WO2002008808A1

WO2002008808A1 - Multilayer plastic sheet for light pipe

Info

Publication number: WO2002008808A1
Application number: PCT/US2001/016523
Authority: WO
Inventors: Donald A. Maclennan
Original assignee: Fusion Lighting, Inc.
Priority date: 2000-07-24
Filing date: 2001-07-11
Publication date: 2002-01-31

Abstract

A flexible sheet (23) for a light pipe (21) includes a substrate covered with a plurality of interface layers with differing respective indices of refraction, thereby providing Fresnel reflection, where each layer is thicker than a wavelength of light in the visible region. For example, each layer is at least 1 micron thick. Preferably, each layer is at least 2 to 10 microns thick. A light pipe (21) includes a conduit formed from the multilayer flexible sheet (23) and an extractor (27) arranged lengthwise along the conduit for directing most of the light which encounters the extractor (27) out of the light pipe (21).

Description

MULTILAYER PLASTIC SHEET FOR LIGHT PIPE

BACKGROUND

1. Field of the Invention The invention relates generally to light distribution systems such as light pipes. More particularly, the invention relates to a novel plastic sheet material for a light pipe.

2. Related Art

In general, the present invention relates to the type of light pipes disclosed in PCT Publication No. WO 00/43815 (the '815 Publication), which is herein incorporated by reference in its entirety.

U.S. Patent No. 5,882,774 discloses a multi-layer optical film and U.S. Patent No. 5,661 ,839 discloses a light pipe which utilizes the multi-layer optical film. The multi-layerfilm consists of numerous quarterwave stacks which collectively provide a high reflectivity over a wide wavelength range and also over all angles of incidence. Each quarterwave stack consists of a low and high index pair of film layers, where each layer pair has a combined optical thickness of 1/2 the center wavelength of the band it is designed to reflect. In the visible and near infrared range, the average thickness of each layer pair is not more than 0.5 microns. The principle of operation of the film is based on interference and birefringence. The resulting film provides an efficient mirror surface which is substantially non-absorptive and non-transmissiveof light. Openings in the film must be provided for extraction of light.

SUMMARY In general, the inventions described herein are related to the light pipes described in the above-referenced '815 publication, and to various improvements and / or modifications thereof. A detailed discussion of such light pipes and the manner of making and using such light pipes may be had by reference to the '815 publication. A flexible sheet for a light pipe includes a substrate covered with a plurality of interface layers with differing respective indices of refraction, thereby providing Fresnel reflection, where each layer is thicker than a wavelength of light in the visible region. For example, each layer is at least 1 micron thick. Preferably, each layer is at least 2 to 10 microns thick.

A light pipe includes a multi-layer sheet with differing indices of refraction among adjacent interface layers providing Fresnel reflection, the multi-layer sheet being formed into a conduit, and an extractor arranged lengthwise along the conduit for directing most of the light which encounters the extractor out of the light pipe.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features, and advantages of the invention will be apparent from the following more particular description of preferred embodiments as illustrated in the accompanying drawings, in which reference characters generally refer to the same parts throughout the various views. The drawings are not necessarily to scale, the emphasis instead being placed upon illustrating the principles of the invention. Fig. 1 is a schematic, longitudinal cross sectional view of a light pipe which utilizes several concentric sheets of plastic.

Fig. 2 is a cross sectional view taken along line 2-2 in Fig. 1. Fig. 3 is a schematic diagram of a multi-layer plastic sheet in accordance with the present invention. Fig. 4 is a graph of reflection versus angle of incidence for several interfacing surfaces with different ratios of indices of refraction.

Fig. 5 is a graph of total single net reflection versus angle of incidence for various numbers of sheets or layers.

Fig. 6 is a schematic, longitudinal cross sectional view of a light pipe in accordance with the present invention, utilizing the multi-layer plastic sheet of the present invention.

Fig. 7 is a cross sectional view taken along line 7-7 in Fig. 6. Fig. 8 is a schematic view of an example light pipe system in accordance with the present invention. Fig. 9 is a schematic view of another example of a light pipe system in accordance with the present invention. Fig. 10 is a schematic view of a light pipe which utilizes several sections of the multi-layer plastic film in accordance with the present invention.

Fig. 11 is a schematic view of a tapered, diffuse light extraction used in a light pipe system in accordance with the present invention.

DESCRIPTION

In the following description, for purposes of explanation and not limitation, specific details are set forth such as particular structures, interfaces, techniques, etc. in order to provide a thorough understanding of the present invention. However, it will be apparent to those skilled in the art having the benefit of the present disclosure that the invention may be practiced in other embodiments that depart from these specific details. In certain instances, descriptions of well known devices and methods are omitted so as not to obscure the description of the present invention with unnecessary detail. The light pipes described in the '815 publication utilize one or more sheets of commercially available plastic (e.g. polycarbonate, acrylic, etc.) to construct light pipes which distribute light efficiently and uniformly with grazing angle reflection. An example structure of this type of light pipe is shown in Figs. 1-2. A light pipe 3 includes a plurality of concentric plastic sheets 5 separated by an air gap 7. A light source 9 directs light into one end of the pipe 3. Narrow angle light is transported down by the pipe by the Fresnel reflection which occurs at the plastic / air interface. Higher angle light exits the pipe either directly or after encountering an extractor 11. Multiple sheets 5 are utilized to increase the angle of light which can be efficiently transported down the pipe. A narrow angle light source 9 (e.g. between about 7° and 14° half angle) is preferably used with the light pipe 3 to effectively utilize the efficient light transport mechanism.

The present invention provides efficient distribution of light with grazing angle reflection and also with higher angle reflection. Advantageously, the present invention provides such light distribution with a single coated sheet of plastic, thereby simplifying the light pipe construction and reducing material and labor cost. Instead of multiple sheets of plastic separated by a suitable gap (as in the '815 publication), the present invention provides a single plastic sheet substrate covered with one or more layers of plastic with differing indices of refraction.

With reference to Fig. 3, a substrate 17 (e.g. a plastic sheet) is covered with several interface layers L^ through L_n of material (e.g. coatings, laminates, adhesives, etc.) with differing indices of refraction. Specifically, each interface layer L| has an index of refraction Ri which is different from the index of refraction of any adjacent interface layer:

Ri-1 <> Ri <> Ri+1 ; where i = 2 to n-1. Preferably R1 is also different from the index of refraction of the substrate 17. There may be an even or odd number of interface layers. Preferably, each interface layer corresponds to one layer of material. However, each interface layer itself may include several layers of the same material or different materials with the same index of refraction. Preferably, the interface layers are bonded to each other, but may alternatively simply comprise alternating sheets of plastic with differing indices of refraction. As a practical matter (e.g. simplification of manufacturing) it may be preferred to use only two materials (other than the substrate) with different indices of refraction for the interface layers.

In contrast to the above-mentioned multi-layer optical film described in the '774 and '839 patents, the layers of the present invention are preferably thicker than a wavelength of light in the visible region (e.g. at least 1 or 2 microns), while remaining relatively very thin as compared to the plastic sheet substrate. For example twenty 10 micron layers is 200 microns thick or about 0.2 mm. By using layers which are thicker than a wavelength of light in the visible region, significant interference effects are avoided and the present invention does not operate as an interference filter. The thickness and uniformity of the layers are not critical as long as the layers are thick enough to avoid interference effects. Accordingly, there are no critical manufacturing steps involving depositing sub-micron coatings to a high level of accuracy, as are required by the complex dichroic coatings of the '774 and '839 patents. For example, the layers may be applied by a UV curing process, as a laminate, or as plastic sheets bonded to each other by thermal bonding or ultrasonic bonding. In further contrast to the '774 and '839 patents, the multi-layer plastic sheet of the present invention is substantially light transmissive for higher angles of incidence (e.g. above 30° from horizontal). The amount of light transmitted varies with the angle of incidence such that grazing angle light is substantially all reflected and light above some other selected angle of incidence is substantially all transmitted (i.e. escapes the light pipe). For the light pipes of the present invention, most of the light which encounters an extractor exits the light pipe. The angle of incidence for light which is transmitted may be selected, for example, depending on the beam angle of the light source and / or the length of the pipe. An example range of high reflectivity for the present invention is between about 0° from horizontal (grazing) to about 15° or 20° from horizontal (14° being presently preferred), with the reflectivity falling off rapidly above those angles.

Advantageously, the present invention may provide a much greater practical number of layers (e.g. > 20) than the light pipes described in the '815 publication, while otherwise utilizing similar principles of operation. Accordingly, light pipes constructed from the multi-layer plastic sheet of the present invention can accept a higher half angle beam of light and can efficiently distribute the light over longer distances, while potentially also reducing labor and material costs. Practical numbers of layers range in tens to hundreds of layers, depending on the thickness of each layer. For example, one hundred 2 micron layers = 200 microns thick = 0.2 mm (about 8 mils).

Another advantage of the present invention is that losses due to bulk absorption are reduced by reducing the total path the light travels through the material. An example process for making a multi-layer plastic sheet in accordance with the present invention includes applying a thin layer of UV curable plastic of one optical index on a substrate, curing it, and then applying a thin layer of another UV curable plastic with a different index of refraction on top of the previously applied layer and curing it. A large number of layers may be applied while maintaining a sufficiently flexible plastic sheet. For example, starting with a plastic sheet, apply a thin layer of a first UV curable coating (e.g. spray, dip, or squeegee) to the sheet. Expose the coated plastic to UV light to cure the coating. Next, apply a thin layer of a second UV curable coating to the plastic sheet in similar manner and cure with UV light. Repeat as necessary for the desired number of layers. The first and second coatings have different indices of refraction and also low absorption.

Another example process according to the invention is as follows. Start with a base sheet of polycarbonate about 10 mils thick. Apply a thin layer of an optically transparent adhesive which has a different index of refraction after drying. For example, the adhesive is squeezed out of a tube and spread evenly with a squeegee. Put down a thin (e.g. 1 mil) sheet of polycarbonate. The glue is transparent with a different index of refraction from the plastic, thus providing the desired optical interface. This process can be performed by hand or may be automated to apply many layers. The resulting multi-layer plastic sheet is thin, light, and flexible.

An example automated process according to the invention is as follows. A roll of plastic sheeting is fed through various processing stations by rollers. First, the sheet passes under a spray station which sprays a UV curable coating on the sheet as it rolls past. The next station is a UV light which cures the coating. Another subsequent station sprays on a UV curable coating with a different index of refraction which is cured by another downstream UV light station. Many such pairs of stations may be alternated to provide the desired number of layers. Alternatively, the sheet may loop through or pass back and forth through the same stations multiple times to provide the desired number of layers. The result is a low cost manufacturing arrangement for the multi-layer plastic sheet.

Fig. 4 is a graph of approximated reflection versus angle for various indices of refraction for two materials providing a single interface. Each line represents the reflection of a single interface. The amount of Fresnel reflection for a given angle of incidence is a function of a ratio of the indices of refraction of the two materials providing the interface. For example, a typical index of refraction for polycarbonate is about n1 = 1.6 while the index of refraction for air is nO = 1.0. The Fresnel reflection is dependent on the ratio n1/n0 = 1.6/1.0 = 1.6. Both polarities (TM and TE) are graphed for the ratio of 1.6. Only the TM polarity is graphed for the ratios of 1.1 and 1.2 (the TE polarity provides higher reflection). Indices of refraction for different commercially available plastic materials are given below in Table 1.

Table 1

Various acrylic materials are commercially available which can be UV cured with a refractive index in the range of 1.5 to 1.6. For example, Loctite of Rocky Hills, CT sells Loctite™ 3100 with a nominal cured refractive index of 1.51. Another Loctite product, SuperBonder™ 498 has a nominal cured refractive index of 1.45. Raymat, Inc. of Tinley Park, IL sells a variety of low refractive index claddings under the name OPTI-CLAD which have indices of refraction in the range of 1.36 to 1.49. Rad-Cure Corporation of Fairfield, NJ provides a UV curable high gloss hard coat for plastics under the name RAD-KOTE 503SP. The index of refraction is in the range of 1.43 to 1.54.

By selecting appropriate materials, a desired ratio n1/n0 may be provided in the range of 1.1 to 1.2. For example, alternating layers of polycarbonate and the polytetrafluoro-ethylene material with the lower index of refraction provides a ratio of 1.60/1.30 = 1.23. Alternating layers of polycarbonate and the OPTI-CLAD material with the lower index of refraction provides a ratio of 1.60/1.36 = 1.18. Alternating layers of polycarbonate and the Loctite SuperBonder 498 provides a ratio of 1.60/1.45 = 1.10. Alternating layers of the RAD-KOTE 503SP material with the high / low indices provides a ratio of 1.54/1.43 = 1.08. Of course, the foregoing are only examples and a wide variety of other materials with a wide range of indices of refraction are available and suitable for use in accordance with the present invention.

In general, a large differential between the two indices is preferred and it is also preferred that the lower index be as low as possible. Higher ratios generally provide higher reflectivity and require fewer layers to provide a desired amount of reflection as compared to a lower ratio. Surprisingly, however, a ratio of n1/n0=1.2 is relatively close in performance to a ratio of 1.6 and even a ratio of 1.1 provides good performance. Increasing the number of layers generally increases the amount of reflection at angles of incidence near normal (i.e. 90° from horizontal), although relatively high reflection at normal (e.g. up to about 30%) does not detract significantly from the operation of the light pipe. Using as few layers as necessary (e.g. a high differential) and lower index of refraction materials reduces the amount of light reflection at angles of incidence near normal (i.e. 90° from horizontal). Fig. 5 is a comparison graph of approximated total single net reflection versus angle for various number of sheets and various numbers of layers. As can be seen from Fig. 5, seven degree half angle light is well transported down a light pipe (e.g. a 30 feet long, 6 inch diameter pipe), even with 3 or 4 sheets of polycarbonate / air interface (n1/n0 = 1.6). However, fourteen degree half angle light is not as well transported, even with 6 concentric sheets. As compared to 2 through 6 sheets of polycarbonate / air interface, 22 layers of materials providing a ratio of n1/n0 = 1.1 provides relatively more reflection at grazing angles of incidence and higher angles of incidence. As compared to the ratio of n1/n0 = 1.1 , the same number of layers of materials providing a ratio of n1/n0 = 1.2 provides relatively more reflection. In general, 85%-90% total net reflection provides good performance in a light pipe. For a given light source (with a specified half beam angle), a suitable number of layers may be determined in accordance with the ratio of indices of refraction. For example, for a 7-9° half angle light source, 22 layers of materials providing a ratio of n1/n0 = 1.1 provides good performance. For a higher angle light source, more layers of such materials would be required. Conversely, for a given multi-layer plastic sheet structure, a suitable light source may be determined. For example, for 22 layers of materials providing a ratio of n1/n0 = 1.2, a light source having a beam half angle of 9-11° is suitable for good performance.

With reference to Figs. 6-7, a light pipe 21 includes a multi-layer plastic sheet 23 having the structure described in connection with Fig. 3 above. An outer shell or carrier 25 support the sheet 23. No air gap is necessary between the sheet 23 and the carrier 25. An extractor 27 is arranged along the length of the pipe to direct light out of the light pipe 21. A light source 29 provides a beam of light into the light pipe 21. The carrier 25 may have a diffuse interior and / or exterior surface for diffusing light which exits the pipe 21. The extractor 27 preferably has an opaque diffuse reflecting surface adjacent to the sheet 23 so that light which is not reflected down the pipe 21 by the sheet 23 is scattered into mostly high angle light which exits the pipe 21 near the point where the light encountered the extractor 27.

In general, the principle of operation of the present invention of the light pipe 21 utilizing a multi-layer plastic sheet 23 with different indices of refraction is similar to the light pipe with multiple concentric plastic sheets separated by an air gap described in the '815 publication except that the reflection is achieved by utilizing materials with different indices of refraction instead of using air to provide the optical interface. As noted above, however, the present invention provides higher reflectivity over a wider range of angles of incidence by utilizing more layers than is practical with the configuration of the '815 publication. With reference to Fig. 8, a light pipe 30 includes a multi-layer plastic sheet 31 suspended by a mounting bracket 33. Respective edges of the sheet 31 are inserted in and secured to slots 35a and 35b of the bracket 33. As compared to the light pipe 21 , the light pipe 30 omits the carrier 25. An extractor 37 is disposed on an outside surface of the sheet 31. An optional strip 39 covers a surface of the bracket 33 which is interior to the light pipe 30. The strip 39 may be specular or diffuse, or may exhibit a combination of specular and diffuse properties such as the white vinyl tape discussed in the '815 publication.

With reference to Fig. 9, a light pipe 40 is similar to the light pipe 30, including a multi-layer plastic sheet 41 , except that the extractor 47 is disposed on an inside surface of the sheet 41. The extractor 47 may be specular or diffuse, or may exhibit a combination of specular and diffuse properties such as the white vinyl tape discussed in the '815 publication. Preferably, the optional strip 49 exhibits similar optical properties as compared to the extractor 47.

In Fig. 10, a light pipe 50 in accordance with the present invention includes a plurality of section 51 , 53, 55 of multi-layer plastic sheet material. A light source 57 directs light into the pipe 50. In accordance with a present aspect of the invention, at least one of sections 51 , 53, and 55 provides a different number of layers as compared to one of the other sections. For example, section 51 may have 40 layers, while section 53 has 20 layers and section 55 has ten layers. The light pipe 50 may be made from a single sheet of plastic which is processed differently in the different sections 51 , 53, and 55 to provide the different number of layers. Alternatively, the sections 51 , 53, and 55 may be separate sheets of multi-layer plastic with seams or couplings therebetween. Of course, different numbers of sections and different number of layers may be utilized to adjust the light output for a given installation.

With reference to Fig. 11 , a light pipe 60 (including a multi-layer plastic sheet) includes an extractor 61 which is tapered along its length with a narrow end of the taper near a light source 63 and a relatively wider end of the taper distal to the light source 63. Adjusting the shape of the extractor affects how much light is scattered out of the light pipe at any given point along the length of the light pipe. Other shapes and configurations for the extractor may be utilized as necessary to provide a more uniform light output.

While the invention has been described in connection with what is presently considered to be the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the inventions.

Claims

CLAIMSWhat is claimed is:

1. A flexible sheet for a light pipe, comprising: a substrate covered with a plurality of interface layers with differing respective indices of refraction selected to provide Fresnel reflection, wherein each layer is thicker than a wavelength of light in the visible region.

2. The flexible sheet of claim 1 , wherein each layer is at least 1 micron thick.

3. The flexible sheet of claim 2, wherein each layer is at least 2 microns thick.

4. The flexible sheet of claim 3, wherein each layer is between 2 and 10 microns thick.

5. A light pipe, comprising: a conduit formed from a multi-layer sheet with differing indices of refraction among adjacent interface layers providing Fresnel reflection, each layer being at least 1 micron thick, and an extractor arranged lengthwise along the conduit for directing most of the light which encounters the extractor out of the light pipe.

6. A method of making a flexible sheet for a light pipe, comprising: providing a flexible plastic sheet for a substrate; and coating the sheet with at least two layers of material having different indices of refraction, wherein each layer is at least 1 micron thick.

7. The method of claim 5, wherein each layer is at least 2 microns thick.