System for correcting shading or non-uniformity in a photosensitive element array

The invention is directed to a system for correcting shading or non-uniformities in the output of a linear photosensitive array. The invention employs a memory circuit having a number of cells corresponding to the number of photoelements positioned along the linear array. The sensor output of each respective element is successively compared with the data value stored in a corresponding memory cell in the memory. The data stored in the memory is updated by determining the larger data value signal and then storing that signal in the corresponding memory cell. This comparison and storage operation continues for each successive output signal produced by the linear array. The data stored in memory for each line represents the non-uniformity of the photosensor system including non-uniformities due to light source, lens, optical transmission and sensor characteristics. The stored data for each line is then converted in accordance with a weighting factor; the resulting converted data is multiplied by the received sensor output to produce sensor output response signals that compensate for the non-uniformities.

BACKGROUND OF THE INVENTION 
This invention relates to a system for correcting shading or non-uniformity 
which occurs in the output of an array of photosensitive elements used in 
picture image apparatus such as electric printers, facsimile or optical 
character recognition (OCR) systems. 
The phenomenon called shading occurs in the output of photosensitive 
elements. In accordance with this phenomenon the photosensitive array 
produces an output which varies as the position of the impinging light 
along the surface of the photoarray. FIG. 1 shows the characteristics of a 
linear sensor array for a given image tone. As shown in FIG. 1, the 
intensity of output signal will be higher at the center of the sensor 
array than at its edges. This produces undesirable results in the 
reproduced printed copy. That is, the original is inaccurately read and 
the printed copy has intensity graduations resulting in inaccurate 
reproduction of the original. In some cases, the output will vary more 
randomly due to the inherent nonuniform characteristics or sensitivity 
among the various array elements. This is a further factor causing an 
undesirable output. 
There are three recognized causes of shading or non-uniformities of sensor 
output: array elements having non-uniform sensitivity; non-uniformity of 
certain light sources (e.g., fluorescent lights); and inherent 
characteristics of lens systems to vary light intensity along surface of 
array. In particular, in constructing a sensor array it is virtually 
impossible to produce a multitude of sensor elements each having identical 
output characteristics; as a result, the output of each element will be 
different for a given image tone and intensity of impinging light. The 
resulting output has a random intensity depending upon the characteristic 
of each adjacent element even though each sensor may receive light from 
the same image tone. Without some correction, the resulting printed copy 
will have graduations in intensity and will be commercially undesirable. 
The second cause of non-uniformity are certain light sources. For example, 
fluorescent light sources produce an output intensity which is constant at 
its center portion but is substantially reduced at its edges. While 
attempts have been made to use a very long fluorescent source whereby only 
the centermost region supplies the desired light, this has necessarily 
resulted in a very large and impractical reproduction facsimile system. 
Some light sources, moreover, have characteristics which change due to 
sputtering of the filament caused by aging. As a result, the output of the 
source will vary nonuniformily along its length to a greater degree, 
producing even more severe degradation in reproduced printed copy. 
Finally, in some reproduction system, a lens system is used to focus the 
light image on the array. Such systems, however, compounded the 
non-uniformity problem discussed above due to the inherent characteristics 
of lenses. That is, a lens has the characteristic of transmitting light of 
varying intensity along its surface (See, e.g., Optics by Hiroshi Kubota, 
Iwanami Shoten Publishing Co., Toyoko, September 1975). 
One prior art method provides some correction for the characteristics shown 
in FIG. 1 by utilizing a correcting board positioned between the document 
and the lens system. The board is opaque and has a particular shape to 
gradually prevent passage of light from the centermost portion of the 
document to the detector. Consequently, the more intense light at the 
center will be reduced to substantially equal the light intensity at the 
edges of the detector. In attempting to achieve uniformity of light with a 
correcting board, it must be specially designed and oriented differently 
for each case since the characteristics of each light source is different. 
Practically, however, the board design has never achieved complete 
uniformity of output intensity for a given tone image and has required the 
costly and time consuming process, during manufacturing of adjusting and 
orienting each board. 
Another prior art method utilizes a lamp shading plate or cover which 
corrects for the non-uniformity of the lamp (see, Japanese Utility Model 
Publication No. 53-15211, Apr. 21, 1978). This method, however, does not 
correct for non-uniformity of the photoelements or aging of the lamp. 
A further prior art method employs an opaque slotted lamp cover plate (see, 
e.g., Japanese Utility Model Publication No. 52-16121, Apr. 12, 1977). The 
width of the slots are varied to correct for the non-uniformity light 
intensity of the lamp. this method does not correct for non-uniformity of 
the photoelements or aging of the lamp. 
A still further prior art method is disclosed in Japanese Utility Model 
Publication No. 28-12984, Dec. 28, 1953. This method utilizes two spaced, 
curved reflector plates, each positioned around a portion of the outside 
surface of the lamp adjacent a respective end. The reflector plate aids in 
slightly increasing the light intensity at the ends of the lamp. While 
some improvement is provided by the reflector plates, there remains 
substantial non-uniformity along the surface of the lamp. 
SUMMARY OF THE INVENTION 
One object of the present invention is to provide a system for correcting 
non-uniformities or shading errors in the output of a photosensor array. 
A further object of the present invention is to substantially eliminate 
shading caused by non-uniformities in the sensitivity of the sensor 
elements. 
A still further object of the present invention is to substantially 
eliminate shading caused by non-uniformities of certain light sources. 
Another object of the present invention is to substantially eliminate 
shading caused by the inherent characteristics of the lens system. 
Another further object of the present invention is to correct shading due 
to aging of the lamps. 
A still further object of the present invention is to correct shading 
without utilizing the costly and time consuming prior art methods 
previously discussed. 
The invention is directed to a system for correcting shading or 
non-uniformities in the output of a linear photosensitive array. The 
invention employs a memory circuit having a number of cells corresponding 
to the number of photoelements positioned along the linear array. The 
sensor output of each respective element is successively compared with the 
data value stored in a corresponding memory cell in the memory. The data 
stored in the memory is updated by determining the larger data value 
signal and then storing that signal in the corresponding memory cell. This 
comparison and storage operation continues for each successive output 
signal produced by the linear array. The data stored in memory for each 
line represents the non-uniformity of the photosensor system including 
non-uniformities due to light source, lens, optical transmission and 
sensor characteristics. The stored data for each line is then converted in 
accordance with a weighting factor; the resulting converted data is 
multiplied by the received sensor output to produce sensor output response 
signals that compensate for the non-uniformities.

DETAILED DESCRIPTION OF THE INVENTION 
The invention will now be explained with reference to the accompanying 
drawings. 
FIG. 3 is a block diagram for explaining one embodiment of the invention 
for correcting non-uniformity of the sensor output. As shown, an array 1 
of photosensitive elements are used to receive the light image. Array 1, 
for example, consists of approximately 1728 Charge Coupled Devices (CCD). 
An array of 1728 elements correspond to an A4 document size, while an 
array of 2048 elements correspond to a B4 document size. The light image 
reflected from the document impinges upon the elements which, in turn, are 
scanned by scanning signals from a scanning controller 2. As a result, 
photosensor output signals for each cell are successively generated. 
Controller 2 comprises a clock generator which synchronously supplies 
scanning pulses to shift the signal stored in each element and, thereby, 
scans array 1. The output signals from the photosensor elements are 
supplied to an A/D converter 3. Converter 3 converts these analog signals 
into digital signals having, for example, an 8 bit byte. 
The output signal of A/D converter 3 is then supplied to a comparing 
circuit 4. Comparing circuit 4 compares the digital signal for each 
element with data stored in a memory 5. Memory 5 has a number of memory 
cells corresponding to the number of photosensor elements. The output of 
each respective element is successively compared with the data value 
stored in a corresponding memory cell. The data stored in memory 5 is 
updated by determining the larger data value signal and then storing that 
signal in the corresponding memory cell. 
As discussed, memory circuit 5 has a number of addresses corresponding to 
the number of photosensor elements. For an A4 document size, the number of 
the elements is 1728 and, therefore, the number of addresses is 1728. For 
a B4 document size, the number of elements is 2048 and, therefore, the 
number of addresses is 2048. The desired cell of memory 5 is addressed by 
a counter 6 which successively addresses cells. Counter 6 receives the 
scanning signals from controller 2 so that the successive addressing of 
memory 5 occurs in synchronism with the scanning of array 1. Counter 6 is 
reset by a start signal from scanning controller 2 at the beginning of a 
scanning line. Therefore, when the output signal corresponding to a 
certain element is supplied to comparing circuit 4 the data value signal 
stored at a corresponding cell in memory 5 is synchronously supplied to 
circuit 4 for comparison. Circuit 4 determines the larger of the two 
signals and supplies that signal to this corresponding cell for storage. 
The comparison and storage operation continues for each successive output 
signal provided by linear array 1. The data stored in memory 5 will 
approximate the curve of FIG. 1; thus, the data represents the 
non-uniformity of the photosensor system including the non-uniformities 
due to the light source, lens optical transmission and sensor 
characteristics. As will be discussed, the system is designed to obtain 
the maximum intensity values at each element location such that the stored 
values represent the intensity values for a particular image tone. In this 
way, stains or noise on the document will not affect the correcting 
process of the invention. This correcting process is accomplished by 
comparision circuit 4 which updates the data value stored in a 
corresponding cell if a larger photoelement data value is detected. That 
is, when the next adjacent line is scanned the digital output signal of a 
photoelement will be stored in the corresponding memory cell if the output 
signal value is greater than the previously stored value for that element. 
Since documents generally include some white background portions or 
borders, these portions will produce the greatest intensity values. Thus, 
the system will generally store data values in memory 5 which represent 
the light intensity received by the array from a uniform white background. 
Thus, memory 5 will store data representing the non-uniformity of the 
photosensor system despite the presence of stains, blemishes, noise or 
other dark tones on the document. 
In summary, the comparison updating process is utilized so that a data 
value is obtained for each element which represents the light intensity 
from a given image tone. While it is not possible in all cases for each 
sensor to receive light, at least once, from a given tone, the updating 
process will permit storage of values which approximate the curve of FIG. 
1. 
The stored data is then converted in accordance with a weighting factor 
stored in ROM 8. Each resulting converted data is then multipled by its 
corresponding sensor output to produce output response signals for line 
that compensate for the non-uniformities of the system. In particular, the 
output of memory 5 is supplied to ROM 8 which stores the weighting factor: 
y=a/x, where a is a constant, x is the intensity level data supplied from 
memory 5, and y is the converted output data. One input of multiplier 8 
receives the output of A/D converter 3 and the other multiplier input 
receives the output of ROM 8. As can be seen from the intensity values I 
of the response curve (FIG. 1) and the weighting factor equation, the 
weighting factor y will be smallest at the center M of the document and 
will uniformly increase toward either end (R, L). Thus, the photosensor 
output signal from the centermost element M will be multiplied by the 
smallest weighting factor Y.sub.M, while rightmost and leftmost element 
outputs will be multiplied by the largest weighting factor Y.sub.R and 
Y.sub.L, respectively. Consequently, for a given tone, the output signals 
of multiplier 7 will be substantially uniform. That is, for a line having 
a given tone, the output signals will be substantially equal. However, 
while the output signals will vary for a line having various tones, the 
difference among the signal levels will be due only to the various tones 
and not to the shading errors. 
Also shown in FIG. 3 is a clear circuit 9, coupled to memory 5, for 
clearing the memory upon turning on the power source. Power switch 10 
turns on the power in response to activating microswitch 11. In 
particular, microswitch 11 senses the presence of a document 12 on 
document reader guide 3 and activates power switch 10 which turns on clear 
circuit 9 for clearing memory 5. 
FIG. 4 describes, in detail, comparing circuit 4 which comprises comparator 
41, gating circuit 42, and inverter 43. Comparator 41 receives an 8 bit 
signal A from the A/D converter 3 and an 8 bit signal B from memory 5; 
signals A and B are compared. If A&gt;B, a high level signal is supplied on 
line 44, and signal A and signal B are supplied to gating circuit 44. If 
A.ltoreq.B, a low level signal is supplied on line 44, and signals A and B 
are supplied to gating circuit 44. 
Gating circuit 42 comprises 8 sets of gates, each gate set corresponding to 
a bit and comprising a first AND gate 45, a second AND gate 46, an OR gate 
47 and an inverter 48. The remaining set of gates are not shown in FIG. 4. 
The first bit line of signal A is supplies to one input terminal 45i of 
gate 45, and the first bit line of signal B is supplied to one input 
terminal 46i of second AND gate 46. The second bit line of signal A is 
supplied to one input of a first AND gate of the next gate set, while the 
second bit line of signal B is supplied to the other input of the first 
AND gate and second AND gate of the second set. The gate signal from 
inverter 43 is supplied via inverter 48 to the other input terminal of AND 
gate 45, and is also supplied to the other input terminal of AND gate 46. 
The output signals of AND gate 45 and AND gate 46 are supplied to memory 5 
via OR gate 47. Therefore, if a high level signal is supplied from 
comparator 41 on line 44, AND gate 45 will be activated so that output 
signal A will be supplied via gate circuit 42 and stored in memory 5. If, 
however, a low level signal is supplied on line 44 from comparator 41, AND 
gate 46 and the other corresponding AND gates for signal B will be 
activated so that the output signal B will be supplied via gate circuit 42 
and stored in memory 5. 
With references to FIGS. 3-4, the operation of the system will be 
explained. The output of memory 5 is an 8 bit signal (0-255). The input to 
ROM 8 is signal x from memory 5 such that ROM 8 produces an output y 
wherein y=a/x. The output y is a 14 bit signal (0-16383), wherein the 
constant a=16,384. If the x address in ROM 8 is 96 (i.e., x=96), then the 
y output of ROM 8 is 171. That is, 16384/96=170.6, where 170.6 is rounded 
off to its nearest whole number. If the x address is 128, the y output 
will be 128. Likewise, if the x address is 192 the y output is 85. 
The remaining values for y are likewise stored in ROM 8 at their respective 
addresses with only the following two exceptions. First, for the x=0 
address, the y value is selected as 0. Second, for the x=1 address, the y 
value is selected as 16383, rather than 16384, since the ROM output is 
designed for 14 bits. 
With reference to FIGS. 1-2, the x=96 address corresponds to element L, the 
x=192 address corresponds to element M. In the case from white paper, 
element L produces an intensity output value from A/D converter 3 of 96 
(i.e., I.sub.L), element N produces a value 128 (i.e., I.sub.N), and 
element M produces an output value of 192 (i.e., I.sub.M). The 
corresponding outputs of multiplier 7 for each of these elements are, 
therefore, 16416 (171.times.96) for element L, 16384 (128.times.128) for 
element N, and 16320 (192.times.85) for element M. Consequently, the 
output signals are substantially equal for a given tone, thus the shading 
errors are substantially eliminated. 
If a gray document with a continuous gray tone boarder immediately follows 
the white document, the output of A/D converter 3 will be 48, 64 and 96, 
respectively, for L, N and M. This is because the gray tones produce half 
the output intensity of the white tones. Since this document immediately 
follows the white document, memory 5 is not cleared, the corresponding 
data values stored in memory 5 are 96, 128 and 192, respectively. Since 
the stored values are greater than the output values of A/D converter 3, 
the existing stored values in memory 5 will not be updated. Consequently, 
the respective output signals from ROM 5 will remain 171, 128 and 85. As a 
result, the respective outputs of multiplier 7 will be half as much as was 
produced for the white document: 8208, 8192 and 8160. Likewise, these 
output signals are substantially equal for the gray tone line; 
consequently the shading errors are substantially eliminated. The pattern 
images which follow the gray border and are subsequently read by the 
sensors will likewise have their corresponding output signals modified to 
reduce the effects of shading errors. The weight of the converted output 
signals produced by ROM 5 will likewise depend upon the particular 
photoelement output signals that are being processed in accordance with 
the expression y=a/x. 
It is desirable in utilizing the correction system of the instant invention 
to store the complete and accurate shading or non-uniformity pattern 
(e.g., FIG. 1) before the image or pattern information is read to 
accurately compensate for the non-uniformities. In most cases, this will 
be done. Generally, documents have an upper horizontal border consisting 
of a non-black single tone (e.g., white) before the image or pattern 
begins. In that case, the initial data values produced and stored in 
memory 5 will correspond to FIG. 1 and accurately portray the 
non-uniformity pattern. If the document has no upper border space, or is 
black with a white pattern, or has excessive noise, it would not be 
possible to obtain complete and accurate shading information until the 
document is nearly completely scanned. As a result, complete compensation 
for shading will not occur. To overcome that problem, the invention could 
incorporate a white roller 62 as shown in FIG. 5. As shown, a roller 62 
having a white surface is positioned along the path of the document. The 
document moves along a path from guide plate 61 via feed rollers 63, 
transport pinch rollers 64 and output supply rollers 66. Roller 62 is 
positioned to reflect light from lamp 65 onto sensor array 1. Thus, as the 
document is being transported toward this reading position, light 
reflected from the single tone surface of roller 62 is processed by the 
system of the invention to record the non-uniformity pattern of the 
system.