CA2034378C

CA2034378C - Uniform illumination of large, thin surfaces particularly suited for automotive applications

Info

Publication number: CA2034378C
Application number: CA002034378A
Authority: CA
Inventors: John M. Davenport; Richard L. Hansler; John L. Henkes
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1990-03-20
Filing date: 1991-01-17
Publication date: 1999-04-20
Anticipated expiration: 2011-01-17
Also published as: JPH07120485B2; JPH04221236A; EP0453092B1; DE69103456T2; US5101325A; CA2034378A1; DE69103456D1; EP0453092A1

Abstract

A thin, efficient and uniform illuminator for large areas comprises a plastic wedge of a series of plastic wedges applied to or formed integrally with a back surface of an area to be illuminated. Light is collected and concentrated from a high efficiency light source. The concentrated light is focused as an input to one or more light guides. The light guides may be optical fibers. The light guides or fibers are fused or mated in some other way to an edge or edges of an optically clear plastic wedge or wedges. The wedge shape increases the angle of the internal reflections of the edge coupled light per unit distance and intercepts more of the light as the wedge gets thinner. To further enhance the uniformity of the illuminating light, light scattering centers may be added to the surface or volume of the wedge. The number of scattering centers over the illuminating surface of the wedge or within the volume of the wedge may he increased along the distance moving away from the light source.

Description

2~3~3'~

-1- LD 100~5 UNIFORM ILLUMINATION OF LARGE, THXN SURFACES
PARTICU~RLY ~Ul'l'~;V Fo~ AU~1~OIIJ~11V~; APPLICATIONS

DESCRIPTION
~ACRGP~OIJND OF THE lN V~ih ~lON
Field of the In~ention The present invention generally relates to tne 5 illumination of large surfacei and, more particularly, to providing uniform illumination over large areas at a shallow depth which is particlllarly suited for automobile~: .

Description of the Prior Art In modern industrial desiyn, there ar~ a numbQr o~
applications which call for illumina~ing large surface areas but ~he distance perpendicular to the surface area to be illuminated which is available for a light source is small. The problem then is to provide 15 uniform illumination of a thin pa.nel at an acceptable cos~ .
In one such applicz~tion, automobile manufacturers have been adding large decora~ive reflec~orized area~
on the r~ar deck lids of som~ of their models. The 20 reflectorized areas ~lend in well with the rear lightin~ on the quarter p~n~l s and have a pleasing z~ppe~rance during the day. At night, however, these areas appear clark and unattractive. To light these large areas wi.th inc~descent lamps presents two 2~13~37~

problems. First, it is difficult to yet even illumination over such a large axea using point sources such as incandescen~ lamps. Secondly, the large accelerations experienced by lamps mounted on the rear deck lid when it is closed are sufficient to deform or even fracture the filament of t~le lamp. Lighk emitting diode (T~Ds) may very well serve as an alternative light source so as to solve such incandescent lamp problems.
In another application, li~lid crystal displays (LCDs) are commonly used for a variety of applications ranging from personal televisions to computer displays. one of the principal reasons for the popularity of LCDs is their small size and low power consumption. Current illuminators for LCDs use fluorescent lamps of high efficiency and light box cavities to provide uniform illumination. To m~ke LCD~
more acceptable, the LCDs are now generally provided with a source of back lighting. In order to retain the advantage of LCDs being used as a thin flat panel display, this back lighting source must also be thin.
This type of design must be of a cert~in ~
thickness due to the lamp size and light box cavity size to achieve a uniform backlighting of the display.
Another type of illuminator which achieves uniform illumination over a large area and yet is thin is manufactured by Lumitex, Inc. Th~ Lumitex device uses a high efficiency light source and collects and concentrates this light by focusing it into an optical fiber bundle. The fibers of the bundle are ~anned out and woven into a flat panel. Light is made to leak from the woven panel by sharp b~n~in~ of the fibers in the weave pattern. The disadvantages of this device are its cost of cons~ruction and the lack of directionality of the leaked light and efficiency when designed to achieve a high degree of uniformity.

2~3~3'~8 SUMMARY OF THE lNv~:NlION
It is therefore a gen~ral objsct of the present invention to provide a device that provides uni~orm illumination over a large area of shallow depth.
It is another object of the invention to provide a thin, efficient and uniform illuminator for larg~
areas.
It is a further object of the invention to provide an ef~lcient mean~ to collect and conduct light from a high efficiency light ~ource and unifo~mly distri~ute and emit this light over a large area.
According to the invention, a plastic wedge or a series of plastic wedge~ are applied to or formed integrally with a back surface o~ an area to be illuminated. Light is collected and concentrated from a high efficiency light source. The concentrat~d light is focused as an input ~o one or more light guides which may be optical fiber~. The light guides or fibers are fused or mated in so~e other way to an edge or edges of an optically clear pla-~tic wedge or wedgesO The wedge shape increases the angle of internal reflections of the edge coupled light as the wedge gets thinner. In addition, the slopin~ back ~urface of the wedge intQrcepts more of the illuminating light bea~ as it progresses through the wedg~ toward its ape~. To further e~h~n~e the uniformity of the illuminating liqht, the number o~
scattering centers o~er the illuminating surface of the ~-wedge or within ~he volume of the wedge are increased along the distance away from the light sollrce.

BRIEF DESCRIPTION OF THE DRAWINGS
T~e ~oregoing and other objects1 aspects and advantage~ will be better unders~ood ~ro~ t~e following detailed description of a preferred e~bodim2nt of the invention with referenc~ to the drawings, in which:

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Figure 1 is an isometric drawing of the basic wedge shaped illuminating device according to khe invention;
Figure 2 is an enlarged cror,s-sectional view of the wedge shaped illuminating devicea shown in Figure 1 illustrating the internal r~flections of light within the device;
Figures 3A and 3B are enlarged portions of the ligh~ emitting surface of the w~dge ~haped illuminating device showing scattering centers on the surface;
Figure 4 is an enlarged croe;s-sectional view of the wedge shaped illuminating device showing scattering centers in the volume of the wedge;
Figure S is a top view of a double wedge embodiment of the invention;
Figure 6 is an end view of the double wedge ~mbodiment shown in Figure 5: and Figure 7 is an enlarged cross-sectional view o~ the double wedge embodiment shown in ~igureS 5 and 6 ~ . -illustrating the internal reflection~ of light within the device.

DETAILED DESCRIPTION OF THE:PRE~ERRED
EMBOD~ Nl~ OF THE lNv~N~lION
Referring now to the drawings, and more particularly to Figure 1, there is shown a basic form of the illuminating device lO according to the invention. The illuminating device includes a transparent plastic wedge 12. The wedge may be made of a moldable thermoplastic such as, for ~xample, acrylics, polycarbona~es or polys~yrenes. Along a ~ .
rectangular edge 13 of the wedge 12 are a plurality of optical fibers 14. The optical fibers 14 terminate in a common bundl~ which is positioned to collect concentrated light from a high ef~iciency source 16 which may be, ~or example, an arc lamp centrally ~ ;
located in a reflector preferably having a spherical '' ':".

2~343 ~8 shape. The coupling between the ends of the fibers 14 and the wedge 12 may be made by meltinq them together -or mated in some other way so as~ to reduce any reflection losses and which provides for the light from the fibers to spread out in a di.rection perpendicular to the direction of the bea~ so as to provide uni~orm illumination over the front or light emitting surfac~
lR. For the various applications of device 10, the front surfac~ 18 is arranged to be co-extensive with the area to be illuminated. The back surface 20, shown in more detail in Figure 2, Of the wedge iS coated With a reflecting coa~ing. Pr~ferably, this reflecting coating is a diffu~e reflector such as Barium Sul~at~, BaS04, as manufactured by KodaX o~ ~ochester, New York, for this purpose. Alternatively, the re~lecting coating may be a specular reflector, such as sputtered aluminum, but this generally does not produce as good a result as a diffuse reflector.
Figure 2 illustrate~ the reflections of light from one of the optical fibers 14 located at the rectangular edge 13 within the volume of the wedgs 12. Figure 2 illustrates the back surface 20 in a differen~ manner than it was shown in Figure 1 in ~hat back surface 20 now converges relative to the light emitting surface 18. Th~ challenge in illuminating a large area ~e~le~ented by illuminating device 18 is to make the illumination fairly uniform. To accomplish this by the practice of the present invention using edge lighting techniques, two principles are co~bined. First, the wedge 12 increases the an~le of the reflections per unit distance as the wedge gets ~h i nn~r and the back or reflecting sur:Eace 20 intercepts more of the light ~ :
beam, shown as rays 19a,...19n emitted from fiber 14 located at sur~Eace 13, as such a beam travels toward the apex of th~ wedge. Second, the number of scattering centers, related to th~ light emittin~

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surface 18, which send light o~f in all directions as shown by groups o~ ~ays l9a...19n being emi~ted from surface 18. The scattering ef~.ect increa~es in th~
dir~ction away from the source 14. The reflector or reflective coating on the back surface 20 ensures that all of the light not absorbed by the reflector is reflected by the reflector and comes out through the front surface 18. Either principle by itself provides some improvement in uniformity so that in some 0 applications, only one or the other might be used.
The scattering sourceg may be ~ormed on the front surface 1~, by scratching or ~rooving or caatln~ th~
surface 18~ where the den~ity of scattering sources i9 low near the soUrCe 14 of the light and increases as t~e distance from the source increas~s. The scattering sources are illustrated in Figures 3A and 3B which show, respectively, y~o~es 17A and pits 17B as scattering sources which preferably increase in frequency of occurrence or density over the surface as their location moves away fro~ the source of illumination 14 located a~ rectangular edge 13.
Alternatively, the scattering sources may be formed within the volume o~ the material, again with the guideline that the related density increases as the distance from the source increases. This is illustrated in Figure 4 which shows scattering particles 17c within the volume of the wedge 12. The scattered light rays are shown a~ ~Lou~S l9a...19c which are composed of individual rays l~a...l9n that are emitted ~ro~ optical fiber 14 at rectangular edge 13. These particles may be pa~ive or inert types mixed with the thermoplastic material and allowed to - :
gradually settle toward the apex end of the wedge before curing the plastic ~hereby producing the .i~
35 increasing density of such light scattering centers as :::
generally illustrated. The particle could be .. . .

3 ~ 3 ~

encapsulated liquid crystals such a~ those described in U.s. Patents No. 4,435,047 and No. 4,616,903 to F~rgason and produced by ~aliq Corp. o~ Sunnyvale, California.
Transparent electrode~ 21 and 2l' are applied over the front and back ~ur~aces 18 clnd 20, respectively, of the wedge 12 for the purpose of applying an electric field. The application of an electric field ha~ the effect of aligning the li~uid crystals parallel to the direction of the field, in contras~ to its normally structurally distor~ed shape in the absence of a field. When an electric field is pre~ent, thQ liquid crystals become more tran~parent, a~ their transparency is a function of the s~rength of the ~lectric field.
If the liquid crystals are nonuniformly distributed through the volume of the wedge, the applica~ion o~ a uniform electric field across the volume controls the light ~cattering effect desired. On the other hand, the use of encapsulated liquid crystals allows for some flexibility of manufacture. Specifically, the encapsulated liquid cry~tals may be uniformly distributed within the volume o~ the wedge 12 and a nonuniform electric field applied across the wedge to produce the effect of an increasing density of light scattering centers. In other words, by appropria ~ly selecting the electric field applied across the envelope, the illumina~ing light emitted from the front surface 18 of the wedg~ 12 may be mad~ more or less uniform as desired by the application.
3a When the illuminating device according to the invention is used to illumin2te, for example, an applique (cutout decoration fastened to a larger pie~e of material) on the rear deck of a car, the applique may be formed by molding plastic such that the back 35 surface farms a series of wed~e~ relative to the front i .
:surfa~e and such that each wedge may be illumin~ed by ~ .

~ 2~3~3 3'~

means of fiber optics at the thick end or edge of the wedge. The back surface ls roughened and coated with the previously discussed diffuse reflector so that all of the light not absorbed by the back surface is reflected by the back sur~ace and comes through the front surface 18 of the applique. The purpose of the wedge shape is ~o provide illumination throuqh the front surface 18 which iS as uniform as po~sible. If it is desired that the appliquz be reflective, it may be made in two layers, the oute~ layer o~ which is provided with the usual corner cube reflectiv~ surface on the back, while the second layer has the wedge construction as generally shown in Figure 1.
For very large areas, such as the back of a car, it may bQ nece~sary to have a serie~ of illuminators, each fed by it~ own sourc~, for examplQ the end o~ an optical fiber. As mentioned and as shown in Figure 1 with regard to re~erence number 16, the source o~ light into the fibers may be a ~isc~rge lamp light source centrally located With ~he reflector or a similar high efficiency light source. Unlike the incandescent lamp discu~s~d in the "Background" section, such discharge lamp may be mounted on the deck lid without fear o~
high acceleration, since there is no ~ilament to fail.
It will be appreciated by those s~illed in the art that th~re are several variables that must be considered in the practice o~ the invention. The first of these i5 th~ nature of the light beam introduced into the plastic wedge 120 Generally, it is preferred that the light bea~ be colli~ated, or nearly so, to achieve the best uniformity of illuminating light e~itted from the front surfac~ 18 of the wedge. ':
Secon~ly, the thickne~ and ~he angle of the wedge should be determined for the specific application. And finally, when us~d, a decision must be ~ade on the location and distribution of light scattering centers, ~"' 2 ~ 7 ~3 either over the emitting surface 18 or within the volume of the wedge 12.
A practical example of ~he in~ention is shown in Figures 5 and 6 which illustratle a double wedge illuminating device 22 fabricatled to illuminate an automobile speed___t~r. The ~p~ee~ er itself Was ~abricated using LCD tech~ology, and the double wedge structure was used to backlight the LCD. In the embodiment shown in Figures 5 and 6, light is coll~cted and concentrated from a high ~f~iciency light source (not shown) as before. The concen~rated light is focused as an input to a pair of light guides 24 which transmit light, shown in Figure 5 as rays 25a...25n, into edges 28 and 30 of the double wedge 26. The edge 28 is comprised of portion~ 2~A, 28B, 28C and 2~D, whereas, edge 30 is comprised of portion~ 30A, 30B, 30C
and 30D. The double wedge shape is be~t seien in Figure 6 in which the two wedge portions 26 are joined a~ a common edge 33. The illumin~tor 2~ having the back (32J and front (34) surfaces of each wedge 26 converge and the front surfaces 34 of the two wedges 26 forming a common illuminating surface con~oxming to a surface that is to be illuminated. A~ shown in Figure 5, th@
eYr~n~ing beams of ligh~ 25a...25n are intercept~d by serrated edge portions 28A...30D which reflect the light rays toward the ~hinn~r central part of the double wedge 26. The sur~ace~ of the serrated edges 28 and 30 are coated with a specular re~lector, such as : sputtered aluminum. In the illustrated emb~ nt, the serrations are designed such that the light beams 25a...25n are divided into seven parts o~ roughly equal lumens. The seven distributed light beams of the serrated edges 28 and 30 are reflected by the back surface 32 (Figure 53 of the double wedge and are .
e~itted from the front surface 3~ (Figure 6) i~ the same manner as the generalized struc~ure shown in :.:

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-lO- LD 10065 Figure 1.
Figure 7 shows in more detai.l the back surface 32 of the double wedge 26 shown in Figure 6. The back surface may be grooved by means o~ grooves 17A, disc~ eA with regards to Figure! 3A, perpendicular to the direction from which the light (shown by rays 25a and 25n) is propa~ated fro~ 50 a~ to form a surface that is stepped with 45' risers 36. The risers 36 intercept light rays 25a and 25n and redirec~ the light through the front surface 34 into groups o~ light rays 27. An alternative to groovinq (17A) the back surface 32 is to texture the front surface 34 by simple rough s~ing. The purpose o~ the s~di~ is to defeat total internal reflection and scatter the light striking this sur~ace thereby allowing the light to escape. Except for the desired illuminating surface 34, the entire plastic form of wedge 22 is coated with a diffuse re~lective coatinq 38 to assure that any liyht which is not totally internally reflected is returned to the plastic cavity of wedge 22 and contributes ~o the output of wedge 22.
While the embodiment shown in Figure~ 5 and 6 is particularly advantageou~ ~or use as a back light source for LCDs, such as ~he automobile speedometer mentioned, this particular embodiment of the invention may ba used wherever uniform illumination of a large .
surface area is desired. Therefore, while the invantion ha~ been described in terms of pre~erred :
~ ~o~iments, those skilled in the art will recogniZe that the invention can be practiced with modification within the spixit and scope o~ the app~n~eA claims.

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Claims

CLAIMS:

1. An illuminator for large surface areas comprising:
a transparent wedge having back and front surfaces and a generally rectangular shaped edge between said surfaces, said front surface being co-extensive with an area to be illuminated;
a high efficiency light source;
at least one light guide optically coupling light from said light source to said rectangular shaped edge so that light entering the wedge is internally reflected from the back surface and emitted from said front surface; and, wherein said wedge is provided with scattering centers throughout its volume and said scattering centers are encapsulated liquid crystals and further comprising means for applying an electric field across said front and back surfaces of said wedge.

2. The illuminator according to claim 1, wherein said wedge is of a plastic material.

3. The illuminator as recited in claim 2, further comprising a reflective coating applied to said back surface.

4. The illuminator as recited in claim 3, wherein said reflective coating is of a diffuse type.

5. The illuminator as recited in claim 2, wherein said plastic is a thermoplastic and said light guide is fused with said wedge so as to reduce any reflection losses and cause light from said light guide to spread out within said plastic wedge.

6. The illuminator as recited in claim 5, wherein said front surface is integrally formed with the surface area to be illuminated.

7. An illuminator for large surface areas comprising:
a transparent wedge having back and front surfaces and a generally rectangularly shaped edge between said surfaces, said front surface being co-extensive with an area to be illuminated;
a high efficiency light source;
at least one light guide optically coupling light from said light source to said rectangular shaped edge so that light entering the wedge is internally reflected from the back surface and emitted from said front surface;
wherein said transparent wedge is provided with scattering centers throughout its volume; and wherein the number of scattering surfaces increases along a distance moving away from said rectangular shaped edge.

8. The illuminator as recited in claim 7, wherein said at least one light guide is coupled to said wedge immediately adjacent and parallel to said rectangular edge and said rectangular edge is serrated to reflect light from said light guide toward said back and front surfaces.

9. The illuminator as recited in claim 8, further comprising a reflective coating applied to said back surface.

10. The illuminator as recited in claim 9, wherein said reflective coating is of a diffuse type.

11. The illuminator as recited in claim 7, wherein said scattering centers are grooves formed in said volume perpendicular to a direction of propagation of the light within said wedge and having a frequency of occurrence which increases away from said rectangular shaped edge.

12. The illuminator as recited in claim 7, wherein said scattering centers are pits formed in said wedge and have a density which increases away from said rectangular shaped edge.

13. The illuminator as recited in claim 7, wherein said back surface is grooved to form stepped risers perpendicular to the direction of light propagating within said wedge whereby light is reflected from said stepped risers toward said front surface.

14. The illuminator as recited in claim 13, further comprising a reflective coating applied to said back surface.

15. An illuminator for large surface areas comprising:
a plurality of transparent wedges having respective front and back surfaces, at least one of respective front and back surfaces converging at a generally rectangular shaped converging edge, said front surfaces of said plurality of said transparent wedges being coextensive with an area to be illuminated;
a high efficiency light source;
at least one light guide optically coupling light from said light source to an edge on each of said plurality of transparent wedges other than said converging edge, said at least one light guide being effective so that light entering each of said transparent wedges is internally reflected from said respective back surfaces and emitted from said respective front surfaces; and wherein said plurality of transparent wedges are provided with scattering centers throughout their volumes, the number of said scattering centers increasing towards said converging edge.

16. The illuminator as recited in claim 15, wherein the number of wedges is at least two and said converging edge occurs along a common edge where said back and front surfaces of each wedge converge and the front surfaces of said two wedges form a common illuminating surface conforming to a surface area to be illuminated.

17. The illuminator as recited in claim 16 wherein there are at least two light guides, one for each of said two wedges, and each light guide is coupled to a corresponding one of said plastic wedges immediately adjacent and parallel to the rectangular edge of that wedge and said rectangular edge is serrated to reflect light from said light guide toward said converging back and front surfaces.

18. The illuminator as recited in claim 17 further comprising a reflective coating applied to said back surfaces.

19. The illuminator as recited in claim 17 wherein said reflective coating is of a diffuse type.