Method and apparatus for controlled emission of light from prism light guide

Method and apparatus for reflecting light constrained to travel within a prism light guide such that the reflected light is refracted by and escapes through a selected portion of the light guide wall at a selected angular orientation with respect to the guide.

TECHNICAL FIELD 
This application pertains to a method and apparatus for controlling the 
angular orientation of light reflected from a selected region within a 
Prism Light Guide as described and illustrated in U.S. Pat. No. 4,260,220 
issued Apr. 7, 1981 for an invention of Lorne A. Whitehead. 
BACKGROUND ART 
The aforementioned U.S. patent describes and illustrates a Prism Light 
Guide for transmitting or "piping" light from a central source to a remote 
location or locations. As used herein the term "light guide" means a light 
guide as described and claimed in the aforementioned U.S. patent. More 
particularly, the term "light guide" as used herein means a hollow 
structure comprised of sections having substantially planar inner and 
outer surfaces which are in "octature" in that each section satisfies all 
of the following conditions: 
1. All of the inner surfaces of each section are either perpendicular or 
parallel to one another; 
2. All of the outer surfaces of each section are either perpendicular or 
parallel to one another; and, 
3. Each of the inner surfaces is at a 45.degree. angle to each of the outer 
surfaces. 
As explained in the aforementioned patent, light may be constrained to 
travel through such a light guide without escaping through the walls of 
the guide if the angle by which the light rays deviate from the 
longitudinal axis vector of the guide does not exceed a maximum angle 
which depends upon the refractive index of the light guide material and 
which can be shown to be 27.6.degree. for light guide material such as 
acrylic plastic having a refractive index of 1.5. 
In many applications it is desirable that light escape from the light guide 
at selected regions along the guide, rather than simply being directed to 
the end of the guide or being allowed to escape continually along the 
length of a given surface of the guide. It is also desirable that light 
escape from the guide at a selected angular orientation with respect to 
the guide so as to achieve the optimum coefficient of utilization with 
minimum glare for interior lighting applications. In many cases it is 
particularly desirable that light escape from the guide at an angle of 
90.degree. with respect to the internal planar surface of the guide 
section through which the escaping light is refracted. It is also 
desirable that the aforementioned objectives be attained with minimal 
attenuation or absorption of light by whatever means may be used to 
facilitate the escape of light from the guide at a particular selected 
region or regions so that light which does not escape from the guide at a 
particular region continues to travel along the guide to provide 
illumination when its escape from the guide is facilitated by further 
means located at another region or regions along the guide. 
One way of achieving the foregoing objectives is to locate a light 
reflecting element such as a mirror in one of the light guide walls or 
within the hollow space inside the light guide. Such elements could be 
oriented to reflect a portion of the light within the guide (presumed to 
be formed of a material having a refractive index of 1.5) at an angle 
(measured between a vector which characterizes the light path and the 
longitudinal axis vector of the guide) in excess of 27.6.degree., thereby 
allowing the reflected light portion to escape by refraction through the 
wall of the guide as explained in the aforementioned patent. However, 
there are several disadvantages to this approach. First, since the 
reflected light may escape through any of the four light guide walls an 
external cover must be provided around those light guide wall sections 
through which it is desired to prevent light escapement. The cover 
reflects light which escapes through the light guide walls adjacent the 
cover back through the light guide walls for eventual direction of the 
light to the uncovered light guide wall section through which it may 
escape so as to provide illumination along the uncovered section of the 
light guide surface. A filter may be provided over the uncovered light 
guide section through which light is allowed to escape to give the 
escaping light a desired angular orientation with respect to the light 
guide surface through which it escapes. 
However, the foregoing technique has some significant shortcomings. In 
practice, it is rarely possible to avoid absorption of less than about 25% 
of the incident light by the light reflecting element and/or reflective 
light guide cover. Furthermore, although filtration as aforesaid 
facilitates some control of the angular orientation of light escaping from 
the light guide, the extent of such control is quite limited. As 
previously indicated it is often desirable that light escaping from the 
light guide be oriented in a relatively narrow range of angles which are 
approximately perpendicular to the internal (planar) surface of the light 
guide section through which the escaping light is refracted, which is not 
possible with state of the art filtration techniques and materials. 
The present invention facilitates precise control of the angular 
orientation, relative to the light guide, of light reflected from a 
selected region inside a light guide and refracted by the guide to a 
region outside the guide. This is accomplished with the aid of light 
reflecting elements which are precisely located within the light guide so 
as to (1) enable a certain fraction of the light to pass unattenuated for 
processing at further region(s) along the light guide; and, (2) reflect 
the rest of the light toward a selected light guide wall section such 
that, when refracted through the wall, the light escapes from the guide at 
a selected angular orientation with respect to the guide. 
DISCLOSURE OF INVENTION 
In a broad aspect, the invention provides a method for reflecting a portion 
of the light constrained to travel within a light guide such that the 
reflected light portion escapes through a selected section of the light 
guide at a selected angular orientation with respect to the guide. The 
method comprises the steps of locating a light reflecting element within 
the guide and orienting the element to reflect a portion of the light 
constrained to travel within the guide at an angle, relative to the 
longitudinal axis vector of the guide, which exceeds the maximum angle at 
which light remains constrained to travel within the guide, and which 
further results in escapement of the reflected light portion at the 
selected angular orientation when it is refracted through the selected 
guide portion. 
The reflected light path inside the guide is characterized by a vector 
I.sub.in. The refracted light path outside the guide is characterized by a 
vector I.sub.out. The orientation of the light reflecting element is 
characterized by a vector I.sub.m, which is perpendicular to and directed 
out of the reflective surface of the light reflecting element. 
The vector I.sub.in has an angular orientation defined by: 
(a) an angle .theta..sub.in measured between the vector I.sub.in and the 
longitudinal axis vector of the guide; and, 
(b) an angle .phi..sub.in measured between: 
(i) a plane which contains the vector I.sub.in and which also contains the 
longitudinal axis vector of the guide; and, 
(ii) a line perpendicular to the selected internal light guide surface 
through which the light is refracted. 
The vector I.sub.out has an angular orientation defined by: 
(c) an angle .theta..sub.out measured between the vector I.sub.out and the 
longitudinal axis vector of the guide; and, 
(d) an angle .phi..sub.out measured between: 
(i) a plane which contains the vector I.sub.out and which also contains the 
longitudinal axis vector of the guide; and, 
(ii) a line perpendicular to the selected internal light guide surface 
through which the light is refracted. 
The method comprises locating a light reflecting element within the guide 
at an angular orientation defined by: 
(e) an angle .theta..sub.m measured between the longitudinal axis vector of 
the guide and a vector I.sub.m perpendicular to and directed out of the 
reflective surface of the reflecting element; and, 
(f) an angle .phi..sub.m measured between: 
(i) a plane which contains the vector I.sub.m and which also contains the 
longitudinal axis vector of the guide; and, 
(ii) a line perpendicular to the selected internal light guide surface 
through which the light is refracted. 
The light reflecting element is located such that: 
EQU .theta..sub.m =(180.degree.+.theta..sub.out)/2; 
EQU .phi..sub.m =.phi..sub.in ; 
EQU .theta..sub.in =.theta..sub.out ; and, 
EQU .phi..sub.in =sin.sup.-1 (n'sin((sin.sup.-1 (1/n sin(.phi..sub.out 
.+-.45.degree.))).-+.45.degree.)); 
where 
EQU n'=n((1-cos.sup.2 .theta..sub.out /n.sup.2)/(1-cos.sup.2 
.theta..sub.out))1/2; 
and n is the refractice index of light guide material. 
The reflecting element may be a mirror, a prismatic reflective or 
refractive element, or any reflective or refractive element which 
redirects light so that it is caused to escape from the light guide in 
accordance with the foregoing relationships, although in practice the 
reflecting element is preferably a dielectric material (such as acrylic 
plastic) having a pair of parallel opposed planar surfaces. The surface 
area and shape of the light reflecting element may be selected with 
consideration given to the internal cross-sectional shape of the light 
guide and the angular orientation of the element within the guide such 
that the element occupies a selected portion of the internal 
cross-sectional area of the guide, thereby causing the desired fraction of 
light to be reflected by the element with the remainder of the light 
passing further along the guide. 
In a particularly preferred embodiment the refractive index of the light 
guide material is 1.5, .phi..sub.in =.+-.25.8.degree. and .theta..sub.n 
=135.degree.. In another particularly preferred embodiment the refractive 
index of the light guide material is 1.5 and the light reflecting element 
comprises first and second light reflecting portions. The element is 
located within the guide such that .phi..sub.in is 25.8.degree. for the 
first portion, and such that .phi..sub.in for the second portion is 
-25.8.degree. and such that .theta..sub.m for the first and second 
portions is 135.degree.. 
Advantageously, a plurality of light reflecting elements may be located in 
the light guide at selected regions within the guide so that light is 
reflected from each such region for refraction by the guide to a region 
outside the guide at a selected angular orientation with respect to the 
guide. Further advantage may be obtained by varying the reflectivity of 
the light reflecting elements such that elements which are close to the 
light guide light source are less reflective than elements which are 
further away from the light source, thereby equalizing the light output at 
each region.

DETAILED DESCRIPTION 
In accordance with the invention a light reflecting element is located 
within a light guide such that light which is constrained to travel 
through the light guide and which encounters the element either passes 
through (or around) the element for further processing at other locations 
along the guide, or is efficiently and precisely reflected toward a 
selected section of the light guide wall at an angle, relative to the 
longitudinal axis vector of the light guide, which enables the selected 
wall section to refract the light so that it escapes through the selected 
wall section at a selected angular orientation with respect to the light 
guide. Preferably, this orientation is perpendicular to the internal 
planar surface of the selected light guide wall section through which the 
light ray is refracted. 
The light reflecting element may be a mirror precisely located within the 
light guide in the manner hereinafter explained and which is of a size 
which will reflect only a portion of the light emanating from the light 
guide light source. Alternatively, the light reflecting element may be a 
dielectric material having a pair of parallel, opposed planar surfaces. As 
a further alternative, the light reflecting element may be a prismatic 
reflective or refractive material (in which case the relationships 
involving the angles .phi..sub.in, .phi..sub.out, .phi..sub.in do not 
change, although the definition of the orientation of the reflecting 
element becomes more complex since the geometry of a prismatic element is 
more complex than that of the planar elements hereinafter described). 
Indeed, the light reflecting element may be any reflective or refractive 
material capable of redirecting light so that it escapes through a 
selected section of the light guide in accordance with the relationships 
hereinafter set forth. 
Preferably, the light reflecting element is formed of a dielectric material 
such as acrylic plastic. Dielectric materials are preferred for their 
partial surface reflectivity characteristics which facilitate transmission 
of approximately 92% of the incident light through the dielectric material 
while essentially all of the remaining incident light is reflected by the 
material. Accordingly, the efficiency attainable with dielectric material 
is very high compared with that attainable with mirrors which are 
comparatively absorptive. Such light transmissive dielectric materials 
facilitate reflection of a relatively small portion of the incident light 
from a relatively large surface area, thereby lowering the luminance of 
the escaping light. Thus, in addition to enhancing the efficiency of light 
transmission/reflection and facilitating control over the directionality 
(i.e., selection of the light guide wall section through which the light 
escapes) and angular orientation of the escaping light, such materials 
also facilitate greater control over the region from which light is 
emitted through the light guide wall and variation of the brightness of 
the emitted light as a function of the position along the light guide at 
which the light is emitted. 
The orientation of a light reflecting element located within a light guide 
can be defined in terms of the direction of the vector I.sub.m which is 
normal to the surface of the reflecting element. One must then consider 
the relationship between three vectors: 
(1) the direction vector I.sub.n for the light ray which is to escape 
through a selected section of the light guide and at a selected angular 
orientation with respect to the guide; 
(2) the corresponding direction vector I.sub.out for a light ray which has 
been reflected by the light reflecting element toward the wall of the 
light guide through which it is to be refracted; and, 
(3) the corresponding direction vector I.sub.m for the normal to the planar 
surface of the reflecting element. 
The most convenient coordinate system (from the point of view of 
simplification of the mathematics) is that shown in FIGS. 1 and 2. As 
shown in FIGS. 1 and 2, I.sub.in is a direction vector which characterizes 
the path traversed within prism light guide 10 by light which has been 
reflected by a light reflecting element positioned within light guide 10 
(the light reflecting element is not shown in FIGS. 1 or 2 but is located 
off the drawing to the right along longitudinal axis vector 12 of light 
guide 10). Light traversing the path characterized by vector I.sub.in 
encounters the upper surface of light guide 10 and is refracted through 
that surface to escape from light guide 10 along a path characterized by 
direction vector I.sub.out. Direction vector I.sub.in has an angular 
orientation defined by: 
(a) an angle .theta..sub.in measured between vector I.sub.in and 
longitudinal axis vector 12 of light guide 10; and, 
(b) an angle .phi..sub.in measured between: 
(i) a plane 14 which contains vector I.sub.in and which also contains 
longitudinal axis vector 12; and, 
(ii) a line 16 perpendicular to the selected internal planar light guide 
surface through which the light is refracted. 
Similarly, vector I.sub.out has an angular orientation defined by: 
(c) an angle .theta..sub.out measured between vector I.sub.out and 
longitudinal axis vector 12; and, 
(d) an angle .phi..sub.out measured between: 
(i) a plane 18 which contains vector I.sub.out and which also contains 
longitudinal axis vector 12; and, 
(ii) line 16 perpendicular to the selected internal planar light guide 
surface through which the light is refracted. 
The angular orientation of a light reflecting element located within light 
guide 10 to reflect light along a path characterized by direction vector 
I.sub.in for refraction by light guide 10 and escapement from guide 10 
along a path characterized by direction vector I.sub.out may be defined 
by: 
(e) an angle .theta..sub.m (not shown) measured between longitudinal axis 
vector 12 and a vector I.sub.m (not shown) perpendicular to and directed 
out of the reflective surface of the planar light reflecting element; and, 
(f) an angle .phi..sub.m (not shown) measured between: 
(i) a plane which contains vector I.sub.m and which also contains 
longitudinal axis vector 12; and, 
(ii) a line perpendicular to the selected internal planar light guide 
surface through which the light is refracted. 
Simple geometric considerations and the reflection characteristics of 
planar reflectors reveals that: 
EQU .theta..sub.m =(180.degree.+.theta..sub.out)/2 (1) 
EQU and that: 
EQU .phi..sub.m =.phi..sub.in. (2) 
For cylindrically symmetrical structures such as light guide 10 the effects 
of refraction in a three dimensional situation as illustrated may be 
solved with the aid of a two dimensional analogy in which the refractive 
light guide material has an effective refractive index which is enhanced 
by the fact that light travels through light guide 10 in the direction of 
longitudinal axis vector 12 (see: "Simplified Ray Tracing in Cylindrical 
Systems" by Lorne A. Whitehead, Applied Optics, 21, p. 3536-3538, 1982). 
Utilizing this approach and Snell's Law, it can readily be verified that: 
EQU .theta..sub.in =.theta..sub.out (3) 
EQU .phi..sub.in =sin.sup.-1 (n'sin((sin.sup.-1 (1/n sin(.phi..sub.out 
.+-.45.degree.))).-+.45.degree.)) (4) 
EQU where: 
EQU n'=n((1-cos.sup.2 .theta..sub.out /n.sup.2)/(1-cos.sup.2 
.theta..sub.out))1/2 (5) 
and where n is the refractive index of the material used to construct light 
guide 10. 
Note that there are two possible solutions to equation (4) above. This is 
because light refracted through the upper surface of light guide 10 may 
emerge through either one of the symmetrically repeated pairs of right 
angled facets 20, 22 which shifts the direction of vector I.sub.out, by 
.+-.45.degree. depending upon which of the two facets the emerging ray 
passes through. 
FIG. 3 is a graph in which the angle .phi..sub.in is plotted as the 
ordinate versus the angle .phi..sub.out as the abscissa for various values 
of .theta..sub.out and assuming that the material used to construct light 
guide 10 has a refractive index of 1.5 (as does acrylic plastic). FIG. 3 
shows that there are many values of .phi..sub.out for which there are two 
different solutions for .phi..sub.in, as predicted by equation (4) above. 
In practice it will usually be desirable to locate the light reflecting 
element within light guide 10 such that light escapes from light guide 10 
along a direction vector I.sub.out which is perpendicular to the internal 
planar surface of light guide 10 through which the light is refracted. 
That is, the angular orientation of direction vector I.sub.out will be 
such that .theta..sub.out =90.degree. and .phi..sub.out =0.degree.. As may 
be seen by solving the above equations, this corresponds to an angle 
.phi..sub.in =.+-.25.8.degree. and an angle .theta..sub.in =90.degree. 
which in turn corresponds to an angle .theta..sub.m =135.degree.. In other 
words, the vector which is normal to the planar surface of the light 
reflecting element located within light guide 10 faces toward the 
direction along which light is propagated through light guide 10 and is 
45.degree. off axis in that direction. 
The actual dimensions of a particular light reflecting element may be 
selected, relative to the internal cross-sectional area of light guide 10 
(i.e., the rectangular area shown in FIG. 2), and relative to the angular 
orientation of the light reflecting element within light guide 10, such 
that the element will occupy a selected portion of the internal 
cross-sectional area of light guide 10. The shape of the light reflecting 
element may then be determined by routine vector algebra which need not be 
presented here. Those skilled in the art will however understand that by 
selectably altering the dimensions and/or shape of the light reflecting 
element as aforesaid one may allow a given quantity of light to pass along 
light guide 10 unaffected by the light reflecting element, with the 
remaining light reflected by the light reflecting element for refraction 
by and escape through a selected wall section of guide 10, while ensuring 
that the light reflecting element is shaped to fit neatly within light 
guide 10. 
FIGS. 4, 5 and 6 illustrate, respectively, end, cross-sectional side and 
partially fragmented pictorial views of a light guide 10 within which a 
light reflecting element 30 is located in accordance with the invention to 
allow part of the light constrained to travel through light guide 10 to 
pass unaffected by element 30 while the remaining light is reflected by 
element 30 toward a selected section of one of the internal surfaces of 
light guide 10 for refraction through that surface section to escape 
therethrough at a desired angular orientation with respect to light guide 
10. 
FIG. 7 illustrates the manner in which light constrained to travel within 
guide 10 may be partially reflected by and partially transmitted through 
element 30 for refraction of the reflected light portion by a selected 
section of guide 10 at an angle of 90.degree. to the internal planar 
surface of guide 10. As may be seen, incident light ray 32 strikes element 
30 which is located within guide 10 and oriented to partially reflect 
light ray 32 along path 34 toward a selected section of the internal 
planar surface of light guide 10 at an angle which deviates from the 
longitudinal axis vector of guide 10 by more than 27.6.degree., thereby 
ensuring that reflected ray 34 will be refracted through the wall of light 
guide 10 and escape therefrom, rather than remain constrained to travel 
within light guide 10 (which would be the case if reflecting element 30 
were not present). Since reflecting element 30 is a dielectric material 
the portion of incident ray 32 which is not reflected by element 30 is 
transmitted through element 30 along path 36 with very little absorptive 
loss and remains constrained to travel within guide 10. Reflected ray 34 
is refracted through the wall of light guide 10 to escape therefrom along 
path 38 at an angular orientation which may be precisely controlled, 
relative to light guide 10, through selective location of element 30 
within light guide 10 according to the relationships described above, so 
that the escaping light has the preferred perpendicular orientation to the 
internal planar surface of light guide 10. 
FIG. 8 is a cross-sectional side view of a light guide 10 through which 
light is propagated from a light source which is not shown but which is 
located off the drawing to the right. Light which travels through light 
guide 10 to the end of light guide 10 (off the drawing to the left) 
encounters a terminal reflective mirror (not shown). The terminal mirror 
reflects light so that it returns through and remains constrained within 
light guide 10. FIG. 8 illustrates that a light reflecting element 30 
located within light guide 10 to reflect light which travels "directly" 
between the light guide light source and element 30 (i.e., without being 
reflected back through guide 10 by the terminal mirror--the situation 
discussed above with reference to FIG. 7) for refractive escape through a 
selected section of the light guide and at selected angular orientation 
with respect thereto, will also reflect light which reaches the element 
"indirectly" (i.e., by being reflected back through guide 10 by the 
terminal mirror) such that the indirect light reflected by element 30 
escapes through the same selected section of the light guide and at the 
same angular orientation with respect thereto as the direct light 
reflected by element 30. More particularly, indirect light ray 40 
reflected off the terminal mirror encounters element 30 which partially 
reflects and partially transmits ray 40 along paths 42, 44 respectively. 
Light traversing path 44 continues along light guide 10 and remains 
constrained therewithin in accordance with the relationships set forth in 
the above-mentioned U.S. patent. Reflected light traversing path 42 is 
refracted through the upper surface of light guide 10 (as viewed in FIG. 
8) and is then reflected by reflector 45 back to the upper surface for 
further refraction therethrough back into light guide 10 along path 46 
which can be shown to have a direction vector identical to the direction 
vector of light reflected by element 30 after traversing a "direct" path 
from the light guide light source. Accordingly, light traversing path 46 
is refracted by the lower wall of light guide 10 and escapes therethrough 
at an angular orientation with respect to the light guide 10 defined by 
the relationships set forth above. 
As explained above there are many values of the angle .phi..sub.out for 
which there are two different solutions for the angle .phi..sub.in. This 
fact is exploited in the embodiment of FIG. 9 which shows a partially 
fragmented section of a light guide 10 within which a light reflecting 
element 50 is located. Element 50 comprises first and second light 
reflecting portions 52, 54 respectively. In constructing element 50, 
portions 52 and 54 are located, relative to one another, such that element 
50 may be located within light guide 10 with the angle .phi..sub.in for 
first portion 52 equaling 25.8.degree. and with the angle .phi..sub.in for 
second portion 54 equaling -25.8.degree.. The angle .theta..sub.m for the 
first and second portions remains 135.degree.. Element 50 thus elegantly 
ensures that light reflected thereby escapes from light guide 10 along 
paths 56, 58 which have the generally preferred perpendicular orientation 
to the internal planar surface 60 of light guide 10 through which the 
light escapes. A significant advantage of element 50 is that it may simply 
rest within light guide 10 on internal surface 60 and is easily aligned to 
facilitate the preferred light escapement aforesaid. By contrast, some 
mounting structure would have to be provided for a single light reflecting 
element 30 of the type illustrated in FIGS. 4 through 8 and it would be 
difficult to attain the precise three dimensional orientation of element 
30 within light guide 10 required to ensure that light escapes through a 
selected section of the light guide surface at the preferred angular 
orientation. 
In practice it is not possible to produce a light guide having perfectly 
planar internal surfaces; there will always be some slight imperfections 
in the internal surfaces. As a result, a small amount of light will in 
practice escape through each of the internal light guide surfaces, not 
just through a selected section thereof. FIG. 10 is an end view of a light 
guide 10 having "imperfectly" planar internal surfaces through each of 
which some small portion of light traveling within guide 10 will escape. 
By covering sides 10A and 10B of guide 10 with diffuse reflector material 
60, covering side 10C of guide 10 with specular reflector material 62, and 
covering side 10D with a transparent filter 64, one may ensure that light 
which escapes through sides 10A, 10B and 10C is reflected back into guide 
10 by reflectors 60 and 62 for eventual refraction through side 10D and 
filter 64 in the generally desired direction of escapement of light from 
guide 10. Although light reflected by reflectors 60 and 62 may not be 
refracted through side 10D and filter 64 at the same desired angular 
orientation as light refracted therethrough after reflection from a light 
reflecting element 66 located within guide 10 in accordance with the 
relationships set forth above, the escaping light is at least not 
completely lost through sides 10A, 10B and 10C but escapes through the 
desired side of light guide 10; namely side 10D. 
A plurality of light reflecting elements may be located at selected regions 
within a light guide to enable light to escape from the light guide at 
selected points. However, if those elements are of equal reflectivity then 
light reflected by the elements farther away from the light guide light 
source will be dimmer than light reflected by those elements which are 
located closer to the light guide light source, due to progressive 
attenuation of the light by the reflecting elements. This may be overcome 
by ensuring that light reflecting elements located farther away from the 
light guide light source are progressively more reflective than elements 
located closer to the light guide light source, thereby equalizing the 
intensity of light which escapes from the light guide at the site of each 
reflecting element. This equalization results from the fact that the light 
reflected by a particular reflecting element for refraction through the 
wall of the light guide travels not only along "direct" paths between the 
particular reflecting element and past or through intermediate elements to 
originate at the light guide light source, but also along "indirect" 
paths; namely, paths traversed by light transmitted through the light 
guide and past or through each of the reflecting elements located 
therewithin, reflected off the terminal mirror at the end of the light 
guide and returned through the light guide and past or through 
intermediate elements to the particular element. The desired progressively 
varied reflectivity may be obtained, for example, by using multi-layered 
dielectric materials to construct the light reflecting elements or by 
providing a metallic coating of varying reflectivity on each light 
reflecting element. 
FIG. 11 is an end view of a light guide 70 which comprises an opaque, 
internally light reflective material on three sides and a prismatic light 
guide material on the fourth side. A light reflecting element 72 may be 
located within light guide 70 precisely in accordance with the above 
relationships to reflect light for refraction through a selected section 
of the light guide material at a desired angular orientation with respect 
to light guide 70. The structure of FIG. 11 has relatively low light 
transmission efficiency, due to light absorption by the opaque light 
reflecting portion, but the structure is relatively cheap and easy to 
construct and may therefore be desirable in some practical applications. 
FIG. 12 is a partially fragmented pictorial illustration of a "light panel" 
comprising a first light guide 80 located within a second light guide 82. 
A light source 84 projects light into first light guide 80. A dual 
reflector 86 directs the light in both directions along the longitudinal 
axis of first light guide 80. One or more first light reflecting elements 
88 are located within first light guide 80 in accordance with the 
relationships set forth above to reflect light from within first light 
guide 80 for refraction through a selected section of first light guide 80 
into the region within second light guide 82. One or more second light 
reflecting elements 90 are optionally located within second light guide 82 
in accordance with the relationships set forth above to reflect light for 
refraction through a selected section of second light guide 82 at a 
desired angular orientation with respect thereto. It will be understood 
that the selected section of second light guide 82 through which light is 
desirably refracted may comprise, for example, the entire surface area 92 
of second light guide 82. That is, it may in practice be desirable to 
configure second light guide 82 as a "light panel" such that light is 
uniformly emitted through the entire surface area 92, although the light 
need not necessarily be emitted from second light guide 82 at any 
particular angular orientation with respect thereto (which is why light 
reflecting element 90 is optional). Similarly, first light guide 80 may be 
configured as a "light bar" such that light is uniformly emitted through 
the entirety of one or more of the surfaces of first light guide 80, for 
maximal uniform illumination of the interior region of second light guide 
82. 
It should be noted that only the three surfaces A, B and C of light guides 
80, 82 comprising the light panel of FIG. 12 need be formed of prismatic 
light guide material. The other surfaces of light guides 80, 82 could also 
be formed of prismatic light guide material, or they could instead be 
formed of a reflective material. Note further that first light guide 80 
may be located anywhere within second light guide 82 and that light source 
84 may be located anywhere along any side or at either end of first light 
guide 80. 
As will be apparent to those skilled in the art in the light of the 
foregoing disclosure, many alterations and modifications are possible in 
the practice of this invention without departing from the spirit or scope 
thereof. Accordingly, the scope of the invention is to be construed in 
accordance with the substance defined by the following claims.