Light fixture providing normalized output

A light fixture providing normalized output. An undirected light source is located in an optical cavity having an optical window. A film with a smooth surface and a structured surface is placed in the optical window with the smooth surface facing the interior of the optical cavity. The structured surface has a plurality of triangular prisms thereon.

TECHNICAL FIELD 
The present application relates to light fixtures, and more particularly to 
light fixtures providing normalized light output from extended linear 
light sources having undirected output characteristics. 
BACKGROUND ART 
A light fixture that emits light in only a narrow range of directions is 
said to provide normalized output. Such light sources are often desirable. 
Because most light sources in common use produce an undirected output, 
some technique must be used to normalize the light they emit. One common 
method of normalizing the output of a light source is the use of a 
parabolic reflector. Such a reflector will produce a collimated beam of 
light from a point source. 
The problem becomes more difficult when the light source is an extended 
linear light source, such as a common fluorescent lighting tube. Light 
from a fluorescent tube may be partially collimated by a reflector which 
is parabolic along a cross-section running perpendicular to the major axis 
of the tube. This technique will normalize the direction of the light that 
is emitted perpendicular to the tube, but not the light that is emitted in 
other directions. 
Another technique that is commonly used with extended linear light sources 
involves positioning louvers in front of the light source. Such louvers 
allow only light traveling in the normalized direction to pass. 
Both of these techniques suffer from common disadvantages. One such 
disadvantage is that light fixtures utilizing them are necessarily bulky. 
Parabolic reflectors, by their very nature, must be deep in relation to 
the size of the aperture. This is particularly true if the parabolic 
reflector is to normalize wide angle light. Similarly, if the louvers are 
to have a significant normalizing effect they must be reasonably deep. 
A second disadvantage of both of these techniques particularly when a 
fluorescent tube is used as a light source for a light fixture which is to 
be used as an area source, is that both will produce brighter areas, 
commonly known as "hot spots". Hot spots normally arise in the portions of 
the light fixture closest to the light source. If such hot spots are to be 
avoided, other techniques must be used. 
DISCLOSURE OF INVENTION 
In the light fixture of the invention an extended linear light source 
having undirected output characteristics is located in an optical cavity 
having an optical window. A transparent film having a smooth surface and a 
structured surface lies in the optical window with the smooth surface 
facing the interior of the optical cavity. 
The structured surface has a plurality of triangular prisms thereon. The 
prisms will totally internally reflect light entering the film 
perpendicular to the smooth surface, causing that light to remain in the 
optical cavity. The combined effect of the smooth surface and the prisms 
on the structured surface will, however, be to normalize the majority of 
the light entering the film at an angle other than perpendicular to the 
smooth surface so that it exits the film at an angle closer to 
perpendicular than the angle at which it entered.

DETAILED DESCRIPTION 
FIG. 1 shows a light fixture 10 according to the invention. Light fixture 
10 includes an optical cavity 12 defined by case 14. Optical cavity 12 has 
an optical window 16. Within optical cavity 12 are extended linear light 
sources 18, 20 and 22. Extended linear light sources 18, 20 and 22 could 
be, for example, conventional fluorescent lighting tubes. The number of 
such tubes included in optical cavity 12 is not critical to the invention. 
The number used should be chosen to provide the desired amount of light 
output. 
In optical window 16 is a transparent film 24. Transparent film 24 may be, 
for example, of a polymer material such as an acrylic or a polycarbonate, 
and has a smooth surface 26 facing the interior of optical cavity 12 and a 
structured surface 28 facing the exterior of optical cavity 12. Structured 
surface 28 has a plurality of triangular prisms, such as prisms 30 and 32, 
forming a series of ridges and grooves thereon. Each of these prisms has a 
major axis that runs parallel to the major axes of extended linear light 
sources 18, 20 and 22. The invention has been found to work when the major 
axes of the triangular prisms run perpendicular to the major axes of the 
extended linear light sources, as well. In a preferred embodiment these 
prisms form right isosceles triangles, although triangles having other 
angles may be used. 
Also in optical window 16, and exterior to film 24, is a film 34 that may 
be transparent or diffusely transmissive to light. Film 34 could be of an 
acrylic material. If film 34 is diffusely transmissive it will help to 
provide a more even appearance to the light exiting optical cavity 12 
through optical window 16. Film 34 also provides physical protection to 
the prisms on structured surface 28 of transparent film 24. Film 34 has an 
exterior surface 36. 
A layer of reflective material 38 is placed on the interior of case 14. 
Reflective material 38 may be a highly specularly reflective material, 
such as a mirror, or may be a diffusely reflective material. Bonded 
nonwoven films of polyolfin fibers have proven to be effective as such 
diffuse reflectors. Reflector 38 could also be the same type of film as 
film 24. If a film having a structured surface like film 24 is used as the 
reflector, the triangular prisms should run perpendicular to the direction 
of extended linear light sources 18, 20 and 22 and the smooth surface of 
the film should face the interior of optical cavity 12. In one embodiment 
reflector 38 is a two layer reflector having a layer of diffusely 
reflective material adjacent to case 14 with a film similar to film 24 
overlying the diffusely reflective material. In the embodiment of the 
invention shown in FIG. 1, a leg 40 is also shown. Leg 40 is attached to 
case 14 by hinge 42. Leg 40 may be placed in the position shown in FIG. 1 
when light fixture 10 is in use or may be folded out of the way to make 
light fixture 10 more compact for storage or transportation. 
When light fixture 10 is to be used, leg 40 is positioned as shown and 
extended linear light sources 18, 20 and 22 are energized. Some of the 
light from light sources 18, 20 and 22 reaches film 24 directly while 
other light reaches film 24 after reflection from reflector 38. Most of 
the light which enters film 24 in a direction other than near perpendiular 
to smooth surface 26 will be refracted so that it emerges from film 24 in 
a direction which is closer to perpendicular to surface 26 of film 24. 
In use a slide, such as an overhead transparency, is placed on surface 36 
of film 34. Light emerging from optical cavity 12 through film 24 and film 
34 will illuminate the slide, allowing it to be viewed without requiring 
the use of a projector. 
The function of film 24 in the invention may be seen by reference to FIG. 
2. FIG. 2 shows a portion of film 24 including triangular prisms 30 and 
32. A ray of light 46 enters film 24 through smooth surface 26. Light ray 
46 strikes facet 48 of prism 32 and is totally internally reflected. The 
reflected ray then strikes facet 50 of prism 32 and is again totally 
internally reflected so that light ray 46 emerges through surface 26. 
Thus, light which enters film 24 perpendicular or near perpendicular to 
smooth surface 26 is reflected back into optical cavity 12 of FIG. 1. 
Light which enters smooth surface 26 at an angle other than perpendicular 
to surface 26, however, is treated differently. Light beam 52 enters film 
24 through surface 26 at an angle other than perpendicular to surface 26. 
As light ray 52 enters film 24 it is refracted at surface 26. Light ray 52 
is again refracted as it emerges through facet 54 of prism 30 of film 24. 
Therefore light ray 52 emerges from film 24 at an angle closer to the 
normal to surface 26 than the angle at which it entered. 
The effect of film 24 provides several advantages to the light fixture of 
the invention. The first advantage is that the refraction of light that 
enters the film at an angle other than near perpendicular to the smooth 
surface will cause that light to exit in a more highly directed beam. A 
second advantage relates to the fact that the light that enters film 24 in 
a direction perpendicular or near perpendicular to the smooth surface is 
reflected back into the optical cavity. Because the portion of the light 
that is so reflected will be greater in the portions of the film closest 
to the light sources this effect helps to provide an even illumination of 
the optical window, eliminating hot spots present in other light fixtures. 
A third advantage occurs for some types of light sources. Some of the 
light that is reflected by film 24 will be reflected back into the light 
source that emitted it. When light is directed back into some types of 
light sources, such as filament sources, the electrical efficiency of the 
light source is improved. Thus the present invention provides several 
advantages over the prior art. 
FIG. 3 shows another embodiment of the invention. The embodiment of FIG. 3 
is also intended for use in viewing slides such as overhead 
transparencies. Light fixture 60 of FIG. 3 has a case 62 defining an 
optical cavity 64. Within optical cavity 64 is an extended linear light 
source 66. Extended linear light source 66 could again be, for example, a 
conventional fluorescent lighting tube. Optical cavity 64 has an optical 
window with a transparent film 68 therein. Transparent film 68 has a 
smooth side 70 and a structured side 72. The structures of structured 
sides 72 of film 68 are the same as the structures of structured side 28 
of film 24 of FIG. 1. As shown in FIG. 3, however, the major axes of the 
triangular prisms of structured sides 72 of film 68 run perpendicular to 
the major axis of extended linear light source 66. The prisms of film 68 
could be placed parallel to extended linear light source 66, but 
experimentation has shown that better results are achieved when the film 
is arranged in the manner shown in FIG. 3. The critical element, however, 
is that structured side 72 must face the exterior of optical cavity 64, 
while smooth side 70 of film 68 must face the interior of optical cavity 
64. 
Additionally shown in optical cavity 64 is reflector 74. Reflector 74 could 
be any of the materials described for use as reflectors on the interior of 
optical cavity 12 of FIG. 1. 
The light fixture of FIG. 3 additionally has a case 76 which defines a 
second optical cavity 78. Optical cavity 78 has an input optical window 
adjacent to the optical window of optical cavity 64. Thus, light emerging 
from optical cavity 64 through film 68 will enter optical cavity 78 
through the input optical window. 
Optical cavity 78 also has an output optical window having a film 80 
therein. Film 80 has a smooth surface 82 facing the interior of optical 
cavity 78 and a structured surface 84 facing the exterior of optical 
cavity 78. Structured surface 84 of film 80 is again the same as 
structured surface 72 of film 68 or structured surface 28 of film 24 of 
FIG. 1. The major axes of the triangular prisms of structured surface 84 
of film 80 run perpendicular to the major axis of extended linear light 
source 66. Finally, light diffusing film 86 overlies structured surface 84 
of film 80 and the output optical window of optical cavity 78. 
Optical cavity 78 will typically have a reflector lining the interior of 
case 76. The reflector on the interior of optical cavity 78 may be any of 
the reflective materials described for use on the interior of optical 
cavity 12 of FIG. 1. Diffusely reflecting materials are preferred, 
however. 
In the preferred embodiment of the invention, case 88 defines a third 
optical cavity having a second extended linear light source therein. This 
third optical cavity cooperates with optical cavity 78 in a manner 
entirely analogous to that of optical cavity 64. Also included in the 
preferred embodiment is leg 90 which functions in a manner similar to leg 
40 of FIG. 1 to stand light fixture 60 in a desired orientation for 
viewing. 
The operation of a light fixture according to the invention may be better 
understood by reference to FIGS. 4, 5 and 6. Turning first to FIG. 4, a 
film 100 is shown having a smooth surface 102 and a structured surface 
104. A point 106 on smooth surface 102 is selected. All light entering 
film 100 through point 106 must pass through theoretical hemisphere 108 
having point 106 at its center. Hemisphere 108 is divided into three 
portions, 110, 112 and 114. Light which enters film 100 at point 106 after 
passing through region 110 of hemisphere 108 will undergo total internal 
reflection at structured surface 104 and will re-emerge through smooth 
surface 102. Light which enters film 100 at point 106 after passing 
through regions 112 or 114 of hemisphere 108, however, will emerge from 
film 100 through structured surface 104. In practice the relative sizes of 
regions 110, 112 and 114 will vary according to the index of refraction of 
the material making up film 100, but the general shape will be as shown 
regardless of the material used. 
FIG. 5 illustrates the light output distribution that will result from the 
light entering the film through regions 112 and 114 of FIG. 4. Light 
emerging from the center of hemisphere 116 of FIG. 5 must pass through 
that hemisphere. The majority of such light will pass through region 118. 
A smaller portion of the light emerging will pass through regions 120 and 
122. The light that passes through regions 120 and 122 will be that small 
percentage of the light that undergoes only a single total internal 
reflection from structured surface 104' of film 100'. None of the light 
will emerge through region 124 of hemisphere 116. 
FIG. 6 illustrates how the input and output characteristics shown in FIGS. 
4 and 5 cooperate in the embodiment of the invention shown in FIG. 3. FIG. 
6 shows films 68 and 80, corresponding to film 68 and 80 of FIG. 3. Light 
emerging from the first optical cavity through film 68 will emerge in 
regions 118, 120 and 122, with the great majority of that light emerging 
in region 118. As may be seen, however, the majority of the light emerging 
in region 118 will be contained within region 110, meaning that that light 
will not be transmitted by film 80. Instead it will be totally internally 
reflected by film 80 back into the second optical cavity 78 of FIG. 3. By 
proper choice of the indices of refraction of films 68 and 80 region 118 
may be totally contained in region 110. As a result that light must be 
reflected at least once by the interior walls of case 76 of optical cavity 
78. This helps to create a uniform light distribution within optical 
cavity 78. A proper choice of reflective materials lining the walls of 
case 76 will help further improve the uniformity of such distribution. A 
diffuse reflector, such as bonded, nonwoven polyolfin fibers, works very 
effectively to produce such a distribution. Alternatively a combination of 
spectral reflectors in some portions of optical cavity 78 and diffuse 
reflectors in other portions may be used. 
The present invention has been described with respect to a light fixture 
which may be used for illuminating slides such as overhead transparencies. 
Those skilled in the art will readily perceive that the invention is not 
limited to such light fixtures. It may be advantageously used in any 
situation in which a light fixture is to have a directed output. For 
example, a light fixture according to the invention could be 
advantageously used in an overhead light fixture where light from the 
fixture is to be preferentially directed to a desk or other work space.