United States Patent [191
`Hathaway et al.
`
`[11] Patent Number:
`[45] Date of Patent:
`
`5,050,946
`Sep. 24, 1991
`
`[54] FACETED LIGHT PIPE
`[75] Inventors: Kevin J. Hathaway, San Jose, Calif;
`Richard M. Knox, Jr., Houston, Tex.;
`Douglas A. Arego, Spring, Tex.;
`Gaylon R. Kornfuerhrer, Cypress,
`Tex.
`[73] Assignee: Compaq Computer Corporation,
`Houston, Tex.
`[21] Appl. No.: 589,325
`[22] Filed:
`Sep. 27, 1990
`
`[51] Int. Cl.5 ........... ................................. .. G02B 6/00
`[52] US. Cl. .................................... .. 385/33; 362/309;
`362/341; 362/27; 362/32; 362/31; 385/37;
`385/146; 385/901; 359/48; 359/50
`[58] Field of Search ............. .. 350/9610, 96.15, 96.18,
`350/9619; 362/309, 341, 27, 31, 32
`References Cited
`U.S. PATENT DOCUMENTS
`
`[56]
`
`4,257,084 3/1981 Reynolds ............................ .. 362/31
`4,277,817 7/1981 Hehr ..... ..
`362/31
`4,323,951 4/1982 Pasco ........ ..
`.. 362/27
`4,528,617 7/1985 Blackington
`..... .. 362/32
`4,706,173 11/1987 Hamada et al. ............... .. 362/341
`4,799,137 l/l989 Aho ................................... .. 362/309
`4,883,333 ll/l989 Yanez ............................. .. 350/9610
`
`FOREIGN PATENT DOCUMENTS
`3825436 3/1989 Fed. Rep. of Germany
`350/9610
`0073206 4/1987 Japan .............................. .. 350/9610
`
`0271301 1l/l988 Japan .............................. .. 350/9610
`0287803 ll/l988 Japan .............................. .. 350/9610
`Primary Examiner-Georgia Epps
`Attorney, Agent, or F1'rm—-Pravel, Gambrell, Hewitt,
`Kimball & Krieger
`ABSTRACT
`[57]
`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, ?uorescent
`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 sur
`face connected by facets, which are angled so that the
`injected light re?ects off the facets and through the
`front surface. A re?ector having a planar, highly re?ec
`tive, highly scattering surface or a sawtoothed or
`grooved upper surface is located adjacent to and paral
`lel with the light pipe back surface to re?ect light escap
`ing 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 liq
`uid 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.
`
`37 Claims, 6 Drawing Sheets
`
`K.J. Pretech Ex. 1019
`
`Pretech_000795
`
`

`
`U.S. Patent
`
`Sep. 24, 1991
`
`SheOt 1 of 6
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`5,050,946
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`U.S. Patent
`
`Sep. 24, 19531
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`Sheet 2 of 6
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`5,050,946
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`108
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`Pretech_000797
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`

`
`US. Patent
`
`Sep. 24, 1991‘
`
`She'et 3 of 6
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`5,050,946
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`U.S. Patent
`
`Sep. 24, 1991
`
`Sheet 4 of 6
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`5,050,946
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`US. Patent
`
`Sep. 24, 1951
`
`Sheet 6 0f 6
`
`5,050,946
`
`F IGJY
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`168
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`Pretech_000801
`
`

`
`1
`
`FACETED LIGHT PIPE
`
`5,050,946
`
`BACKGROUND OF THE INVENTION
`1. Field of the Invention
`The invention relates to backlighting systems used
`with liquid crystal displays, and more particularly to
`light pipe systems.
`2. Description of the Related Art
`Liquid crystal displays (LCD’s) are commonly used
`in portable computer systems, televisions and other
`electronic devices. An LCD requires a source of light
`for operation because the LCD is effectively a light
`valve, allowing transmission of light in one state and
`blocking transmission of light in a second state. Back
`lighting the LCD has become the most popular source
`of light in personal computer systems because of the
`improved contrast ratios and brightnesses possible. Be
`cause conventional monochrome LCD’s are only ap
`proximately 12% transmissive and color LCD’s are
`only approximately 2% transmissive, relative large
`amounts of uniform light are necessary to provide a
`visible display. If power consumption and space were
`not of concern the necessary level and uniformity of
`25
`backlight could be obtained.
`However, in portable devices power consumption,
`which directly effects battery life, and space are major
`concerns. Thus there is a need to obtain a sufficiently
`uniform and bright backlight level with as little power
`as possible in as little space as possible at, of course, as
`low a cost as possible.
`Numerous designs exist which trade off various of
`these goals to achieve a balanced display. Several of
`these designs, such as light curtains and light pipes, are
`shown in the ?gures and will be described in detail later.
`The designs generally trade off uniformity of backlight
`ing for space or efficiency. The designs utilize various
`scattering means and a ?nal diffuser before the light is
`presented to the LCD. The scattering means and the
`diffusers both allow loss of light and thus reduce the
`efficiency of the transfer from the light source 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 pres
`ent, as is a desire for the use of less space.
`
`45
`
`SUMMARY OF THE INVENTION
`The present invention is a faceted, parallel surface
`light pipe design. Light sources, preferably re?ector or
`apertured ?uorescent lamps, but alternatively uniform
`lamps, supply light to one or both ends of a light pipe.
`The front surface of the light pipe, on which is posi
`tioned a low loss diffuser, which in turn is in contact
`with the LCD, is planar, while the back surface of the
`light pipe is generally parallel to the front surface, but
`has a stair stepped or faceted surface. The facets are
`preferably formed at an angle so that the light injected
`into the ends of the light pipe is re?ected off the facets
`and through the front surface. The pitch or step length
`of the facets is such that the faceting structure is not
`‘visible to the human eye. The step height of the facets is
`preferably in the micron range and may increase with
`the distance from the lamp. A planar, white, diffuse
`re?ector, which is highly re?ective and high scattering, ,
`is positioned parallel to the back surface of the ligh
`tpipe. This allows light leaving the back surface to be
`re?ected back through the front surface of the light
`pipe. Alternatively, the re?ector can have a sawtoothed
`
`50
`
`65
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`2
`or grooved surface. The axis of the sawtooth ridges is
`preferably slightly askew the axis of the facets to reduce
`the effects of moire pattern development. The re?ec‘
`tions can be satisfactorily controlled so that little light is
`returned to the light source, little light leaves the other
`end of the light pipe and little light is 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
`sufficient so that a conventional diffuser is not required
`before the LCD, thus allowing further relative in
`creased light transmission and efficiency. However, a
`low loss diffuser is preferably located between the light
`pipe and the display to overcome moire pattern devel
`opment. Various designs of the end of the light pipe and
`the actual facet profile and pitch can be used to alter
`speci?c aspects of the transmission to vary the light
`output.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`A better understanding of the prior art and the pres
`ent invention can be obtained when the following de
`tailed description of the preferred embodiment is con
`sidered in conjunction with the following drawings, in
`which:
`FIGS. 1-4 are views of various backlighting systems
`of. the prior art;
`FIG. 5 is a view of a backlighting system according
`to the present invention including a light pipe and light
`sources;
`FIGS. 6 and 7 are greatly enlarged views of portions
`of the backlighting system of FIG. 5;
`FIGS. 8, 9A, 9B and 10 are greatly enlarged views of
`portions of the light pipe of FIG. 5 showing light ac
`tion;
`FIG. 11 is a greatly enlarged view of an alternate
`injector according to the present invention;
`FIG. 12 is a greatly enlarged, view of a facet of the
`light pipe of FIG. 5;
`FIG. 13 is an alternate single source backlighting
`system according to the present invention; and
`FIGS. 14 to 17 are alternative designs for a lamp
`re?ector according to the present invention.
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENT
`Prior to discussing the present invention, it is consid
`ered appropriate to further discuss various designs in
`the prior art to explain the present technology and thus
`make clear the scope of the present invention.
`FIG. 1 generally discloses a conventional light cur
`tain system used in providing backlight to an LCD.
`Two uniform output cold cathode ?orescent lamps 20
`and 22 are the basic light source for the system S1. A
`re?ector 24 generally having a white re?ective surface
`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 spots which would
`occur directly over the lamps 20 and 22 to provide a
`first level of uniformity to the light. The blocking layer
`26 is 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. This variable opacity
`is generally provided by a printed pattern on the surface
`
`Pretech_000802
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`‘5,050,946
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`of the blocking layer 26. However, because the light is
`not suf?ciently uniform after passing through the block
`ing layer 26, a diffuser 28, which is generally a translu
`cent plastic material, is used to further diffuse the light
`and produce a more uniform display. However, the
`diffuser generally reduces the light transmission by
`approximately l0% to 50%, which greatly reduces the
`ef?ciency 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 economi
`cally manufacture the system S1.
`Two variations of similar light pipe systems are
`shown in FIGS. 2 and 3 and are generally referred to as
`systems S2 and S3. Both systems again generally use
`uniform emission lamps 20 and 22, but the lamps are
`located at the ends of a light pipe 30. White re?ectors 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 struc
`ture so that the light is projected out the front surface 36
`of the light pipe 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 lo
`cated in the light pipe 30 to perform the scattering func
`tion. Preferably the density of the particles is greater
`near the center of the display and lesser near the ends of
`the display near the lamps 20 and 22 to produce a uni
`form light because of the effective light density, which
`reduces approaching the center of the light pipe 30. A
`30
`mirrored or fully re?ective surface 38 is applied to the
`back surface 37 of the light pipe 30 so that any light
`which is scattered in that direction is re?ected in an
`attempt to have the light transmitted through the front
`surface 36 of the light pipe 30. However, this light
`might again be scattered and so various losses can oc
`cur. The backlighting system S3 uses a scattering struc
`ture printed on the front surface 42 of the light pipe 40
`to provide the scattering effect. In both systems S2 and
`$3 a diffuser 28 is required to provide a sufficiently
`uniform light source to the LCD 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
`FIG. 4, and is generally referred to by S4. In this case a
`double quadratic wedge light pipe 44 is used in contrast
`to the parallel light pipes 30 and 40 of the systems S2
`and S3. The back surface 46 of the light pipe 44 is a
`relatively constant, diffuse surface, with the front sur
`face 47 being a clear or specular surface. The curve
`formed by the back surface 46 is a quadratic curve such
`that more light which impinges on the back surfaces is
`reflected through the front surface as the light ap
`proaches 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 adequately
`uniform source. This design has problems in that some
`light does leak out at low angles out the back and in
`some cases light is sent back to the source. Additionally,
`there are some problems at the exact center of the dis~
`play.
`.
`Thus while the light pipe designs S2, S3 and S4 are
`generally thinner designs than the light curtain system
`S1, they have problems related to having to turn the
`light generally ninety degrees and thus have a lower
`ef?ciency than the light curtain design, which in turn
`has the drawback it is a relatively thick design which
`
`4
`limits the design possibilities in portable computer sys
`tems and television applications.
`_
`A backlight system according to the present inven
`tion, generally referred to as S5, is shown in FIG. 5. A
`faceted, dual source light pipe 100 is coupled to an LCD
`D. FIG. 5 shows two alternate lamp variations. In one
`variation a uniform dispersion lamp 102 may be located
`adjacent to an optional separate injector 104. The lamp
`102 is preferably surrounded by a re?ector 106. The
`separate injector 104 is used to couple the transmitted
`light from the lamp 102 into the light pipe 100. The
`second and preferred embodiment of the light source is
`a lamp 108 which is a cold cathode, re?ector ?orescent
`lamp having an aperture located adjacent to the end 105
`of the light pipe 100. A reflector 106 may be used with
`the lamp 108. For use with monochrome displays D a
`cold cathode lamp is preferred to keep power consump
`tion at a minimum, the backlight S5 being suf?ciently
`ef?cient that the added light output is not considered
`necessary. However, if a color display D is used, a hot
`cathode lamp is preferred because of the need for maxi
`mum light output. Additionally, a re?ector lamp is
`preferred to an aperture lamp for lamps of the diameter
`preferably being used in the preferred embodiment. A
`re?ector lamp has a ?rst internal coating of the re?ec
`tive material, which then has an aperture developed and
`is ?nally completely internally coated with phosphor.
`The aperture lamp is ?rst coated internally with the
`re?ective material, then with the phosphor and ?nally
`the aperture is developed. Given the relatively large arc
`of the aperture, the additional phosphor present in the
`re?ector lamp more than offsets the lower 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 100.
`As shown the upper surface of the light pipe 100 is
`planar, specular and is adjacent a low trapping and low
`scattering diffuser 111. The diffuser 111 preferably pro
`duces less than 10% brightness drop and is used to
`reduce the effects of any moire pattern developed be
`tween the light pipe 100 and the LCD display D be
`cause of the pitch and alignment variations between the
`items. The LCD display D is located over the diffuser
`111. A back surface re?ector 126 is located parallel to
`the back surface 112 of the light pipe 100 to re?ect light
`through the back surface 112 back through the light
`pipe 100 and out the front surface 110. In the macro
`scopic view of FIG. 5 the back surface 112 of the light
`pipe 100 appears to be a'straight wedge or planar sur
`face but in the enlarged views shown in FIGS. 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 series of
`facets 116 leading to the next parallel portion 114. FIG.
`6 is the enlarged view showing the coupling of the
`apertured lamp 108 with the light pipe 100, while FIG.
`7 shows the central portion of a dual source light pipe
`100. Preferably the lamp 108 is a ?uorescent type lamp
`with an aperture height approximating the thickness of
`the light pipe 100. The light pipe 100 preferably has a
`thickness of 5 mm or less at the outer edges and a thick
`- ness of 1 mm in the center. The thickness of 1 mm is
`preferred because the light pipe 100 is preferably made
`of polymethyl methacrylate (PMMA) and so this mini
`mum thickness is provided for mechanical strength
`reasons. Other materials which can develop and main
`tain the faceted structure may be used to form the light
`
`45
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`Pretech_000803
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`pipe 100. The light pipe 100 is restrained to a thickness .
`of approximately 5 mm so that when combined with the
`LCD D, the re?ector 126 and the diffuser 111 of the
`preferred embodiment, the overall unit has a thickness
`of less than i of an inch, not counting the lamp 108, thus
`saving a great deal of space as compared to prior art
`light curtain designs. The lamp 108 can have a diameter
`greater than the thickness of the light pipe 100, allowing
`a narrower aperture, as shown in FIGS. 5 and 6, or
`preferably can have a diameter approximately equal to
`the thickness of the light pipe 100 as shown in FIGS. 5
`and 11, with an angularly larger aperture.
`If the preferred cold cathode lamp is used as 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
`re?ector 106 which is also insulating. Four alternate
`embodiments are shown in FIGS. 14-17. In the embodi
`ment of FIG. 14, a U-shaped insulator 150 is used. In
`side the insulator 150 and before the light pipe 100 can
`be a white re?ective material 152. This material 152 can
`be adhesively applied if needed, but preferably the insu
`lator 150 is formed of a white, re?ective material. The
`presently preferred material is a high density polysty
`rene foam, but silicone, polyethylene, polypropylene,
`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 raising
`the lamp operating temperature and, as a result, its effi
`ciency. It is desireable that the insulator 150 and associ
`ated materials be able to withstand 100° C. for extended
`periods and have a moderate ?re resistance.
`In the variation of FIG. 15, an expanded polystyrene
`block 156, or similar material, is combined with two
`strips of foam tape 158 to form the insulating re?ector
`106. Preferably the adhesive surface of the tape 158
`includes a mylar backing for strength. In the variation
`of FIG. 16 foam tape 158 is again used, but this time
`longitudinally with the lamp 108 to form a U-shape.
`Preferably the inside of the U is covered by a re?ective
`tape 160, while the foam tape 158 is ?xed 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 is at
`tached to the light pipe 100 by a suitable adhesive layer.
`The outer surface 166 of the acrylic material 164 is
`coated with metallizing material 168 so that the outer
`surface 166 is a re?ector. In this manner light which is
`emitted from the lamp 108 at locations other than the
`aperture is re?ected through the acrylic material 164
`into the light pipe 100, instead of through the lamp 108
`as in FIGS. 14 to 16. While the acrylic material 164 will
`provide some insulation, it may not be suf?cient to raise
`the lamp 108 temperature as desired and so foam insulat
`ing tape 158 may be used over the acrylic material 164
`for better insulation. In this case the entire inner surface
`of the foam tape 158 may be adhesive coated as the
`re?ective layer is present on the acrylic material 164.
`A separate injector 104 may be used to couple the
`light being emitted by the lamp 108 into the light pipe
`100, but preferably the end 105 of the light pipe 100 is
`considered the injector. The injector 104 or end 105 is
`preferably a ?at surface which is polished and specular,
`that is non-diffuse, and may be coated with anti-reflec-I
`tive coatings. A ?at, specular surface is preferred with a
`light pipe material having an index of refraction greater
`than 1.2, which results in total internal re?ection of any
`
`5,050,946
`6
`injected light, which the facet structure will project out
`the front surface 110.
`Several other alternatives are available for the injec
`tor, such as index matching material 107 to match the
`lamp 108 to the light pipe 100 to eliminate surface re
`?ections. The index matching material 107 is a clear
`material, such as silicone oil, epoxy or polymeric mate
`rial, which contacts both the lamp 108 and the end 105.
`Alternatively, the injector 118 can be shaped to con
`form to the lamp 108 with a small air gap (FIG. 11).
`This curved surface of the injector 118 helps 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. Alter
`nate lenses can be developed on the separate injector
`104 or end 105 which in combination with the facets 116
`- can effect the output cone of the light as it exits the light
`pipe 100. Preferably the output cone is the same as the
`20
`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 sys
`tem.
`FIG. 8 shows a greatly enlarged view of a portion of
`one facet 116 and several parallel portions 114 of the
`light pipe 100. As can be seen the parallel back surface
`portions 114 are parallel with the front surface 110, both
`of which are specular, so that the light pipe 100 prefera
`bly utilizes only specular re?ections and does not utilize
`diffuse re?ection or refraction, except in minor
`amounts. By having primarily only specular re?ections
`it is possible to better control the light so that it does not
`leave the light pipe 100 in undesired directions, thus
`allowing better focusing and less diffusion. Thus the
`basic propagation media of the light pipe 100 is that of
`a parallel plate light pipe and not of a wedge or qua
`dratic. The facet 116 preferably has an angle a of 135'
`degrees from the parallel portion 114. This is the pre
`ferred 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 range from 90 to 180 degrees depending
`upon the particular output characteristics desired. 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 ends of the light
`pipe 100 near the lamps 108 to 1000 lines per inch at the
`center so that more reflections toward the front face 110
`occur at the middle of the 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 practi
`cally manufacture the light pipe 100 in large quantities,
`given the limitations of compression or injection mold-'
`ing PMMA. If the diffuser 111 is utilized, the pitch can
`go lower than 200 lines per inch because of the scatter
`ing effects of the diffuser 111. The limit is dependent on
`the particular diffuser 111 utilized. Thus the use of the
`diffuser 111 can be considered as changing the limit of
`visual threshold. In one embodiment of the present
`invention the facet height H (FIG. 8) ranges from ap
`proximately I micron near the 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
`
`45
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`65
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`Pretech_000804
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`5,050,946
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`the pitch for illustrative purposes. The preferred mini
`mum facet height is 1 micron to allow the light pipe 100
`to be developed using conventional manufacturing pro
`cesses, while the preferred maximum facet height is 100
`microns to keep 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 extrac
`tion ef?ciency, so that by changing the pitch P, facet
`height H and facet angle a varying pro?les and varia
`tions in uniformity of the light output from the front
`surface 110 can be developed as needed.
`While the desire is to use purely specular re?ective
`effects in the light pipe 100, some light will be split into
`transmitted and reflected components. Even though
`there is total internal re?ection of light injected into the
`light pipe 100 by the front surface 110 and parallel por
`tions 114, when the light strikes a facet 116 much of the
`light will exceed the critical angle and develop transmit
`ted and reflected components. If the light is re?ected
`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 re?ective coating 122 may be ap
`plied to the facet 116. This re?ective material 122 then
`redirects any light transmitted through the facet 116.
`This is where the greatest amount of transmission is
`likely to occur because of the relatively parallel effects
`as proceeding inward on the light pipe 100.
`A design trade off can be made here based on the
`amount of light exceeding the critical angle being re
`?ected back from the front surface 110, through the
`back surface 112 or through the facets 116. If there is a
`greater amount of this light which will be transmitted
`out the back surface 112 and lost, it may be desirable to
`fully coat the back surface 112 as shown in FIG. 10 so
`that the entire back surface 112 is coated by a re?ector
`material 124. Because the re?ector material is prefera
`bly aluminum or other metals the ef?ciency of the re
`?ector 124 is not 100% but typically in the range of
`40
`80% to 90%, some re?ective loss occurs at each point.
`Thus there is some drop in ef?ciency at each time the
`light impinges on the re?ector 124, but based on the
`amount of high angle light present, more light may
`actually be transmitted through the front surface 110,
`45
`even with the re?ective losses. If the lamp transmits
`much more parallel light, then the coating of the paral
`lel portions 114 with re?ective material may not be
`necessary.
`In the embodiments shown in FIGS. 9A and 98 no
`re?ective coatings are actually applied to the light pipe
`100 but instead a re?ector plate 126A or 126B is located
`adjacent the back surface 112 of the light pipe 100. In
`the preferred embodiment shown in FIG. 9A, the re
`?ector plate 126A is planar and has a white and diffuse
`surface 170 facing the back surface 112 of the light pipe
`100. The surface 170 is highly re?ective and high scat
`tering to re?ect the light passing through the back sur
`face 112 back through the light pipe 100 and out the
`front surface 110. The thickness of the re?ector plate
`126A is as needed for mechanical strength.
`In an alternate embodiment shown in FIG. 9B, the
`front or light pipe facing surface 132 of the reflector
`plate 126B has a sawtoothed or grooved surface, with '
`the blaze angle B of the sawtooth being in the range of
`65
`30 to 60 degrees, with the preferred angle being approx
`imately 40 degrees. The pitch W of the sawteeth is
`different from the pitch P of the light pipe facets to to
`
`8
`reduce the effects of moire pattern development be
`tween the light pipe 100 and the reflector 126B. The
`pitches are uniform in the preferred embodiment and
`are in the range of l-lO mils for the facets and 1-10 mils
`'for the re?ector grooves, with the preferred facet 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 a constant pitch is considered preferable
`from a manufacturing viewpoint. The thickness of the
`re?ector plate 126B_ is as needed for mechanical sup
`port.
`Additionally, the longitudinal axis of the sawteeth is
`slightly rotated from the longitudinal axis of the facets
`to further reduce the effects of moire pattern develop
`ment. The sawtooth surface 132 is coated with a re?ect
`ing material so that any impinging light is re?ected back
`through the light pipe 100 as shown by the ray tracings
`of FIG. 9. Further, the sawteeth can have several differ
`ent angles between the preferred limits to better shape
`the light exiting the light pipe 100.
`The majority of the light which impinges on the
`sawtooth surface 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 is
`effectively a parallel plate because the facet area is only
`a very small percentage as compared to the ?at portion
`of the back surface 112. Thus the light which exits the
`back surface 112 of the light pipe 100 is re?ected back
`through the light pipe 100 to exit the front surface 110
`and contribute to the emitted light with little loss.
`Additionally, the actual facet pro?le 116 is not neces
`sarily planar. As shown in FIG. 12, the actual facet
`pro?le may be slightly concave 128 or slightly convex
`130. The facets 116 then form a lenticular array and can
`be curved as desired to help tailor the output pro?le of
`the light cone. Additionally, the facet 116 surface may
`be roughened to increase scattering if desired.
`While the design 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 single source con?guration as shown in FIG.
`13. The end opposite the light source 102 is then the
`thinnest portion of the light pipe 100’ and a re?ective
`surface 134 is provided to limit losses from the end of
`the light pipe 100'. The light pipe 100' still has the pla
`nar front surface 110, a faceted back surface 112, a re
`?ector plate 126 and a low loss diffuser 111 and the
`other variations described above are applicable. The
`facet pitch and height are preferably varied as previ
`ously 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 above, various modi
`?cations of the techniques, procedures, material and
`equipment will be apparent to those in the art. It is
`intended that all such variations within the scope and
`spirit of the appended claims be embraced thereby.
`We claim:
`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 p