Collector for an LED array

A light collector for an LED array for efficiently collecting and collimating light emitting from the LEDs and projecting the light into an optical wave guide which directs that light onto a photoreceptor surface. Each LED is centered in a hemispherical cavity in the collector array in order that radiation from the LED enters the collector essentially unrefracted. The collector array provides a convex lens portion and a parabolic reflecting surface portion. Light that exits from the LED that is substantially perpendicular to the substrate supporting the LED is applied to the convex lens and is collimated. Light exiting substantially parallel to the substrate strikes a parabolic reflecting surface at greater than the critical angle and is also collimated. The two concentric collimated beams are combined and applied to the photoreceptor surface via a light pipe or optical wave guide secured to the collector.

The present invention relates to a light collector for an LED array, in 
particular a light collector for an array used in an electrophotographic 
machine for patch generation on a photoreceptor surface and for pitch and 
edge erasure on the surface. 
Light emitting diodes (LEDs) are low cost, reliable light sources that emit 
light patterns that are generally broad. The broad light patterns of LEDs 
often impede the efficient of the LEDs with light pipes, optical wave 
guides or other optical transmission media. 
It is known to optically couple an LED to a light pipe by abutting the end 
of the light pipe against the LED. Since the LED source is small, however, 
proper alignment is difficult. Furthermore, when the acceptance angle of 
the light pipe is small, coupling is inefficient. 
Lenses and reflectors have been used to focus and collimate a wide emission 
LED. However, with the use of lenses and reflectors alignment is often 
difficult and therefore expensive. For example, as disclosed in U.S. Pat. 
No. 4,257,672, a spherical lens structure is used as an optical coupler 
for coupling a relatively wide emission light source to an optical 
transmission line. In particular, the wide emission light source is 
mounted adjacent the center of one end of a cylinder. The spherical lens 
structure is mounted on the opposite end of the cylinder at a 
predetermined distance from the light source and in an orientation to 
maximize the amount of light entering the optical transmission line. A 
difficulty with this type of construciton, particularly in using an array 
of LEDs, is the need to align each discrete LED with a spherical 
structure. In addition, only light striking the spherical surface is 
captured in the light pipe. It would be desirable, therefore, to provide a 
relatively inexpensive, reliable and simple means to optimize the 
collection of light from an array of LEDs at one end of a light pipe. 
Other prior art references such as U.S. Pat. No. 4,255,042, teach the use 
of an LED array as an erase lamp to discharge portions of a 
photoconductive surface. Conventionally, in electrophotographic machines, 
erase lamps have been incandescent or fluorescent lamps in which the lamp 
illumination has been attenuated by shields to the photoconductor to 
obtain sharp edge delineation of the erased charge on the photoconductor. 
Generally, since LEDs produce a relatively small quantity of light as 
compared to other types of lamps, LEDs have generally been used with 
optical wave guides to transmit a sufficient amount of light to the 
photoconductor. It is taught in U.S. Pat. No. 4,255,042 to provide a light 
channel having one end next to an array of discrete light emitting diodes 
for channeling the light to a photoconductive surface located at the 
opposite end of the channel. Light is emitted from the LEDs into the light 
pipes and internally reflected and propagated down the light pipes to the 
photoconductor. 
A difficulty with prior art systems using LED arrays is that there is 
generally a loss of edge light that is not captured by the light pipes 
with the resultant inefficient light transmission. Another difficulty 
arises from imperfect or uncollimated light entering the wave guide 
resulting in light exiting the wave guide in an uncontrolled fashion and 
discharging portions of the image. In addition, it is relatively complex 
and expensive to mount and package a large array of discrete LEDs. It 
would be desirable, therefore, to provide an efficient and simple means to 
collect and propagate light in a well controlled manner from an array of 
LEDs to a photoconductive surface. 
It is, therefore, an object of the present invention to provide a new and 
improved light collector for an LED array. It is another object of the 
present invention to provide a light collector for an LED array for use in 
discharging selected portions of a photoconductive surface. 
It is still another object of the present invention to provide a light 
collector for an LED light source that forms a hemispherical cavity about 
an LED and provides both a convex lens and a parabolic reflecting surface 
for collecting and directing light emitted from the LED. 
It is another object of the present invention to provide a means to collect 
radiation emitted from the sides of an LED into a collimated beam of 
light. 
Further objects and advantages of the present invention will become 
apparent as the following description proceeds and the features of novelty 
characterizing the invention will be pointed out with particularity in the 
claims annexed to and forming a part of this specification. 
Briefly, the present invention is a light collector for an LED array for 
efficiently collecting and collimating light emitting from the LEDs and 
projecting the light into an optical wave guide which directs that light 
onto a photoreceptor surface. Each LED is centered in a hemispherical 
cavity in the collector array in order that radiation from the LED enters 
the collector essentially unrefracted. The collector array provides a 
convex lens portion and a parabolic reflecting surface portion. Light that 
exits from the LED that is substantially perpendicular to the substrate 
supporting the LED is applied to the convex lens and is collimated. Light 
exiting substantially parallel to the substrate strikes a parabolic 
reflecting surface at greater than the critical angle and is also 
collimated. The two concentric collimated beams are combined and applied 
to the photoreceptor surface via a light pipe or optical wave guide 
secured to the collector.

With reference to FIG. 1, there is shown a typical prior art device for 
increasing the light output efficiency of a light emitting diode (LED). A 
light emitting diode 20 is positioned at the focal point of a suitably 
coated reflector 22. The light rays emitted from the sides of the LED are 
relected from the reflector 22 into a generally collimated light beam. 
However, the light rays emitted from the front of the LED, as illustrated 
by the arrows, do not strike the reflector and are therefore scattered and 
lost. To compensate for those light rays that are not reflected, it is 
known to place a suitable lens 24 in front of the LED to collect the 
unreflected rays and produce a collimated beam. Unfortunately, while the 
lens 24 may collimate the scattered light rays, it tends to disperse those 
light rays already collimated by the reflector 22. 
In accordance with the present invention, there is provided an integrally 
molded light collector, generally shown as 26, for collecting and 
collimating the light rays projected from an LED as shown in FIG. 2. In 
particular, an LED 28 is suitably mounted on a substrate 30. The integral 
collector 26 is preferably any appropriate transparent plastic material 
such as styrene, acrylic or polycarbonate. The collector is comprised of a 
convex center lens portion 32 with a concentric surrounding leg portion 34 
appending therefrom. An edge portion of the leg 34 is rigidly secured to 
the substrate 30 concentric with the LED 28. Preferably, substrate 30 is a 
porcelain coated metal or ceramic material. A semicircular air pocket 40 
separates the LED 28 from the collector 26. The cavity around the LED 
provides mechanical protection and insulation from dirt and other foreign 
particles that might diminish light output. It should be noted the cavity 
or air pocket 40 could be filled with any suitable optical transparent 
filler in order to increase the output of the LED through index of 
refraction matching. Another air pocket 42 is formed between the leg 34 
and the lens 32. 
In operation, light rays projected from the sides of the LED 28 traverse 
the air pocket 40, enter the end portion of the leg 34 and are reflected 
from the parabolic outside surface 34a of the leg 34 and form generally 
parallel light paths or collimated beams of light through the leg 34. 
Light rays projected from the top of the LED 28 traverse the air pocket 40 
and enter portion 32. The convex surface lens 32, as illustrated, refracts 
the light beams into a collimated beam into and through the air pocket 42. 
In a preferred embodiment, an array of LEDs is used in a strip to 
selectively dissipate the charge on a photoreceptor. With reference to 
Figure 3, there is shown a substrate 30 supporting a plurality of LEDs 44 
aligned with an optical wave guide 46 comprising a plurality of light 
pipes 48. The optical wave guide 46 is rigidly secured to the substrate 
30, in order that one end of each of the segmented light pipes 48 is 
securely fastened in alignment with one or more of the LEDs 44. Light 
pipes of various widths could be provided in alignment with one or more 
LEDs to perform designated functions. For example, light pipes could be 
used for edge fadeout, pitch erase, and patch generators in an 
electrophotographic machine. 
With reference to FIG. 4, there is shown in more detail the alignment of 
the optical wave guide with the substrate supporting the LEDs. In 
particular, an LED 28 is shown mounted on substrate 30. Also secured to 
the substrate 30 is the collector 26 connected to the optical wave guide 
46. It should be understood that the collector 26 is an integral unit 
comprised of several subassemblies having a lens portion 32 and a leg 
portion 34, although only one assembly is illustrated in FIG. 4. 
The optical wave guide 46 includes a connector portion 52, a reflector 
surface 54 and a light pipe portion 48 terminating adjacent to a 
photoreceptor surface. The light rays projecting from the LED 28 are 
reflected through the leg portion 34 or refracted through the lens surface 
32 into well collimated, concentric beams of light that are reflected from 
the surface 54 into the light pipe 48. It should be noted that the light 
rays projected from the LED either from the side or from the top of the 
LED are reflected by either the legs or refracted by the lens 32 into two 
collimated beams. Since the two beams are well collimated, reflection from 
surface 54 can be by total internal reflection, thus avoiding the need to 
coat surface 54 with a reflective material. FIGS. 5, 6 and 7 show in more 
detail a top view, an end view cross section and a side view cross section 
of an LED collector array. 
In operation, there is shown in FIG. 8 a photoconductor 60 in an 
electrophotographic process. The photoconductor 60 is illustrated as 
rotating in a clockwise direction to receive first a uniform charge under 
a charging device 62. Upon receiving an image at station 64, the 
photoreceptor continues to rotate to the LED array and segmented light 
pipes illustrated at 66. By selective activation of LEDs, the light pipes 
can be used to discharge edge portions or pitch portions on the 
photoreceptor surface or to provide a test patch for system correction. 
Next, the photoreceptor advances to the development station illustrated at 
68 at which toner is placed on the image and on the test patch if present, 
and then to the transfer station 70 at which the image is transferred to a 
copy sheet. Not shown are the usual steps of fusing of the image to the 
copy sheet and the placement of the copy sheet in an output tray. 
While there has been illustrated and described what is at present 
considered to be a preferred embodiment of the present invention, it will 
be appreciated that numerous changes and modifications are likely to occur 
to those skilled in the art, and it is intended in the appended claims to 
cover all those changes and modifications which fall within the true 
spirit and scope of the present invention.