Uniform Illumination With Edge Lighting

Abstract

Uniform illumination is provided from an edge lighted optical medium of a given thickness by diffusing a surface portion normally providing internal reflection of the light radiated into the edge such that at least some of the light rays striking the diffused area are reflected through the opposite surface of the medium and some are refracted through the one surface. A cleared area about the source of light free of the diffusion is provided and treated by application of coatings on opposite surface portions of the medium defining the cleared area. These coatings have an index of refraction less than that of the medium such that light is channeled by internal reflection between the surfaces of the medium and inhibited from escaping from the medium in the vicinity of the cleared area. This cleared area has a radius of preferably three to four times the thickness of the optical medium. The resulting light emmination from the surface of the medium is substantially uniform beyond the cleared area.

Description

This invention relates generally to lighting technique and more particularly to an improved system for providing substantially uniform illumination from edge lighted optical media.

BACKGROUND OF THE INVENTION

There are many instances in which it is desired to provide a planar surface emitting substantially uniform illumination; for example, x-ray viewers, light tables for facilitating tracing operations, building business directories in which stencils or cut outs are placed in front of the light emitting surface, edge lighted panels having an opaque surface with etched nomenclature at various points on the surface to indicate control functions such as used on aircraft, and so forth. In most of these applications, it is desirable to provide a relatively large surface area for emitting the light with a minimum depth for the optical system and emitting structure.

In the case of x-ray viewers, light tables, and the like, present practice in a great many cases is simply to provide a multiplicity of light sources beneath a diffused translucent type surface. While such systems are effective for most purposes, there still is not absolute uniformity simply because of the presence of individually separated light sources in spite of the smoothing effect of the diffuse optical covering. Further, a relatively deep depth dimension is required to accommodate the optical sources with the result that the final product is relatively bulky. In the case of edge lighted panels, one or more light sources are disposed adjacent edge portions of the panel and the light radiated into the panel. In this instance, the panel itself constitutes an optical medium which serves as a light pipe trapping the illumination between its opposite parallel surfaces. Because of imperfections in the optical medium itself, some light escapes from the flat surfaces. However, most of the light is simply trapped within the optical medium and is radiated from the remaining edges. As a result, many light sources are required and still the illumination from the surface is fairly poor compared to the light energy provided.

The primary advantage of edge lighted panels is the minimization of the depth dimension while sitll providing illumination from a relatively large flat surface. As a result, edge lighting techniques for providing flat surfaces of illumination find wide application in those instances in which bulk must be kept to a minimum; for example, aircraft panels. It would be desirable, however, to be able to take advantage of the small depth dimensions realizable by edge lighting technique for all applications in which uniform illumination from a flat surface is desired.

In the particular field of edge lighted panels and the like, efforts have been made to increase the illumination from the flat surfaces of the optical medium so that etched nomenclature and the like will be more clearly visible without increasing unduly the number of light sources. One such technique contemplates providing specular surfaces; that is, perfectly smooth mirrored surfaces about the edges of the panel and the back surface of the panel to retain light in the panel and prevent waste by escaping light rays from the edges and rear surface. While such reflecting techniques generally increase the intensity of light passing from the one surface, the major portion of light is still trapped within the optical medium simply because of the internal reflection of the rays occuring as a result of the relatively thin nature of the panel and the fact that the light is radiated into an edge. Further techniques have been proposed such as diffusing a surface of the optical medium or panel to cause irregular reflections within the medium whereby many of the light rays strike the opposing surface at less than the critical angle so that they are refracted out of the medium.

While improvements have resulted from the foregoing techniques, the resulting illumination from the surface is still not uniform; that is, it tends to fall off with increasing distance from the light source measured in a plane parallel to the plane of the panel or optical medium itself.

BRIEF DESCRIPTION OF THE PRESENT INVENTION

A primary object of the present invention is to provide a technique for further improving the emission of substantially uniform light from a large surface wherein the advantages of edge lighting can be utilized and yet wasted light is kept at an absolute minimum. More particularly, an object is to provide uniform illumination from an edge lighted optical medium in which the light escaping from the medium is substantially uniform with increasing distance from the lighted edge as measured in a plane parallel to the plane of the medium all to the end that the applications for edge lighted techniques are greatly expanded.

Briefly, the improvement of the present invention contemplates providing uniform illumination from an edge lighted optical medium by diffusing one surface portion normally providing internal reflection of the light such that at least some of the light rays striking the diffused surface are reflected through the opposite surface and some are refracted through the one surface to escape out of the medium. An important feature of the invention then contemplates providing a cleared area about the source of light free of the diffusion but treated in a manner such that light is channeled by internal reflection between the surfaces and inhibited from escaping from the medium in the vicinity of the cleared area. The treatment of the surfaces defining the cleared area comprises the provision of a coating of material having an index of refraction less than that of the medium thereby assuring internal reflection and proper channeling of the light rays. The channeled light rays expand towards the diffused surface portion so that the light escaping from the medium as a consequence of the diffused surface portion is substantially uniform with increasing distance from the lighted edge as measured in a plane parallel to the plane of the medium.

By adding mirrored surfaces to the edges and beneath the diffused portions, the intensity of light from the surface opposite the one surface is increased with the desired uniformity still being retained.

The provision of a cleared area about the source is of the utmost importance and in accord with the invention is defined by a radius of from three to four times the thickness of the panel although for certain applications in certain types of optical media having abnormal indexes of refraction the radius of the cleared area may vary as much as from one to ten times the thickness of the medium itself.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the invention will be had by now referring to the accompanying drawings, in which:

FIG. 1 is a perspective view, partly diagramatic in form, of an optical medium in the form of a panel with an edge lighting source useful in explaining certain problems encountered in the prior art;

FIG. 2 is an enlarged fragmentary cross-section taken in the direction of the arrows 2--2 of FIG. 1 showing the action of light rays in the medium;

FIG. 3 is a view similar to FIG. 1 of an edge lighted optical medium wherein improved lighting is realized by diffusion techniques;

FIG. 4 is an enlarged fragmentary corss-section of a portion of the panel of FIG. 3 showing the action of light rays in the panel;

FIG. 5 is another perspective view showing an edge lighted optical medium in accord with the present invention;

FIG. 6 is a fragmentary cross-section of the panel of FIG. 5 useful in explaining the operation of the invention;

FIG. 7 is a fragmentary perspective view of an edge lighted optical medium wherein the light source is completely surrounded by the medium;

FIG. 8 is a perspective view of a specific embodiment of the invention; and

FIG. 9 is a fragmentary corss-section taken in the direction of the arrows 9--9 of FIG. 8.

DETAILED DESCRIPTION OF THE INVENTION:

Referring first to FIG. 1 there is shown an optical medium 10 in the form of a flat panel of light conducting material which may constitute glass, or any number of different types of plastic acrylics normally having a refractive index of from 1.42 to 2.00. The optical medium is defined by first and second optically smooth flat surfaces 11 and 12. Adjacent an edge portion of the medium 13 is a light source 14 connected to a battery 15 or equivalent energizing means.

Light rays from the source 14 radiate into the edge of the optical medium 10 and by internal reflection will normally all pass out the surrounding edges such as indicated by the arrows 16 at the far edge. Very little light passes through the flat surfaces 11 and 12 and any light that does escape is a consequence of imperfections in the optical medium itself.

Qualitatively shown above the panel 10 in FIG. 1 is a graph illustrating at 17 the relative illumination intensity with increasing distance from the light source as measured in a plane parallel to the plane of the optical medium. As is evident, a substantial amount of light will pass perpendicularly from the panel in the immediate vicinity of the light itself simply by direct radiation from the source. Thereafter, there is little if any illumination from the surface 12 of the panel.

Referring now to FIG. 2, the reason for lack of substantial illumination from the panel surfaces will be evident. As indicated by the ray lines 18, 19, 20, and 21, light radiated from the source 14 is completely internally reflected by the opposite first and second surfaces 11 and 12. In instances where the edge of the panel forms a right angle with the opposite surfaces of the panel and the refractive index of the panel material is greater than 1.42, it is physically impossible for any light to be refracted out of the panel. The relationship is defined by Snell's law which states:

sin i/sin r = N2/N1

where i is the incident angle of the light ray from the source 14 into the edge of the panel, r is the angle of refraction in the medium, n1 is the index of refraction of air which is substantially unity, and n2 is the index of refraction of the optical medium 10 all as shown in FIG. 2. It will be immediately appreciated that when the critical angle is 45.degree. or less no light rays can escape from the opposite surfaces 11 and 12 when the light source radiates into the edge of the panel.

As stated heretofore, some light as a practical matter does escape from the opposite surfaces and thus edge lighted optical media such as panels used for aircraft have been useful, the required illumination levels being achieved by providing a high density of individual light sources adjacent to the edges of the panels and/or embedded in bores through the panel throughout its area. However, this solution is unsatisfactory simply because of increased bulk and expense and the high probability factor of light bulb failure.

Referring now to FIG. 3, there is shown another optical medium in the form of a panel 23 having flat opposite surfaces 24 and 25 which may be similar to the optical medium or panel 10 of FIG. 1. However, in this instance, the one surface 24 is completely diffused as by sandblasting. In addition, a highly reflective surface or mirror 26 is positioned beneath the diffused surface 24 and also highly reflective or specular surfaces or coatings are provided about the edges such as indicated at 27.

With the arrangement illustrated in FIG. 3, a substantial increase in illumination from the opposite or second surface 25 is realizable, the mirrors 26 and 27 retaining light within the panel and the diffused surface portion 24 causing irregular internal reflections so that light can escape through the second surface. In FIG. 3, the escaping light is indicated by the arrows 28 and the relative illumination with increasing distance from the source is indicated by the graph 29 wherein the improvement over the former graph reproduced in dotted lines at 17' in the case of FIG. 1 is readily evident.

The reflection of light rays from the panel of FIG. 3 can be seen by referring to FIG. 4 wherein the irregular or diffuse surface 24 results in reflection of some of the internally reflected rays such as indicated by the lines 30, 31, 32, and 33 in a direction perpendicular to the second surface 25 as indicated by the arrows 28. LIght rays passing directly into the medium from the edge and thus traveling parallel to the opposite surfaces as indicated by the ray 34 will simply be reflected back into the medium by the end mirror 27 described in FIG. 3.

The diffuse surface 24 not only reflects portions of the light rays upwardly to escape from the second surface of the medium but also refract some of these light rays out of the bottom surface or one surface of the medium. These refracted rays are simply reflected back into the panel by the bottom mirror 26 so that substantially all of the light radiated into the edge of the panel is eventually passed out of the second surface 25. However, there still results a non-uniformity in the illumination intensity as evidenced by the falling off of the illumination with increasing distance from the source.

Referring now to FIG. 5, there is shown a preferred embodiment of the present invention wherein an optical medium in the form of a panel 35 has first and second surfaces 36, and 37, a major portion of the surface 36 being diffused as indicated. As in the case of the panel of FIG. 3, preferably a specular reflecting surface 38 is provided beneath the diffused one surface 36 or may simply be incorporated as a part of the diffused surface by sandblasting the mirror itself. An end mirror, such as indicated at 39, is provided at the end edge and also at the other edges with the exception of the edge portion receiving light from the source 14.

In the embodiment of FIG. 5, as opposed to that of FIG. 3, there is provided a cleared area 40 about the light source 14; that is, a portion of the one surface 36 in which there is no diffusion but rather an optically smooth surface. This cleared area has a circular end boundary defined at least in part by a circle with the light source 14 at the center of the circle. With this arrangement, a substantially uniform illumination is provided beyond the end boundary of the cleared area from the upper surface 37 of the panel as indicated by the arrows 41. This uniform illumination is depicted in the graph above the panel by the straight line 42. Comparison with the reproduced curve 29 of FIG. 3 shown at 29' and the reproduced curve 17 of FIG. 1 shown at 17' graphically illustrates the improvement provided by the cleared area. In a sense, the cleared area holds light in the panel which would otherwise escape in the vicinity of the source. This light is then utilized to render uniform the remaining portion of the illumination curve. Qualitatively speaking, the shaded portion 43 in the graph of FIG. 5 constitutes a quantity of light which effectively is distributed over the remaining distance from the light source to result in the uniform illumination shown by the curve 42.

Referring to FIG. 6, the manner in which the foregoing improvement is achieved will be understood. As shown, a given sector of light rays from the source 14 passing into the edge of the panel will be initially internally reflected from the upper surface 37 as indicated by the rays 44. The interception of the edge rays defining the ray sector with the upper surface is indicated at 45. The internally reflected rays 44 then impinge on the cleared area 40 and are again internally reflected since this cleared area is free of any of the diffusion. Because of the expansion of the rays with increasing distance of travel, the interception of the internally reflected rays 44 occupy a distance indicated at 46 on the lower surface of the panel. These internally reflected rays will then pass to the upper surface 37 intercepting over a distance indicated by the line 47. The rays will then be again internally reflected onto the initial portion of the diffused surface 36 intercepting an ever widening area as indicated at 48. The expanding light rays continue as indicated by the even wider interception area 49 but because of the diffused portion, irregular reflections and refractions occur so that the light rays can now escape out of the top of the panel as indicated by the arrows 41.

Essentially, the cleared area results in an initial channeling of the light rays from the source to an extent that the same may expand towards the diffused surface portion so that the light escaping from the medium as a consequence of the diffused surface portion is substantially uniform with increasing distance from the lighted edge as measured in the plane parallel to the plane of the medium. Thus, light that would have initially escaped in the vicinity of the cleared area in the absence of such cleared area is utilized to render uniform the remaining illumination above the diffused area from the optical medium. The shaded portion 43 of the graph in FIG. 5 is meant to represent the quantity of light that might otherwise be lost in the vicinity of the light source if the cleared area were not provided.

The extent of the cleared area from the source is important. While some variation is possible, for a given panel thickness and a given index of refraction, the preferred distance of the cleared area about the source as measured by the radius R of the circle defined in part by the end boundary of cleared area is between 3T and 4T where T is the thickness of the panel. This dimensioning is ideal for a panel or optical medium of index of refraction of 1.49. It should be understood, however, that the dimension of the cleared area as indicated by the radius R could vary in certain circumstances from a value equal to the thickness of the panel to a value of 10 times the thickness of the panel. Providing a cleared area beyond these outer limits would be impractical and would substantially decrease the area of uniform illumination relative to the entire area of the panel.

FIG. 7 illustrates a portion of a panel 50 provided with a diffused surface portion 51 with a cleared area 52 free of any diffusion about a light source 53 disposed within a bore 54 formed in the panel. The purpose of the showing in FIG. 7 is simply to illustrate the fact that the light source may be disposed essentially within the panel and still radiate into an edge of the panel, the interior wall of bore 54 constituting the edge in question. In the structure of FIG. 7 the cleared area takes the form of a complete circle with radius R and again the relationship between R and the thickness of the panel T would be within the limits defined for the structure of FIGS. 5 and 6.

FIG. 8 illustrates a specific embodiment incorporating the principles of the present invention in the form of an edge lighted panel as might be used for aircraft instruments. The optical medium making up the panel is designated at 55 and may constitute an acrylic. Various lights are embedded in the acrylic as indicated at 56 and 57, there being provided cleared areas in accord with the teachings of FIG. 7. Nomenclature such as indicated at 58, 59 and 60 may be provided on the second surface of the optical medium; that is, the top surface. By utilizing the principles of the present invention, the number of individual lights such as 56 and 57 required to uniformly illuminate the nomenclature may be vastly reduced over those heretofore required.

The actual physical make-up of the panel of FIG. 8 will be understood by now referring to FIG. 9. In FIG. 9, the thicknesses of various coatings provided on the acrylic panel are greatly exaggerated for purposes of clarity. Thus there is shown the basic acrylic panel material 61 provided with a coating 62 of material having an index of refraction less than that of the panel 61. In the particular embodiment shown, this coating covers the entire second or top surface of the panel. Overlying this material is a coating of acrylic 63 which is of the same material as the acrylic 61 making up the bulk of the panel. This coating also covers the entire second or top surface of the panel. On top of the acrylic coating 63 there is then provided a vinyl coating 64 which is white and translucent. Finally there is provided a black or opaque vinyl coating 65 overlying the white vinyl 64.

Referring to the first surface of the panel 61 or bottom surface, it will be noted that there is defined a cleared area indicated by the radius R about the light source 56. On the bottom surface of this cleared area there is provided a coating 62 of material having an index of refraction less than that of the optical medium or acrylic 61. This coating 62 is the same material as the coating 62 on the top surface and is co-extensive with the cleared area defined by the radius R. Over the coating of material 62 there is then provided an acrylic coating 63 of the same material as the coating 63 on the top surface. This acrylic coating 63 extends beyond the radius R of the cleared area by a first given distance designated D1. Overcoating this acrylic is an opaque or black vinyl coating 66 which terminates short of the cleared area by a second given distance D2. The remaining first or bottom surface of the panel is then coated with a white vinyl 64 which serves to define the diffused surface area and over this white vinyl coating is a black or opaque vinyl coating 65. The white vinyl coating covers the entire bottom surface and the black or opaque vinyl coating 65 overcoats the entire bottom surface area. The white vinyl 64 and black vinyl 65 correspond to the white and black or opaque vinyl 64 and 65 on the top surface and thus are designated by the same numerals.

As shown in FIG. 9, the opaque or black vinyl coating 65 on the top surface has certain portions removed to define the desired nomenclature 59. Light can thus escape through these portions of the top surface as indicated by the arrows 67.

In the provision of edge lighted panels, it is conventional to provide a white translucent vinyl coating on the panel itself followed by a black or opaque vinyl coating which is etched away to define the nomenclature so that light can escape through the etched or removed portions. However, it would not be possible to take advantage of the principles of the present invention if these vinyl coats were applied directly to the acrylic surfaces. This is because the desired internal reflection of the light rays in the cleared area will not take place in the absence of an air interface or an interface at the cleared area with a material having an index of refraction less than that of the acrylic material of the panel. Since there is no interface because of the application of the vinyl coatings, it is necessary to provide the coatings 62 described in FIG. 9 which provide a material of index of refraction less than that of the acrylic about the cleared area. Internal reflection is thus assured in the cleared area by the provision of this particular material. The addition of the acrylic coatings 63 overlying the material layer 62 is merely provided to enable a vinyl such as a vinyl paint to adhere or bond. Thus, while the material coating 62 of index of refraction less than that of the acrylic can be bonded to the acrylic, in those instances in which it is not possible to bond the vinyl directly to this material, the layers or coatings 63 provide a base for the vinyl coatings. If a material 62 were selected which was heat bondable to both acrylic and vinyl, then the coatings 63 would not be required.

It is preferable to extend the acrylic coating overlying the material 62 slightly beyond the cleared area radius as indicated by the distance D1. The provision of the opaque vinyl 66 on the first or bottom surface of the cleared area terminating short of the radius of the cleared area provides a black spot which has been found to eliminate halo effects from the light source 66.

As already mentioned above, the white vinyl serves as a diffusing coating in place of sandblasting and it will be noted in FIG. 9 that this diffusion coating covers the entire bottom surface. The final bottom black or opaque vinyl coating 65 blocks any light from escaping from the bottom of the panel. The same coating 65 on the top surface, of course, also blocks light from escaping from the top surface except at those places where the vinyl has been removed to define the nomenclature.

The various rays from the light source 56 will thus be channeled in the clear area by internal reflection and will then provide a desired uniform illumination throughout the remaining surface portion of the panel all as described in conjunction with FIGS. 5 and 6.

In the actual embodiment of the panel described in FIGS. 8 and 9, the radius R of the cleared area would preferably lie between 3T and 4T. The first given distance D1 would preferably be between .25R to .75R and the second given distance would be from .1R to .5R.

From the foregoing description, it will be evident that the present invention has provided a marked improvement in the provision of uniform illumination from an edge lighted optical medium with the desirable result that problems heretofore encountered are overcome. The resulting arrangement permits the use of edge lighted optical mediums for x-ray viewing, light tables, business directories, and other applications wherein sufficient uniform illumination was not heretofore available to permit such use. Further, by utilizing the techniques of the present invention in edge lighted panels for aircraft, the number of light sources can be substantially reduced and the illumination of nomenclature etched on an opaque covering on the top surface will be uniform over varied distances from the light source.

While specific examples of uses of the invention have been set forth, for purposes of illustration, it should be understood that the techniques involved are applicable to a wide variety of lighting uses. The invention, accordingly, is not to be thought of as limited to the specific examples set forth.

Claims

What is claimed is:

1. A method of providing substantially uniform illumination from an edge lighted optical medium of thickness T, comprising the steps of:

a. diffusing one surface portion normally providing internal reflection of the light such that at least some of the light rays striking the diffused surface are reflected through the opposite surface and some are refracted through the one surface to escape out of the medium;

b. providing a cleared area about the source of light free of the diffusion; and,

c. treating the cleared area by providing an interface for the surfaces of the optical medium defining the cleared area, said interface having an index of refraction less than that of the medium to assure internal reflection of light rays in the cleared area such that light is channeled by internal reflection between the surfaces and inhibited from escaping from the medium in the vicinity of the cleared area, the channeled light rays expanding towards the diffused surface portion so that the light escaping from the medium as a consequence of the diffused surface portion is substantially uniform with increasing distance from the lighted edge as measured in a plane parallel to the plane of the medium.

2. The method of claim 1 including the step of providing a highly specular surface under the diffused portion of the optical medium to reflect light refracted through the one surface back into the medium so that the level of the uniform illumination from the opposite surface is substantially increased.

3. The method of claim 2 including the additional step of providing a highly specular surface about the edges of the optical medium except the edge portion admitting light to block light from escaping from the edges.

4. The method of claim 1, in which said cleared area has a circular end boundary defined at least in part by a circle with the source of light at the center of the circle, said circle having a radius which lies between T and 10T.

5. The method of claim 4, in which said radius is from three to four times the thickness T of the optical medium.

6. The method of claim 1, in which said interface is in the form of a coating of material having an index of refraction less than that of the medium.

7. A means for providing substantially uniform illumination over a large surface area comprising:

a. an optical medium having opposite surfaces defining a first surface and a second surface parallel to each other and separated by a distance T defining the thickness of said medium; and,

b. at least one light source at an edge of said medium for radiating light into the edge of the medium between said surfaces;

c. said first surface having a diffused surface portion over a large surface area for reflecting and refracting internally reflected light radiated between said surfaces from said source out of said medium;

d. said first surface having a cleared area about said source free of diffusion, the periphery of said cleared area being at a distance from said source greater than T and less than 10T; and,

e. a coating of material having an index of refraction less than that of said medium on the surface portions of said medium defining said cleared area to thereby assure internal reflection of light rays in said cleared area.

8. The subject matter of claim 7, in which said diffused surface portion includes total reflecting means for reflecting back into said medium any refracted light otherwise escaping out of said medium through said first surface so that light is directed out of said second surface.

9. The subject matter of claim 7, including total reflecting means about the edges of said medium except for the edge portion into which light is radiated from said source.

10. The subject matter of claim 7, in which said cleared area has a circular end boundary defined at least in part by a circle with the light source at the center of the circle, the radius of the circle lying between 3T and 4T and in which said optical medium has a refractive index of between 1.42 and 2.00.

11. The subject matter of claim 7, in which said medium comprises an acrylic panel, said coating of material being co-extensive with said cleared area on said first surface, said material being transparent and covering the entire second surface of said acrylic panel; a coating of acrylic overlying said coating of material on said entire second surface; a coating of translucent white vinyl overlying said coating of acrylic on said second surface; and a coating of opaque vinyl overlying said coating of white vinyl, portions of said opaque vinyl being removed on said second surface to define nomenclature, the said first surface having a coating of acrylic overlying said coating of material on said cleared area and extending beyond said cleared area a first given distance; a coating of opaque vinyl overlying a portion of said coating of acrylic on said cleared area and terminating short of said cleared area by a second given distance, said diffused surface being provided by a coating of white vinyl on said first surface outside said cleared area; and a coating of opaque vinyl overlying the entire first surface whereby an edge lighted panel is provided in which light emitted through the openings defined by said nomenclature is substantially uniform.

12. The subject matter of claim 11, in which said cleared surface has a circular end boundary defined at least in part by a circle with said light source at the center of the circle, the circle having a radius R between 3T and 4T said first given distance being from 0.25R to 0.75R and said second given distance being from 0.1R to 0.5R.