Linear diffuse light source

A light source providing a diffuse, uniform intensity of light over a length of the device. A prism of material having a relatively high index of refraction has a shape to capture light emitted longitudinally therein by light emitting diodes molded into the ends thereof. A window, such as a frosted surface of the prism, allows the light to be emitted from the prism. The window has a shape which tapers toward the light emitting diodes so that a linearly uniform amount or intensity of light is emitted along the length of the window.

BACKGROUND OF THE INVENTION 
This invention relates in general to light sources and more particularly to 
diffused light sources. More particularly still this invention relates to 
diffused light sources which have a relatively uniform quantity of 
emission along a length of the light source. 
DESCRIPTION OF PRIOR ART 
Among the many uses of diffused light sources are uses which require a 
uniform dispersion of light over a predetermined length. Often these light 
sources must be confined to narrow or small spaces. For example, the use 
of charge-coupled devices to read slits or the like which allow light to 
pass therethrough from a light source require this type of lighting. Each 
pixel along the length of the charge-coupled device is desired to be acted 
upon by the same intensity of light. This is especially true in position 
sensing devices such as that shown in U.S. Pat. No. 4,074,258 where 
differences in light intensity or direction may cause a misreading of the 
position sensed by the light source and charge-coupled device. 
Among the devices which are currently available for diffused, linear light 
emission are electoluminescent panels. Such panels are commercially 
available from Luminescence Systems and from Quantex. However, because of 
the manner in which the light is generated by these devices, they require 
a high alternating current voltage and have a short life. Further, these 
devices fade over their lives so that the intensity of light emitted is 
not constant. Still Further, the brightness of these devices is relatively 
low. 
SUMMARY OF THE INVENTION 
It is accordingly an object of the present invention to provide an improved 
light source having a diffuse linear emission. It is also an object of the 
present invention to provide such a device which has a low, direct current 
power requirement and a relatively long life. 
Another object of the present invention is to provide a light source of the 
type described which has a relatively low cost. Yet another object of the 
present invention is to provide such a device which is relatively constant 
in its light intensity over its life and which is relatively bright. Still 
yet further, it is an object of the present invention to provide a device 
which has a relatively small package so that it can be used in 
applications requiring this feature. 
In accordance with these objects, the present invention provides a light 
source for producing a relatively uniform dispersed light emissions along 
a length portion thereof. The device includes a prism having two ends, an 
emission face and a plurality of optically reflective faces. The prism is 
formed of an optically transparent material having a relatively high index 
of refraction such as polycarbonate. It is shaped to capture light emitted 
longitudinally therein. A light emitting means is disposed to emit light 
longitudinally in the prism so as to be captured therein. To provide 
emission of the light captured in the prism, the prism has an optically 
transparent and light dispersing window disposed along a length of its 
emission face. This window is shaped so as to narrow as it extends toward 
the light emitting means in a manner which provides a longitudinal uniform 
light intensity along the window. 
Preferably, the light emitting means includes first and second light 
emitting diodes disposed at ends of the prism. The window comprises an 
emission face which is frosted so as to reduce reflection and to increase 
dispersion of the light captured in the prism. The window tapers linearly 
toward each end of the prism. To improve the capture of light it is 
preferable that the prism have an hexagonal cross section with right angle 
prism sides adjacent the emission face. 
For a further understanding of the invention and further objects, features 
and advantages thereof, reference may now be had to the following 
description taken in conjunction with the accompanying drawings.

DESCRIPTION OF PREFERRED EMBODIMENTS 
Referring now to FIG. 1, a device constructed in accordance with the 
present invention is shown generally at 11. The device includes a prism 13 
mounted on a circuit board 15. Light emitting diodes 17 an 19 are molded 
into the ends of the prism 13 so that their leads extend therefrom. Leads 
21 and 23 extend from light emitting diode 17 and leads 25 and 27 extend 
from light emitting diode 19. Leads 21 and 25 are connected to circuit 
path 29, lead 23 is connected to circuit path 31 and lead 27 is connected 
to circuit path 33. By connecting a direct current power source across the 
contact points 35 and 37 of circuit path 31 and 33, respectively, the 
light emitting diodes 17 and 19 are illuminated by their being in series 
connection to this power source. 
As shown in FIGS. 1-4, the prism 13 has an elongated parallepiped shape. As 
shown in FIG. 2 the prism 13 has a hexagonal cross section. Thus, the 
prism 13 has a first end surface 39, a second end surface 41, a base 
surface 43, an upper surface 45 and side surfaces 47, 49, 51 and 53. The 
end surfaces 39 and 41 are planar and hexagonal in shape. The side 
surfaces 47, 49, 51 and 53 are planar and rectangular in shape. The base 
surface 43 and the upper surface 45 are planar and rectangular in shape 
with a width which is approximately twice the width of the side surfaces. 
All of the surfaces are polished or molded in a smooth way so as to be 
reflective. 
The shape of the prism 13 is designed to capture light emitted 
longitudinally into the ends thereof. Particularly, sides 51 and 53 join 
at a right angle 55 to produce a right angle prism reflection of light 
rays directed from the interior of prism 13. Similarly, sides 47 and 49 
join at a right angle 57 to have a right angle prism reflection of light 
rays directed from the interior of prism 13. Together the right angle 
prism 55 and the right angle prism 57 face each other across the prism 13 
so as to capture the light reflected back and forth across the sides of 
the prism 13. 
The sides 47, 49, 51 and 53 join the base surface 43 and the upper surface 
45 at interior angles of 135 degrees. This insures that light reflected 
off of one of the sides will continue to be reflected as it traces a path 
along the interior of prism 13. The light emitting diodes 17 and 19 are 
disposed at the center of the hexagonal ends 39 and 41 so that most of the 
light emitted from these LED's will pass longitudinally into the prism or 
at least at an angle sufficient to be reflected as opposed to transmitted 
through the surfaces 43, 45, 47, 49, 51 and 53. Thus, the effect of the 
placement of the surfaces of prism 13 is to provide a light capturing tube 
or pipe of material. To further this object it is desirable to form the 
prism 13 of a substance having a relatively high index of refraction with 
high transparency. Polycarbonate is ideal for this purpose especially 
since polycarbonate is moldable and can be molded about the light emitting 
diodes 17 and 19. In fact, commercially available light emitting diodes 17 
and 19 can have polycarbonate outer shells, as well. 
The upper surface 45 of prism 13 has a window 59 disposed at its center and 
extending between the diodes 17 and 19. The window 59 is formed by 
frosting the otherwise smooth, reflective surface of surface 45. This 
frosting can be achieved by sandblasting or chemical etching of the mold. 
The frosting forms a surface which transmits as opposed to reflects and 
transmits in a diffused manner as opposed to a directed manner, light 
which encounters the frosted surface from the interior of the prism 13. 
The desired shape can be achieved by forming the correct shape in the mold 
and by injection molding of the prism. 
The window 59 has a wide diamond shape with truncated ends adjacent the 
LED's 17 and 19. In other words, the window 59 is widest at the 
longitudinal center of surface 45 and tapers linearly toward the ends 39 
and 41. The truncated ends 61 and 63 of window 59 are disposed so that 
light emitted from LED's 17 and 19 which encounters surface 45 prior to 
the ends 61 and 63 will be reflected internally in the prism 13. 
The amount of tapering of the window 59 toward its ends 61 and 63 is 
empirically determined so as to provide a uniform linear emission of light 
along the length of window 59. Even though light is repeatedly reflected 
internally in the prism 13, the close proximity of the ends 63 and 61 to 
their respective light emitting diodes 17 and 19 requires that the windows 
be narrower at this point than at the center of the window 59. The amount 
of taper is determined so that the intensity of light emitted along the 
length of the window 59 is uniform. 
The light emitting diodes 17 and 19 can be of a variety of types. A type 
which is especially suited for use with a charge-coupled device in a 
position sensing machine would be an infrared LED, for example Siemen's 
SFH487P. This device is a 50 milliamp, 1.5 volt, 20 milliwatt infrared 
emitter. Other light emitting diodes can be chosen for emitting light of 
different frequencies or having different intensities. 
Although the present invention can be made in a variety of sizes, the one 
depicted in the drawings and described herein is approximately 21/2 inches 
long, 1/8 inch wide and 1/3 inch in height. As can be see, this size is 
ideal for providing a relatively long, diffuse, linearly uniform emission 
of light. This is achieved while maintaining a relatively small package. 
Because light emitting diodes are relatively stable, have long life and 
have the desired frequency characteristics, the present invention has all 
the advantages of the light emitted by light emitting diodes while also 
producing a diffuse, linearly uniform emission. A very large proportion of 
the light emitted by the LED's is emitted through the window 59. This is 
achieved with a single piece, exceptionally sturdy construction. If 
desired, two or more LED's can be provided in the each end of the prism 13 
to provide a greater intensity of light. Also, if it is not desired to 
mold the LED's into the ends of the prism 13, the LED's can be attached to 
the ends of the prism by epoxy cement or the like. 
Thus, the present invention provides a linear type of lambertian light 
source. This is achieved at a relatively low cost. Thus, the device of the 
present invention is well adapted to achieve the objects and advantages 
mentioned as well as those inherent therein. It will be appreciated that 
the instant specification and claims are set forth by way of illustration 
and not of limitation, and that various changes and modifications can be 
made without departing from the spirit and scope of the present invention.