CA2035301A1

CA2035301A1 - Faceted light pipe

Info

Publication number: CA2035301A1
Application number: CA002035301A
Authority: CA
Inventors: Kevin J. Hathaway; Richard M. Knox, Jr.; Douglas A. Arego; Gaylon R. Kornfuehrer
Original assignee: Compaq Computer Corp
Current assignee: Compaq Computer Corp
Priority date: 1990-09-27
Filing date: 1991-01-30
Publication date: 1992-03-28
Also published as: AU646061B2; JPH04234729A; EP0478102A2; IL99091A0; BR9104149A; EP0478102B1; ATE145288T1; DE69123117D1; MX173611B; US5050946A; EP0478102A3; AU8165691A; KR920006895A

Abstract

FACETED LIGHT PIPE
Abstract of the Disclosure A light pipe used for backlighting liquid crystal displays has a planar front surface and a stairstepped or faceted back surface. Light is injected from the ends of the light pipe from cold or hot cathode, apertured, fluorescent lamps. The cold cathode lamps are preferably insulated to raise their operating temperature. The back surface has a series of planar portions parallel to the front surface connected by facets, which are angled so that the injected light reflects off the facets and through the front surface.
A reflector having a planar, highly reflective, highly scattering surface or a sawtoothed or grooved upper surface is located adjacent to and parallel with the light pipe back surface to reflect light escaping from the back surface back through the light pipe to exit the front surface. The axis of grooves is preferably slightly skewed from the facet axis to reduce moire pattern development. A low scattering or loss diffuser is located adjacent to and parallel with the light pipe front surface to reduce moire pattern development. The liquid crystal display is located over the low scattering diffuser. A separate injector may be located between the lamp and the light pipe to better couple the light into the light pipe.

Description

2~53~1 FACETED LIGHT PIPE

The invention relates to backlighting systems used with liquid crystal displays, and more particularly to light pipe ~ystem~.

Liquid crystal displays (LCD I 5) are commonly used in portable computer system~, televisions and other electronic deviceQ. An LCD requires a source of light for operation because the LCD is effectively a light valve, allowing transmission o~ light in one state and blocking transmission of l$ght in a second state.
Backlighting the LCD has become the most popular source lS of light in personal computer systems because of the improved contrast ratios and brightnesses possible.
Because conventional monochrome LCD's are only approximately 12% transmissive and color LCD's are only approximately 2% transmissive, relative large amounts of uniform light ar- necessary to provide a visible display. If power consumption and space were not of concern the necessary level and uniformity of backlight could be obtained.
However, in porta~le devices power consumption, wh~ch directly effects battery life, and space are ma~or concern~. Thu~ there i~ a need to obtain a sufficiently uniform and bright backlight level with as littlc power as possible in as little space as possible at, of course, as low a cost as possible.

2~3~3~

Numerous designs exist which trade off various of these goals to achieve a balanced display. Several of these designs, such as light curtain~ and light pipes, are shown in the figures and will be described in detail later. The design~ generally trade off uniformity of backlighting for space or efficiency.
The designs utilize various scattering means and a final diffuser before the light is presented to the ~CD. The scattering means and the diffusers both allow loss of light and thu~ reduce the efficiency of the transfer from the light ~ource to the LCD. While the designs are adequate in some cases, the demands for longer battery life with monochrome LCD's or equal battery life with color LCD's are present, as is a desire for the use o~ less space.

The present invention is a faceted, parallel surface light pipe design. Light sources, preferably reflector or apertured fluorescent lamps, but alternatively uniform lamp~, supply light to one or both ends of a light pipe. The front surface of the light pipe, on which i~ positioned a low loss diffuser, which in turn i~ in contact with the LCD, is planar, while the back surface Or the light pipe i~ generally parallel to the front surface, but ha~ a stair stepped or faceted surface. The facets are preferably ~ormed at an angle so that the light injected into the ends of the light pipe is reflected o~f the facets and through the front surface. The pitch or step length of the facets i~ such that ~he faceting structure i~ not visible to the human eye. The step height of the facets is preferably in the micron range and may increase with the distance fro~ the lamp. A planar, white, diffu~e reflector, which i~ highly reflective and high sca~tering, is positioned parallel to the bacX

2~3~3~

surface of the lightpipe. This allows light leaving the back surface to be reflected back through the front surface of the light pipe. Alternatively, the reflector can have a sawtoothed or grooved surface.
The axi~ of the sawtooth ridges i3 preferably slightly askew the axis of the facets to reduce the effects of moire pattern development. The reflections can be satisfactorily controlled 50 that little light is returned to the light source, little light leaves the other end of the light pipe and little light $g trapped in the light pipe.
This design is in contrast to the low efficiency of the various scattering techniques of the prior art which allow the losses described. The pitch and step height are suf~icient so that a conventional diffuser is not required before the ~CD, thus allowing further relative increased light transmis~ion and efficiency.
However, a low loss diffuser is preferably located between the liqht pipe and the display to overco~e moire pattern development. Various designs of the end of the light pipe and the actual facet profile and pitch can be used to alter specific aspects of the transmis6ion to vary the light output.

A better understanding of the prior art and the present invention can be obtained when the following detailed description of the preferred embodiment i8 considered in con~unction with the following drawings, in which:
Figures 1 - 4 are views of various backlighting systems of the prior art;
Figure 5 i5 a view of a backlighting system according to th~ present invention including a light pipe an~ light sources;

20353~1 Figures 6 and 7 are greatly enlarged views of portions of the backlighting system of Figure 5;
Figures 8, 9A, 9B and 10 are greatly enlarged views of portions of the light pipe of Figure 5 showing light action; Figure 10 appears with Figure 8;
Figure 11 is a greatly enlarged view of an alternate injector according to the present invention;
as shown with Figure 8;
Figure 12 is a greatly enlarged view of a facet of the light pipe of Figure s; as shown with Figure 8;
Figure 13 is an alternate single source backlighting system according to the present invention;
as shown with Fi~ure 8; and Figures 14 to 17 are alternative designs for a lamp reflector according to the present invention.

Prior to discussing the present invention, it is considered appropriate to further discuss various designs in the prior art to explain the present technology and thus make clear the scope of the present 2~ invention.
Figure 1 generally discloses a conventional light curtain system used in providing backlight to an LCD.
Two uniform output cold cathode florescent lamps 20 and 22 are the basic light source for the system Sl. A
reflector 24 generally having a white reflective suxface facing the lamps 20 and 22 is used to redirect the light being emitted by the lamps 20 and 22 in directions other than towards the LCD D. A light blocking layer 26 is used to reduce any hot, nonuniform gpots which would occur directly over the lamps 20 and 22 to provide a first level of uniformity to the light.
The blocking layer 26 i8 preferably formed of a variable opacity mylar material, with the material . being very opaque near the lamps 20 and 22 and becoming more translucent or transparent away from the lamps.

2~3~3 i This variable opacity i8 generally provided by a printed pattern on the surface of the blocXing layer 26. However, because the light is not sufficiently uniform after passing through the blocking layer 26, a diffuser 28, which is generally a translucent plastic material, i5 used to further diffuse the light and produce a more uniform display. However, the diffuser generally reduces the light tranamission by approximately 10~ to 50%, which greatly reduces the efficiency of the overall backlighting system S1. The light curtain system S1 is relatively thick and as the lamps are placed closer to the blocking layer alignment problems increase, reducing the capability to economically manufacture the system Sl.
Two variations of similar light pipe systems are shown in Figures 2 and 3 and are generally referred to as systems S2 and S3. Both ~ystems again generally use uniform emission lamps 20 and 22, but the lamps are located at the ends of a light pipe 30. White reflectors 32 and 34 are provided around the lamps 20 and 22 so that the uniform light is directed into the light pipe 30. The light pipe 30 includes a variable density scattering structure so that the light is pro~ected out the front surface 36 of the light pipQ
30, through the diffuser 28 and through the LCD D. In the backlighting system S2 the light pipe 30 uses titanium oxide particles or other particles located in the light pipe 30 to perfcrm the scattering function.
Preferably the density of the particles i8 greater near the center of the display and lesser near the ends of the display near the lamps 20 and 22 to produce a uniform light because of the effective light density, which reduces approaching the center of the light pipe .30. A mirrored or fully reflective surface 38 is applied to the back surface 37 of the light pipe 30 so 2~3~3~1 that any light which is scattered in that direction is reflected in an attempt to have the light transmitted through the front ~urface 36 of the light pipe 30.
However, this light might again be scattered and 60 various losses can occur. The backlighting system S3 uses a scattering structure printed on the front surface 42 of the light pipe 40 to provide the scattering effect. In both systems S2 and S3 a diffuser 28 is required to provide a sufficiently uniform light source to the L D D. In these designs light can become trapped in the light pipe 40 and can readily be transmitted from one end to the other and thus be lost, reducing overall efficiency.
An alternate prior Art light pipe design is shown in Figuro 4, and i~ generally referred to by S4. In thig case a double quadratic wedge light pipe 44 is used in contrast to the parallel light pipes 30 and 40 o~ the systems S2 and S3. The back surfacs 46 of the light pipe 44 i~ a relatively constant, diffuse surface, with the ~ront surface 47 being a clear or speculax surface. The curve formed by the back ~urface 46 is a quadratic curve such that more light which impinges on the back surfaces i8 reflected through the ~ront surface as the light approaches the center of the light pipe 44. In this way a relatively uniform light source can be developed, but a diffuser 28 is still required to provide an adeguately uniform source. This design has problem~ in that some light does leak out at low angles out the back and in some cases light is sent back to th2 ~ource. Additionally, there are some problems at the exact center of the display.
Thus while the light pipe designs S2, S3 and S4 are generally thinner de igns than the light curtain system Sl, they have problems related to having to turn the light generally ninety degrees and thus have a 2~3~3~

lower efficiency than the light curtain design, which in turn has the drawback it is a relatively thick design which limits the design possibilities in portable computer ~ystems and television applications.
A backlight system accordinq to the present invention, generally referred to as S5, i8 shown in Figure 5. A faceted, dual source light pipe lO0 is coupled to an LCD D. Figure 5 shows two alternate lamp variations. In one variation a uniform dispersion la~p 102 may b~ located adjacent to an optional separate injector 104. The lamp 102 is preferably surrounded by a re~lector 106. The separate injector 104 i8 used to couple the transmitted light from the lamp 102 into the light pipe 100. The second and preferred embodiment of the light ~ourc~ i~ a lamp 108 which is a cold cathode, reflector florescent lamp having an aperture located adjacent to the end 105 of the light pipe 100. A
reflector 106 may be used with th~ lamp 108. For use with monochrome displays D a Gold cathode lamp i5 preferred to keep power consumption at a minimum, the backlight S5 being suffic~ently efficient that the added light output i5 not considered necessary.
However, if a color display ~ i5 used, a hot cathode lamp i~ preferred because of the need for maximum light output. Additionally, a reflector lamp is preferred to an aperture lamp for lamps of the diameter preferably being used in the preferred embodiment. A reflector lamp has a first internal coating of the reflective material, which then has an aperture developed and is finally completely internally coated with phosphor.
The aperturs lamp is first coated internally with the reflective material, then with the phosphor and finally the aperture is developed. Given the relatively large arc of the aperture, the additional phosphor present in the reflector lamp more than offsets the lower 2~3~3~

brightness because the light must travel through the phosphor coating the aperture. An index matching material 107 may optionally be provided between the lamp 108 and the light pipe lO0.
As shown the upper surfac~ of the light pipe 100 i~ planar, spQcular and is adjacent a low trapping and low scattQring diffuser 111. The diffuser 111 preferably produces less than 10~ brightness drop and i8 used to reduce the effects of any moire pattern de~eloped between the light pipe 100 and the LCD
display D because of the pitch and alignment variations between the items. The LCD display D is located over the diffuser 111. A back surface re~lector 126 is located parallel to the back surface 112 of the light pipe 100 to reflect light through the back surface 112 back through the light pipe 100 and out the front surface 110. In the macroscopic view of Pigure 5 the back surface 112 of tho light pipe 100 appears to be a straight wedge or planar surface but in the enlarged views shown in Figures 6 and 7 the stair stepped or faceted structure is clearly shown.
The back surface 112 consists of a series of portions 114 parallel with the front surface 110, with a serie~ of facets 116 leading to the next parallel portion 114. Figure 6 is the enlarged view showing the coupling o~ the apertured lamp 108 with the light pipe 100, while Figure 7 shows the central portion of a dual source light pipe 100. Preferably the lamp 108 is a fluorescent type lamp with an aperture height approximating the thickness of the light pipe 100. The light pipe 100 preferably ha~ a thickness of 5 mm or less at the outer edges and a thickness o~ 1 mm in the center. The thickness of 1 mm is preferred because the light pip~ 100 i5 preferably made of polymethyl methacrylate ~PMMA) and so this minimum thickness is 5~3~3~;~

provided for mec~anical strength xeasons. Other materials which can develop and ~aintain the faceted structure may be used to form thQ light pipe 100. The light pipe 100 is restrained to a thickness of approximately 5 mm so that when combined with the LCD
Dl the reflector 126 and the diffuser 111 o~ the pre~erred embodiment, the overall unit has a thickness Or less than 1/2 of an inch, not counting t~e lamp 108, thus saving a great deal of space as compared to prior art light curtain de~igns. The lamp 108 can have a diameter greater than the thickness o~ the light pipe 100, allowing a narrower aperture, a shown in Figs. 5 and 6, or preferably can have a diameter approximately equal to th~ thickness of the light pipe 100 as shown in Fig~. S and 11, with an angularly larger aperture.
I~ the preferred cold cathode lamp is used a~ the lamp 108, the lamp 108 may run at temperatures below the optimum efficiency temperature because of the small size of the lamp 108. Therefore it is preferable to use a reflector 106 which is also insulating. Four alternate embodiments are shown in Fig~. 14-17. In the embodiment of Fig. 14, a U-shaped insulator 150 is used. InsidQ the insulator 150 and be~ore the light pipe 100 can be a white reflective material 152. This material 152 can be adhesively applied i~ needed, but preferably the insulator lS0 i~ formed of a white, re~lective material. The presently preferred material is a high density polystyrene foam, but silicone, polyethylene, ~olypropylene, vinyl, neoprene or other similar materials can be used. A double sided adhesive layer 154 is used to retain the insulator 150 to the light pipe 100. The insulator 150 traps the heat produced by the lamp 108, thus rai ing the lamp operating temperature and, as a result, it~ ef~iciency.
It is desireable that the insulator 150 and associated ~3~

materials be abl~ to withstand 100C for extended periods and have a moderate fire resistance.
In the variation of Fig. 15, an expanded polystyrene block 156, or similar material, is combined S with two strips of foam tape 158 to form the insulating reflector 106. Preferably the adhesive surface of the tape 158 includes a mylar backing for strength. In the variation of Fig. 16 foam tape 158 i~ again used, but this time longitudinally with the lamp 108 to form a U-~hape. Preferably the inside of the U is covered by areflective tape 160, while the foam tape 158 is fixed to the light pipe 100 by a double sided metallized mylar tape 162.
Yet another variation is shown in Fig. 17. A
clear acrylic material 164 surrounds the lamp 108 and i~ attached to the light pipe 100 by a suitable adhesive layer. The outer surface 166 o~ the acrylic material 164 is coated with metallizing material 168 so that the outer surface 166 i9 a reflector. In this manner light which is emitted from the lamp 108 at locations other than the aperture i~ reflected through the acrylic material 164 into the light pipe 100, instead of through the lamp 108 a8 in Figures 14 to 16.
While tho acrylic material 164 will provide some in~ulation, it may not be ~ufficient to raise the lamp 108 temperature a~ dQsired and 80 foam insulating tape 158 may be u~ed over the acrylic material 164 for better insulation. In this case the entir- inner surfac~ of the foam tape 158 may be adhesive coated as the reflective layer is present on the acrylic materia~
164.
A separate in~ector 104 may be used to couple the light ~eing emitted by the lamp 108 into the light pipe 100, but preferably the end 105 of the light pipe 100 is considered the in~ector. The injector 104 or end ~3~3~:~

105 is preferably a flat surface which is polished and specular, that i8 non-diffuse, and may be coated with anti-reflective coatings. A flat, specular surface is preferred with a light pipe material having an index of refraction greater than 1.2, which results in total internal r-flection of any in~ected light, wh~ch the ~acet structure will pro~ect out the front ~urface 110.
Several other alternatives are available for the in~ector, ~uch as index matching material 107 to match the lamp 108 to the light pipe 100 to eliminate surface reflections. ~he index matching material 107 is a clear material, such as silicone oil, epoxy or polymeric material, which contacts both the lamp 108 and the end 105. Alternatively, the injector 118 can be shaped to conform to the lamp 108 with a ~mall air gap (Fig. 11). This curved surface of the injector 118 help~ locate the lamp 108. Additionally, a cylindrical fresnel lens can be formed on the end 105 or separate injector 104 to help focus the light being emitted from the lamp 108. Its noted that a cylindrical fresnel lens is preferred over a true cylindrical lens to limit leakage of the light. Alternate lenses can be developed on th~ ~eparate in~ector 104 or end 105 which in combination with the facets 116 can effect the output cone Or the light as it exits the light pipe 100. Preferably the output cone is the same as the viewing angle of the LCD D so that effectively no light is being lost which is not needed when viewing the LCD
D, thus increasing effective efficiency of the ~ystem.
Figure 8 show~ a greatly enlarged view of a portion of one facet 116 and several parallel portions 114 of th~ light pipe 100. As can be seen the parallel back ~urface portions 114 are parallel with the front gurfac~ 110, both of which are specular, so that the light pipe 100 preferably utilizes only specular .~35~i reflections and does not utilize diffuse reflection or refraction, except in minor amounts. ~y having primarily only ~pecular reflections it i~ possible to better control th~ light so that it does not leave the light pipQ 100 in undesired direction~, thus allowing better focusing and less diffusion. Thug the basic propagation media of the light pipe 100 i8 that Or a parallel plate light pipe and not of a wedge or quadratic. The facet 116 preferably has an angle ~ of 135 degrees from the parallel portion 114. This is the preferred angle because then light parallel to the faces 110 and 114 is transmitted perpendicular to the light pipe 100 when exiting the front face 110.
However, the angle can be in any ranga from 90 to 180 lS degrees depending upon the particular output characteristics de~ired. The pitch P (Fig. 6) or distance between successive facets 116 is related to and generally must be less than the visual threshold of the eye which, while proportional to the distance the eye is from the LCD D, has preferred values of 200 to 250 lines per inch or greater. In one embodiment without the diffuser 111 the pitch P is varied from 200 lines per inch at the end~ of the light pipo 100 near the lamps 108 to 1000 lines per inch at the center 80 that more reflQctions toward the front face 110 occur at tha middle of th- light pipe 100 where the light intensity has reduced. The pitch in the center is limited to 1,000 lines per inch to provide capability to practically manufacture the liqht pipe 100 in large quantitie~, given the limitation~ of compression or in~ection molding PMMA. If the diffuser 111 is utilized, the pitch can go lower than 200 line per inch because of th~ scattering effects of the diffuser 111. The limit is dependent on the particular diffuser 111 utilized. ThU5 the use of the diffuser 111 can be ~3~3~

considered as changing the limit of visual thresholdr In one embodiment of the present invention the facet height H (Fig. 8) range~ from approximately 1 micron near t~e end 105 to 10 microns near the middle, the farthest point from a lamp. In the drawings the facet height is greatly enlarged relative to th~ pitch for illustrative purposes. The preferred minimum facet height is 1 micron to allow the light pipQ 100 to be developed using conventional manufacturing processes, while the preferred maximum facet height is 100 microns to Xeep overall thickness of the light pipe 100 reduced. It is noted that increasing the facet height of a facet 116 at any given point will increase the amount of light presented at that point, referred to as the extraction efficiency, QO that by changing the pitch P, facet height H and facet angle ~ varying profiles and variations in uniformity of the light output from the front surface 110 can be developed as needed.
While the desire is to use purely specular reflective effects in th~ light pipe 100, ~ome light will be split into transmitted and reflected components. Even though there i- total internal reflection of light in~ected into the light pipe 100 by the front ~urface 110 and parallel portions 114, when the light ~trike~ a facet 116 much of the light will exceed the critical angle and develop transmitted and reflected components. If the light is reflected from the facet 116, it will preferentially be transmitted through the front surface 110 to the viewer. However, the transmitted component will pass through the back surface 112. Thus a reflective coating 122 may be applied to the facet 116. This reflective material 122 then redirects any light transmitted through the facet 116. This is where the greatest amount of transmission 3 ~ :~

i~ likely to occur because of the relatively parallel effects as proceedinq inward on the light pipe lOo A design trade off can be made here based on the amount of light exceeding the critical angle being reflected back from the front 6urface 110, through the back surface 112 or through the facets 116 If there iB a greater amount of thi~ light which will be transmitted out the back surface 112 and lost, it may be desirable to fully coat the back surface 112 a~
shown in Figure 10 80 that the entire back surface 112 is coated by a reflector material 124 Because the reflector material i5 preferably aluminum or other metal~ the efficiency of the reflector 124 is not 100%
but typically in the range of 80% to 90~, some reflective 10B~ occurs at each point Thus there is some drop in efficiency at each time the light impinges on the reflector 124, but based on the amount of high angle light present, more light may actually be transmitted through the front surface 110, even with the reflective 1055Q~. If the lamp transmits much more parallel light, then the coating of the parallel portions 114 with reflective material may not be necessary.
In the embodiments shown in Figures 9A and 9B no reflectiv- coatings are actually applied to the light plp- 100 but in~tead a refle~tor plate 126A or 126B is located ad~acent the back surface 112 of the light pipe 100 In the preferred embodiment shown in Fig 9A, the reflector plate 126A ie planar and has a white and diffuse surface 170 facing the back sur~ace 112 of the light pipe 100 The surface 170 is highly reflective and high scattering to reflect the light passing through the back surface 112 back through the light pipe 100 and out the front surface 110 The thickness ~3~

of the reflector plate 126A is as needed for mechanical strength.
In an alternate embodiDent shown in Fig. 9B, the front or light pipe facing surface 132 of the reflector plate 126B has a sawtoothed or grooved 6ur$ace, with the blaze angle ~ of the ~awtooth being in the range of 30 to 60 degrees, with the preferred angle being approximately 40 d~grees. The pitch W of the ~awteeth i8 different from the pitch P of the light pipQ facets to reduce the effects of moire pattern development between the light pipe 100 and the reflector 126B. The pitches are uniform in the preferred embodiment and are in the range of 1-10 mils for the facets and 1-10 mil~
for the reflector grooves, with the preferred facat pitch P being 6 mils and the sawtooth pitch W being 4 mils. The sawtooth pitch W can be varied if the facet pitch P varies, but ~ constant pitch is considered preferable from a manufacturing viewpoint. The thickness of the reflector plate 126B is as needed for mechanical ~upport.
Additionally, the longitudinal axis of the ~awteeth i~ lightly rotated from the lcngitudinal axis of the facets to further reduc- the efrects of moire pattern development. The ~awtooth surface 132 i~
~5 co~ted with a reflecting material ~o that any imp~nginq light i- reflected back through th~ light pipe 100 as shown by the ray tracing~ of Fig. 9. Further, the sawteeth can have several different angles between the preferred limit~ ts better ~hape the light exiting the light p~pe 100.
The ma~ority of the light which impinges on the sawtooth ~urface 132 or the diffuse surface 170 will proceed directly through the light pipe 100 and emerge from the front face 110 because the light pipe 100 i~
effectively a parallel plate because the facet area is ~3~"9 ~.

only a very small percentage as compared to the flat portion of the back ~urfac~ 112. Thu~ the light which exits the back surface 112 of the light pipe 100 is reflect~d back through the light pipe 100 to exit the front surface 110 and contribute to the emitted light with little 1088.
Additionally, the actual facet profile 116 i8 not necessarily plan~r. As shown in Figure 12, the actual facet profile may be slightly concave 128 or ~lightly convex 130. The facets 116 then form a lenticular array and can be curved as desired to help tailor the output profilQ of the light cone. Additionally, the facet 116 ~urface may be roughened to increase scattering if desired.
While the de~ign of the light pipe 100 illustrated in Fig. 5 use lamps at both ends in a dual light source arrangement, light could be provided from only one end in a sinqle source configuration as shown in Fig. 13.
The end oppo~ite the light ~ource 102 is then the thinnest portion of the light pipe 100' and a reflective ~urface 134 i~ provided to limit losses from the end of the light pipe 100'. The light pipe 100' still has the planar front surface 110, a ~aceted back surface 112, a reflector plate 126 and a low loss diffu~er 111 and the other variations described above are applicable. The facet pitch and height are preferably varied ~g previously described to develop greater light redirection to help compensate for the lesser total amount of light supplied by the light source 102.
Having described the invention abov~, various modific~tions of the techniques, procedures, material and eguipment will be apparent to those in the art. It i~ intended that all such variations within the scope and spirlt of the appended claims be embraced thereby.

Claims

1. A system for backlighting a liquid crystal display, comprising:
a light pipe having a generally planar front surface for providing light to the liquid crystal display, having a faceted back surface wherein said back surface includes a plurality of generally planar portions parallel to said front surface and a plurality of facets formed at an angle to said front surface and located connecting said back surface parallel portions, and having at least one end surface for receiving light to be transmitted through said front surface;
light source means located adjacent each said end surface for receiving light of said light pipe for providing light to said light pipe; and reflector means located adjacent to and generally parallel to said light pipe back surface for reflecting light back through said light pipe.

2. The system of claim 1, wherein said reflector means is generally planar and has a front surface adjacent to said light pipe back surface, said reflector means front surface including a series of grooves, the longitudinal axis of said grooves extending somewhat parallel to the longitudinal axis of said facets.

3. The system of claim 2, wherein the longitudinal axis of said grooves is somewhat askew of the longitudinal axis of said facets.

4. The system of claim 1, wherein said reflector means is generally planar and has a front surface adjacent to said light pipe back surface, said reflector means front surface being highly reflective and highly scattering.

5. The system of claims 3 or 4, further comprising:
injector means between said light source means and said light pipe for coupling light produced by said light source means to said light pipe.

6. The system of claim 5, wherein said injector means has a flat surface facing said light source means.

7. The system of claim 6, wherein said injector means flat surface is coated with an anti-reflective coating.

8. The system of claim 5, wherein said injector means includes index matching material located between and contacting said light source means and said light pipe.

9. The system of claim 5, wherein said injector means is shaped to generally conform to the surface of said light source means.

10. The system of claim 5, wherein said injector means includes a surface having a fresnel lens developed thereon.

11. The system of claim 10, wherein said fresnel lens is a cylindrical fresnel lens.

12. The system of claims 3 or 4, wherein each said end for receiving light of said light pipe has a flat surface.

13. The system of claim 12, wherein said end is coated with an anti-reflective coating.

14. The system of claim 12 wherein said end has a fresnel lens developed thereon.

15. The system of claim 14, wherein said fresnel lens is a cylindrical lens.

16. The system of claims 3 or 4, wherein each said end for receiving light of said light pipe is shaped to generally conform to the surface of said light source means.

17. The system of claims 3 or 4, wherein said light source means includes fluorescent lamps.

18. The system of claim 17, wherein said lamps are reflector lamps.

19. The system of claim 17, wherein said lamps are aperture lamps.

20. The system of claim 17, wherein said lamps are cold cathode lamps.

21. The system of claim 20, wherein said lamps are partially encompassed by insulation.

22. The system of claim 21, wherein said insulation includes a reflective surface facing said lamps.

23. The system of claim 17, wherein said lamps are hot cathode lamps.

24. The system of claims 3 or 4, wherein said light source means includes uniform dispersion fluorescent lamps.

25. The system of claim 24, wherein said light means further includes reflectors formed around said lamps to reflect light to said light pipe.

26. The system of claims 3 or 4, wherein said light pipe is formed of polymethyl methacrylate.

27. The system of claims 3 or 4, wherein the angle of said facets from said parallel portion is between 90 and 180 degrees.

28. The system of claim 27, wherein the angle is approximately 135 degrees.

29. The system of claims 3 or 4, wherein the pitch defined by the distance from successive facets is less than that required to exceed the visual threshold of a human being.

30. The system of claim 29, wherein said pitch is randomly varied.

31. The system of claim 29, wherein said pitch is uniformly varied to a maximum of approximately 1000 per inch.

32. The system Or claims 3 or 4, wherein the facet height between successive parallel portions is varied between two limits.

33. The system of claim 32, wherein said facet height limits are approximately 1 and 100 microns.

34. The system of claims 3 or 4, further comprising a diffuser located adjacent to and generally parallel to said light pipe front surface.

35. The system of claim 1, further comprising a low scattering diffuser located adjacent to and generally parallel to said light pipe front surface.

36. The system of claims 3 or 4, wherein said facets are generally planar.

37. The system of claims 3 or 4, wherein said facets are portions of a generally cylindrical surface.