Narrow led printheads and gradient index lens array for use therewith

A light-emitting diode (LED) printhead has associated therewith a gradient index fiber lens array for collecting light from the LED's and focussing same onto a photosensitive surface for recording. The fiber lens array includes a segment that is spaced in overlying relationship to the printhead and has a series of optical fibers for conveying light generally parallel with the plane of the LED's. Various configurations of inputs to said array are described for collecting light from the LED's. The relationship between the array and the printhead provides for a narrow construction allowing space for other recording components to fit more readily about the photosensitive surface.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to non-impact printheads for recording and more 
particularly to a radiation-emitting printhead having a plurality of 
recording elements and whose radiation is required to be focussed onto a 
recording element. 
2. Description Relative to the Prior Art 
Non-impact printers such as those using light-emitting diodes are well 
known. In such known printers, one or more extended rows of light-emitting 
diodes (LED's) are arranged so as to selectively emit light to expose a 
photosensitive surface to record images. With regard to the recording of 
images on electrophotographic recording elements such as photoconductive 
drums, the printheads are required to be placed proximate to the drums. 
Room must also be provided around the drum for electrostatic chargers, one 
or more developing stations and transfer devices for transferring images 
to recording members. In order to provide more compact printers such as 
those suited for portability and for table top operation, the drum can be 
made smaller thereby requiring less availability of room for placement of 
the various members adjacent to the drum surface. The prior art is replete 
with suggestions for making LED printheads more compact but the 
suggestions provided by the prior art are far from satisfactory from a 
manufacturer's point of view in making such printheads. Typically, such 
printheads in addition to the one or more rows of LED's will include a 
series of integrated circuit driver chips that are connected to the LED's 
arranged in a row. The chip arrays may then be assembled end to end to 
form a single row of several thousand LED's. The driver chips may each 
include circuitry for receiving data signals and enabling the LED's 
selectively in accordance with such signals. Each driver chip may be 
suited for driving one half of the LED's in a chip array so that typically 
two driver chips are employed for driving a respective chip array of 
LED's. When these driver chips are mounted to either side of the row of 
LED chip arrays one group of the driver chips is used to drive 
odd-numbered LED's and the other group is used to drive even-numbered 
LED's. 
It is preferred from a manufacturing standpoint to mount the driver chips 
and LED's to a common surface of a support. In one example, it is known to 
mount three LED chip arrays of say 128 LED's each to a metal or ceramic 
tile with a corresponding respective number of driver chips for driving 
even and odd LED's. This assembly forms a module which may be tested and 
those modules deemed satisfactory may be mounted one after the other upon 
a printed supporting surface to form the printhead. 
One approach noted in the prior art is illustrated in U.S. Pat. No. 
4,767,172. In this approach, each LED is centered in a hemispherical 
cavity in a collector array in order that radiation from the LED enters 
the collector unrefracted. The collector array includes 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 a photoreceptor 
via a light pipe or optical wave guide secured to the collector. 
As noted in this patent this recorder is directly used with LED's that form 
broad light patterns and as such, are used for patch generation and for 
pitch and edge erasure on the surface of a photoreceptor. In the use of 
LED's for recording pictorial or alphanumeric images, LED's may be spaced 
300 or more to the inch. Providing a lens array as disclosed in the above 
reference thus represents many difficulties from the manufacturing 
standpoint. 
It is further known, see for example U.S. Pat. No. 4,907,034, to employ a 
gradient index fiber lens array such as a Selfoc lens, trademark of Nippon 
Sheet Glass Co., Ltd. to collect light from LED's and focus the light upon 
a receptor. An advantage of these arrays is that no one particular fiber 
needs to be registered or aligned with a particular LED. However, the 
spacing between the LED's and the object side of the lens array needs to 
be accurately made as well as does spacing between the image side of the 
array and the surface of the photoreceptor. Any errors in these spacings 
may be accommodated through the lens depth of focus capability. Light from 
each LED is collected by groups of fiber and focussed upon the 
photoreceptor. 
Thus, it is an object of the invention to preserve the convenience and 
desirability of using a gradient index fiber lens array in combination 
with LED's or other radiation-emitting recording elements in a narrow 
printhead. 
In addition, it is a further object of the invention to preserve the 
manufacturing convenience of continuing to manufacture LED arrays or other 
radiation-emitting recording elements and their associated driver chips 
upon a common surface of a substrate. 
It is, therefore, a further object of the present invention to provide a 
new and improved light collector for radiation from a plurality of 
recording elements. 
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. 
SUMMARY OF THE INVENTION 
In accordance with the invention, there is provided an apparatus for 
exposing a photosensitive member, the apparatus comprising a 
longitudinally extending printhead having a plurality of recording 
elements supported along the length thereof, driver means for providing 
driving current to said recording elements, support means for supporting 
said driver chips and recording elements, said recording elements and said 
driver means being supported by said support means substantially coplanar. 
A gradient index fiber lens assembly is provided extending longitudinally 
with said printhead in spaced overlying relationship therewith and having 
a series of optical fibers for conveying light generally parallel with the 
plane of said recording elements.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Because electrophotographic reproduction apparatus are well known, the 
present description will be directed in particular to elements forming 
part of or cooperating more directly with the present invention. Apparatus 
not specifically shown or described herein are selectable from those known 
in the prior art. 
In the following description similar reference characters refer to similar 
elements or members in all of the figures of the drawings. 
With reference to FIG. 1, printhead 20 contains a horizontally abutting 
series of modules. These modules include LED chip arrays 25 and driver 
chips 35 that are each mounted on a top surface of a tile or plate 65 
serving as a support for the module as well as a heat sink. The LED's and 
driver chips are shown enlarged relative to the other elements of the 
printhead to facilitate this description. Typically these chips are 
secured through use of a thin conductive adhesive layer (well known and 
not specifically shown) that has a good thermal conductance and if 
required (such as by the diodes) a good electrical conductance and which 
is applied to the underside of each chip and to appropriate locations on 
the top surface of the tile plate. The tile plate, in turn, is abutted 
against the top side edge surface of a base plate serving as a heatsink. A 
thin layer of conductive thermal paste (not shown) is situated 
therebetween. To facilitate air cooling, if needed, base plate 60 may have 
a number of downwardly projecting fins that run along its length. An 
intermediate plate may also be provided between the base plate and the 
tile plate. Each module contains, as will be described in detail below, a 
number, here three, of horizontally aligned LED arrays and accompanying 
driver circuits coupled together by tape automated bonds or by wire bonds. 
The diode arrays are situated along a central transverse axis of each 
module. 
To appropriately focus light generated by each individual diode onto a 
separate corresponding location along a transverse line on a surface of a 
rotating photoconductor, such as a photoconductive drum, D, a lens, 
L.sub.1, containing a transversely oriented array of optical fibers may be 
placed over and have a segment thereof in horizontal alignment with the 
vertically oriented LED arrays which form a horizontally aligned row of 
LED's. This optical fiber array is preferably a SELFOC graded index 
optical fiber array manufactured by Nippon Sheet Glass, Limited of Japan 
(which also owns the trademark SELFOC). 
While not shown, an interface board may be mounted to and modified as will 
be described further below to one end portion of the base plate 60 and 
contains appropriate input connectors and various signal processing and 
line driver integrated circuits (all of which are conventional, well known 
and for simplicity not shown in the figure). Alternatively, the interface 
board may be mounted along one or both main faces of the base plate 60. 
The interface board routes via spreader boards to be described appropriate 
digital data, clock and power signals to each of the modules that forms 
the printhead in order to energize individual LEDs therein in a proper 
temporal and positional sequence so as to provide an electrostatic charge 
pattern on the surface of the photoconductive drum, D, that, during a 
subsequent toning pass, will produce a desired visual image of 
alphanumeric or pictorial information on a piece of paper. A suitable 
termination board (not shown) may be similarly attached to still another 
end of base plate 60 at the opposite end of the printhead and is 
connected, also by wire bonds, to the opposite end of the series of 
spreader boards as is the interface board. The termination board contains 
well known line terminations, such as resistors or resistor/capacitor 
pairs or other electronic components, designed to balance the transmission 
line characteristics of certain individual daisy-chained signal lines 
which operate at a sufficiently high frequency that, if left unterminated, 
would suffer from well known unbalanced transmission line effects, such as 
impedance mismatches and signal reflections. The termination board may 
also contain power line decoupling capacitors. 
Signals to the driver chips from the interface board are distributed 
through spreader boards 40, two of which are associated with each module. 
To either side of the odd or even numbered LEDs, a series of vertically 
oriented spreader boards 40 are connected to each other in a daisy-chained 
arrangement, using for example wire-bonds or tape automated bonding (TAB). 
Wire bond pads (henceforth also referred to as "interconnect" pads) are 
provided along both vertical sides of each spreader board 40 to facilitate 
the formation of daisy-chain connections using relatively short wire bonds 
between adjacently situated spreader boards and between a first spreader 
board and an adjacently situated interface board 50 and between a last 
spreader board and an adjacently situated termination board. For a more 
complete discussion of tape automated bonding, the reader is referred to 
U.S. Pat. No. 4,851,862 issued July 25, 1989 and entitled "LED Array with 
TAB Bonded Wiring" which is owned by the present assignee and which is 
incorporated by reference herein. Further description of a printhead with 
the signal distribution referred to herein may be found in U.S. 
application Ser. No. 07/455,125, filed in the names of Beaman et al on 
Dec. 22, 1989, the contents of which are incorporated herein by this 
reference. These daisy-chained connections are used to distribute digital 
signals, such as data and clock signals, to the individual drive circuits 
contained within the module. Wire bond pads are also located along the top 
edge of each spreader board for use in connecting appropriate drive 
circuit terminations thereto. To substantially reduce the incidence of 
current starvation that may occur among individual LEDs along the 
printhead, power is distributed among the individual modules not by 
daisy-chained connections extending between adjacent spreader boards but 
rather through use of bus bars (not shown) that are connected in parallel 
to all the spreader boards used in both the odd or even halves of the 
printhead. These bus bars are connected to each spreader board near its 
bottom edge thereof. Each spreader board includes a multi-layered 
metalized cross-over wiring pattern that matches a pitch associated with 
appropriate terminations on the drive circuits to a pitch associated with 
the daisy-chained wire bond pads. Within each module, the LED chip arrays, 
illustratively three in number, are mounted directly to the substantially 
rectangular metallic, typically stainless steel, tile 65 in abutting 
alignment and along a common central transverse axis of that tile. 
Corresponding integrated circuit driver chips 35, illustratively six in 
number, are also mounted directly to the tile with three such driver chips 
35 located on each side of the LED arrays 25. The spreader boards, 
illustratively two in number, are mounted vertically one on each side edge 
of the tile 65 outward of the driver circuits. Wire bonds 41a, 41b, 
respectively, interconnect the spreader boards 40 with the driver circuits 
35 and the driver circuits with the LED arrays 25. The driver circuits and 
LED arrays 25 are all mounted to a common surface of a tile, with the 
opposite surface of the tile abutting against the top side edge surface of 
base plate 60. The printhead will include several thousand LED's arranged 
in a row which is directed perpendicular to the plane of the Figures 
shown. Each tile provides a common cathode connection to the LEDs mounted 
thereon as well as a path with a low thermal resistance (as compared to 
that possessed by a ceramic tile) to quickly conduct heat from the LED 
arrays and driver circuits through the tile 65 and into the base plate 60. 
The interface board is connected to the first module via its respective 
spreader board through wire bonds. Similar wire bonds, existing on the 
other side of spreader board interconnect this spreader board to its 
neighboring spreader board abuttingly situated thereat for distribution of 
signals to the next adjacent module. In this fashion, successively 
occurring modules running towards the rear end of the printhead and the 
termination board are interconnected with their immediately adjacent 
neighboring modules through wire bonds situated therebetween such that all 
the modules in the printhead receive their signals from the daisy-chained 
spreader boards, with the frontmost and rearmost spreader boards being 
respectively daisy-chained connected to the interface and termination 
boards, for purposes of propagating digital data and clock signals thereto 
from the interface board through all the modules to the termination board. 
As noted above, only certain data and clock signals that possess a 
sufficiently high frequency extend past the modules to and are terminated 
by the termination board. The above-noted three individual bus bars each 
have a relatively wide cross-sectional shape, as compared to the metalized 
leads on the spreader boards. Parallel connections are provided between 
the bus bars and each of the spreader boards to route power signals from 
the interface board, illustratively two different voltage levels (V.sub.cc 
and V.sub.dd) and ground, to each of these spreader boards. Identical 
daisy-chained wire bonds and identical bus bar assemblies are used in both 
the even and odd halves of the printhead to interconnect the spreader 
boards therein. 
As noted in FIG. 1, a Selfoc lens array, L.sub.1, (SLA) has been cut into 
two segments, A.sub.1, B.sub.1, respectively, as shown and mated back 
together where it may be secured along a common surface connection plane S 
by a suitable transparent adhesive. A mirror is coated upon surfaces 
P.sub.1, P.sub.2 of each segment, A.sub.1, B.sub.1, respectively, which 
surfaces align so as to be coplanar. Light rays from the LED's are 
collected by the first segment, B.sub.1, of the SLA which is horizontally 
directed in and out of the plane of the figure. This light is then 
reflected from the mirrored surface onto the vertically directed segment 
A.sub.1 of the SLA and focussed upon the photoconductive surface of the 
drum, D. As the LED's are selectively illuminated, based on signals from 
the driver chips, an appropriate electrostatic latent image is formed by 
modulation of the uniform electrostatic charge on the drum. This latent 
image may be developed with electroscopic toner and transferred to plain 
paper to form a permanent record of the image. 
In the embodiment of FIG. 2, an SLA, L.sub.2, has been also divided into 
two segments, A.sub.2, B.sub.2, as shown, but in this example a mirror has 
been placed between the horizontal and vertical segments of the SLA. The 
mirror directs light exiting from the first segment and directs such light 
into the second segment. The segments may be supported in the orientation 
by an angle bracket 70 to which the segments A.sub.2 and B.sub.2 are 
adhesively attached. The bracket 70 being attached to the printhead 20 
adjacent the ends thereof. 
In the embodiment of FIG. 3 an SLA, L.sub.3, has been also divided into two 
segments A.sub.3, B.sub.3 as shown but in this example, a prism has been 
placed between the horizontal and vertical segments of the LSA. The prism 
directs light exiting from the first segment and directs such light into 
the second segment. The prism may be secured to an end face of the 
respective segments of the SLA to secure the assembly without having an 
air interface. 
With reference now to the embodiment of FIG. 4, an SLA, L.sub.4, has been 
cut into two segments A.sub.4, B.sub.4, as shown with one surface of 
segment B.sub.4 being then coated with a mirror. It will be noted that the 
entire SLA is now oriented vertically and may be positioned closer to the 
LED's to provide a very narrow printhead construction. The mirrored 
surface reflects light from the LED's to the input end 75 of the object 
side of the segment A.sub.4. 
The embodiment of FIG. 5 is similar to that of FIG. 4 except that the input 
end 76 at the object side of the SLA segment A.sub.5 is cut with a convex 
curvature to enhance light collection. This embodiment also has a mirror 
surface on segment B.sub.5 to reflect light from the LED's into the input 
end 76 of the SLA. End 76 also collects light directly from the LED's . 
The segment A.sub.5, B.sub.5 may be secured by adhesive to a plate P which 
extends the length of the SLA and is coupled to the printhead 20. 
Similarly, such a plate may be used on the embodiment of FIG. 4 to secure 
segments A.sub.4, B.sub.4. 
The invention has been described in detail with particular reference to 
preferred embodiments thereof, but it will be understood that variations 
and modifications can be effected within the spirit and scope of the 
invention.