials, means for scanning each of the different photosensitive materials by a light beam in synchronism with the scanning of the copy, -and means for modulating the latter light beam in accordance with the magnitude of the current produced using each of the different lmasks to vary the exposure-ofthe `photosensitive rmaterial in accordance with the chromaticity.

8. `In mechanism for color reproduction, means for scanning colored copy by a light beam, means for measuring the chromaticity of the Vcopy at individual points `of scanning, a kinescope, means for deccting the position of the electron beam 4in the Ykinescope in response to the chromaticity coordinates at the copy at the point of scanning and thereby developing a light beam having a position determined by the chromaticity of the point scanned, a set of masks each individual to a diierent color of printing ink other than black, each varying in optical density over its active area, each active area on the mask corresponding to diiferentrectilinear chromaticity coordinates and each mask having iso-optical density lines each corresponding to a particular content of one colored printing ink in binary combination with all other printing inks having adjoining chromaticities in all possible combinations, means for passing the light beam from the kinescope through the particular mask at a particular area corresponding to the chromaticity, photoelectric means for producing an electric current responsive to the beam of light passing through the mask, a set of photosensitive materials, means for scanning each of the photosensitive materials by a light beam in synchronism with the scanning of the copy, using a different mask for scanning each photosensitive material, and means for modulating the latter light beam in accordance with the magnitude of the current to vary the exposure of each photosensitive material in accordance with the chromaticity.

9. In mechanism for color reproduction, means for scanning colored copy by a light beam, means for measuring the chromaticity of the copy at individual pointsof scanning, a kinescope, means for deecting the position of the electron beam in the kinescope in response to the chromaticity coordinates of the copy at 'the point of scanning and thereby generating a light beam having a position determined by the chromaticity of the point scanned, a set of masks, one for each color of printing ink than black, each varying in optical density over its area in a predetermined manner characteristic of the particular color of printing ink, means for passing the light beam from the kinescope through the mask at a particular area corresponding to the chromaticity, a set of photosensitive materials, means for scanning the photosensitive materials with light in synchronism with the scanning of the copy, means for modulating the light scanning each photosensitive material in accordance With the light transmitted through a mask for a particular color or" printing ink other than black, means for comparing reflectance of the copy at each of the points scanned With retlectance of a binary combination of the printing inks having the same chromaticity, further photosensitive material for making a black printing plate, means for scanning the further photosensitive material in synchronism With the scanning of the copy, and means for modulating the exposure of the further photosensitive material in accordance with the relationship of the reectances.

10. In mechanism for color reproduction, means for scanning copy by a first light beam, means for measuring the chromaticity of the copy at individual points of scanning, a color printing kinescope, means for deecting the position of the electron beam in the color printing kinescope in response to the chromaticity coordinates of the copy at the point of scanning and thereby generating a second light beam having a position determined by the chromaticity of the points scanned, a set of masks, one for each printing ink color other than black, each varying in optical density over its area, each having active areas corresponding to rectilinear chromaticity coordinates and each having isooptical-density lines corresponding to a particular content of one color of printing ink other than black in binary combination with other colors of printing ink having adjoining chromaticities in all possible combinations, means for passing the second light beam from the color printing kinescope through the particular mask at a predetermined area corresponding to the chromaticity, photoelectric means for producing an electric current responsive to the second beam of light passing through each particular mask, a set of photosensitive materials, one to be exposed for making the printing plates for each color of printing ink other than black, means for scanning each photosensitive material by a third light beam in synchronism with the scanning of the copy while using the mask for the particular color of printing ink, means for modulating the third light beam in accordance with the magnitude of the current to vary the exposure of the photosensitive material in accordance with the chromaticity, a black printing kinescope, means for deecting the position of the electron beam in the black printing kinescope in response to the chromaticity coordinates of the copy at the point of scanning and thereby generating a fourth light beam having a position determined by the chromaticity of the points scanned, a black printing mask varying in optical density over its area, having active areas corresponding to the rectilinear chromaticity coordinates, whose optical densities correspond with the reflectance of binary combinations of printing inks of adjoining chromaticities having the chromaticity corresponding to the rectilinear coordinates, means for passing the fourth light beam from the black printing kinescope through the black printing mask, means for generating a further current responsive to the light of the fourth beam passing through the black printing mask, means for comparing the further current with a current responsive to the reflectance of the copy at the particular point of scanning, further photosensitive material, means for scanning the further photosensitive material with a fifth light beam in synchronism with the scanning of the copy and means for modulating the fth light beam in accordance with the comparison of the currents.

ll. In mechanism for color reproduction, means for scanning colored copy by a light beam, means for measuring the chromaticity of the copy at individual points of scanning, a kinescope, means for detlecting the position of the electron beam in the kinescope in response to the chromaticity coordinates of the copy at the point of scanning and thereby generating a light beam having a position determined by the chromaticity of the point scanned, a mask having optical density areas corresponding to rectilinear chromaticity coordinates Within the gamut of a set of printing inks and rectilinear chromaticity coordinates outside of that gamut, the mask having within the gamut, optical densities corresponding to chromaticities within the gamut and having outside the gamut optical densities which produce chromaticities Within the gamut, whereby a compromise is eiected in showing chromaticities outside the gamut of the printing inks, means for passing the light beam from the kinescope through the mask at a particular area corresponding to the chromaticity, photosensitive material, means for scanning the photosensitive material by a light beam in synchronism with the scanning of the copy and means for modulating the latter light beam in response to the light which passes through the mask.

12. In mechanism for color reproduction, means for scanning colored copy by a light beam, means for measuring the chromaticity of the copy at individual points of scanning, a kinescope, means for delecting the position of the electron beam in the kinescope in response to the chromaticity coordinates of the copy at the point of scanning and thereby generating a light beam having a position determined by the chromaticity of the point scanned, a mask varying in optical density over its active area in a predetermined manner and establishing optical densities which definitely depart from those which produce chromaticities corresponding to those on the colored copy, means for passing the light beam from the kinescope through the mask at a particular area corresponding to the chromaticity and means for reproducing on photosensitive material the variations in the intensity of the light beam passing through the mask.

13, ln mechanism for color reproduction, means for scanning colored copy with a light beam, means for measuring the chromaticity of the copy at individual points of scanning, means for varying the indication of the chroautres@ maticity` thus measured, a` kinescope, meansl for deflecting the position of the electron beam inl the kinescope in response to the varied chromaticity measurements and thereby generating a light beam having a position determined by the modified chromaticity of the point scanned, a` mask varying in optical density over its active area in a predetermined manner, means for passing the light beam from the kinescope through the mask at a particular area corresponding to the modified chromaticity, and means for reproducing on photosensitive material the variations in the light transmitted through the mask.

114. A chromaticity record mask for use in mechanism according to claim 1, having varying optical density in different active areas, each active point corresponding to different rectilinear chromaticity coordinates, and having an optical density value individual to the particular chromaticity at each rectilinear coordinate position.

15. A chromaticity record mask for use in mechaism according to claim 1, having varying optical density in different active areas, each active point corresponding to dilerent rectilinear chromaticity coordinates, the mask having a plurality of isooptical-density lines each corresponding to a particular content of one colored printing ink other than black in binary combination with other colored printing inks having adjoining chromaticities in all possible combinations.

16. A color reproduction mask for use in mechanism according to claim l, having active areas of individual optical densities, points on the mask being arranged in sequence according to chromaticity variations on rectilinear coordinates.

17. A set of color reproduction masks for use in mechanism according to claim 6 each having individual optical densities arranged in accordance with chromaticity according to rectilinear coordinates, and each having optical density values at different chromaticities which correspond to the densities of printing of different colored inks of a set.

18. A color separation mask for use in mechanism according to claim 1, having variations in optical density arranged in accordance with variations in functions of X and y trichromatic coordinates.

19. A chromaticity record mask for use in mechanism according to claim 1, having varying optical densities in different active areas, in which different active points correspond to diterent rectilinear chromaticity coordinates, having isooptical-density lines each corresponding to a particular content of one colored printing ink other than black in binary combination with other colored inks of adjoining chromaticities in all possible combinations, including active points over a range within the chromaticity gamut of a set of inks.

20. A chromaticity record mask for use in mechanism according to claim 1, having varying optical densities in different active areas, in which different active points correspond to different rectilinear chromaticity coordinates, having isooptical-density lines each corresponding to a particular content of one colored printing ink other than black in bineary combination with other colored inks of adjoining chromaticities in all possible combinations, including active points within the chromaticity gamut of a set of printing inks and active points outside the chromaticity gamut of the set of printing inks which have optical densities producing chromaticities corresponding to chromaticities within such gamut.

21. A chromaticity duplicating mask for use in mechanism according to claim 1, having varying optical densities in dilerent areas, having isooptical-density lines each corresponding to a particular content of one colored printing ink other than black in binary combination with other inks of adjoining chromaticities in al1 possible combinations, various points on the mask having optical densities which correspond to a` function of the chromaticity on rectilinear coordinates. but are different from the chromaticity.

27; A set of color duplicating masks for use in mechanism according tot claim 6 each for a different color of printing ink other than black, each having varying optical densities at diierent points, each having optical densities at rectilinear coordinate points which are a function of chromaticity, each having isooptical-,density lines on each mask, corresponding to particular contents of different colored printing inks in binary combination with other colored printing inks of adjoining chromaticities other than black in all possible quantities,

23. A binary luminance record mask for use in mechanism according to-claim 9 having varying optical densities in different areas any particular active point on the mask corresponding to dierent rectilinear chromaticity coordinates, the mask having at any chromaticity coordinatesr an optical density corresponding to the reflectance of a binary combination` of colored printing inks which produce that chromaticity.

24. A binaryluminance record mask for use in mechanism according to claim 9. having varying optical densities in different areas, any particular active point on the mask correspondingl to different rectilinear chromaticity coordinates, the mask having at any chromaticity coordinates an optical density corresponding to the reectance of a binary combination of colored printing inks which produce that chromaticity, and the mask having lines of iso-optical density extending thereover.

25. A set of color reproduction masks for use in mechanism according to claim 9 each having varying optical densities in different areas, each having active points corresponding to rectilinear chromaticity coordinates, a plurality of the masks in thev set being chromaticity duplicating masks each of which has iso-optical density lines corresponding to a particular content of a diiferent colored printing ink other than black in binary combination with colored inks of adjoining chromaticities in all possible combinations and one of the masks being a binary luminance record mask having at each set of chromaticity coordinates a density corresponding to the reflectance of the binary mixture of printing inks which produces that chromaticity.

26. A color separation mask for use in mechanism according to claim l, having variations in optical density arranged in accordance with variations in chromaticity and having opaque area-s between areas at which variations in density appear.

References Cited in the le of this patent UNITED STATES PATENTS 2,165,168 Hardy July 4, 1939 2,272,638 Hardy Feb. 10, 1942 2,278,940 Murphy Apr. 7, 1942 2,316,581 Hardy Mar. 13, 1943 2,413,706 Gunderson Jan. 7, 1947 2,434,561 Hardy Jan. 13, 1948 2,573,143 Jacob Oct. 30, 1951 2,607,845 Clark Aug. 19, 1952 2,627,547 Bedford Feb. 3, 1953

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