Image reader for an image recording apparatus

An image reader for an image forming apparatus has an array of CCD sensors in the form of chips which are arranged in a zigzag configuration along an image reading line. A reference white plate is located in association with the position of and outside of the effective image reading range of one of two CCD sensors which are situated at opposite ends of the array. The CCD sensor with which the reference white plate is associated produces a reference white signal by reading the reference white plate. The output gain of this CCD sensor is controlled on the basis of the reference white signal. The reference white signal is added to output signals of the other CCD sensors in the form of a pilot signal, whereby the output gains of the other CCD sensors are controlled.

BACKGROUND OF THE INVENTION 
The present invention relates to an image reader for use in an image 
recording apparatus. 
An electrophotographic copier, facsimile machine, laser printer or similar 
image recording apparatus has an image reader for reading an original 
image to be recorded. A predominant type of image reader is implemented as 
a one-dimensional line sensor in the form of a CCD array. A problem with 
this kind of image reader is that the output of the line sensor associated 
with an original image is often effected by various occurrences such as a 
change in the quantity of light issuing from a light source which 
illuminates the image. In light of this, it has been customary to locate a 
reference white plate in association with a portion of the line sensor 
outside of an effective pixel reading range and to control the gain of the 
output voltage of the line sensor by a white signal which is derived from 
the reference white plate. Another type of image sensor heretofore 
proposed is a fragmentary image sensor which is usable with a 1:1 
magnification optics type image reader. Elaborated to increase the 
effective pixel reading length, the fragmentary image sensor has a 
plurality of CCD chips which are arranged in a zigzag array. This kind of 
image sensor also needs the above-stated gain control and, in this 
respect, has a problem unsolved. Specifically, assume a fragmentary image 
sensor made up of four independent CCD sensor chips by way of example. 
Then, while two of the four CCD chips located at opposite ends of the 
array can have their gain controlled by locating two reference white 
plates at the opposite ends of the array, the gain control is not 
practicable with the other two CCD chips intervening between the end chips 
because reference white plates would interfere with the read-out of an 
original image in the effective pixel reading range. Hence, should the 
quantity of light issuing from the light source change, the output 
voltages of the intermediate chips would change resulting in the quality 
of a recording being degraded. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide an image 
reader for use in an image recording apparatus which prevents the quality 
of a recording from being degraded. 
It is another object of the present invention to provide an image reader 
for an image recording apparatus which controls the gains of the outputs 
of individual sensor fragments with accuracy. 
It is another object of the present invention to provide a generally 
improved image reader for an image recording apparatus. 
An image reader for an image forming apparatus of the present invention 
comprises a fragmentary image sensor comprising a plurality of sensors 
which are arranged in an array along an image reading line, a reference 
white plate located in association with a particular position of and 
outside of an effective pixel reading range of one of two of the sensors 
which are situated at opposite ends of the image reading line, the one 
sensor producing a reference white signal by reading the reference white 
plate, and control circuitry for controlling an output gain of the one 
sensor in dependence on the reference white signal.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1 of the drawings, a one-dimensional line sensor 
implemented as a CCD array and constituting an image sensor of an image 
reader embodying the present invention produces an image signal V.sub.op. 
As shown, the image signal V.sub.op is generated while being controlled as 
to the timing by shift pulses V.sub.sp, carrier pulses .PHI..sub.1a, 
.PHI..sub.2a and .PHI..sub.2b, and reset pulses P.sub.r. Usually, the 
image signal V.sub.op is an analog voltage having an offset voltage 
V.sub.off and which is outputted together with a signal generally called 
reset noise. The image signal V.sub.op is headed by a dummy signal 
representative of seventy-two pixels, i.e., twelve pixels D0 to D11 for 
idle feed, forty-eight pixels D12 to D59 for light shield, and twelve 
pixels fD60 to D71 for idle feed. The dummy signal is followed by a color 
effective pixel signal representative of 2,688 pixels which are associated 
with green (G), blue (B) and red (R). Hence, a one line outputting time is 
associated with 2,760 pixels in total. 
As shown in FIG. 2, in the illustrative embodiment, a fragmentary CCD array 
10 for reading pixels has a framework 10a and a plurality of CCD chips, 
e.g. four CCD chips CCD1 to CCD4 as illustrated. The CCD chips CCD1 to 
CCD4 are arranged in a zigzag configuration along a pixel reading line of 
the framework 10a while overlapping each other. The CCD chips CCD1 to CCD4 
have the same effective reading length l, the sum of the lengths l 
defining an effective reading length L (=4l). The CCD chips CCD1 to CCD4 
produce outputs at the same time. 
Referring to FIGS. 3A and 3B, there is shown the construction of the image 
reader including the CCD array 10. As shown, while a fluorescent lamp 14 
illuminates an original document 12, a reflection from the document 12 is 
incident to the CCD array 10 through a lens array 16. In the illustrative 
embodiment, a reference white plate 18 is associated with, of the two CCD 
chips CCD1 and CCD4 which are located at opposite ends of the array, the 
CCD chip CCD1. Specifically, the reference white plate 18 is located in 
association with one end portion of the CCD chip CCD1 which is outside of 
the effective image region of the latter, i.e., a non-image signal region, 
so that it may be illuminated by the lamp 14 in place of the document 12. 
Hence, while the lamp 14 is turned on, the part of the CCD chip CCD1 which 
is associated with the reference white plate 18 continuously produces a 
reference white signal. 
As shown in FIG. 3A, the output V.sub.os1 of the CCD chip CCD1 is applied 
to a variable-gain amplifier 20 (1) which then outputs a voltage 
V.sub.out1. The output V.sub.out1 of the amplifier 20 (1) is fed back to 
the amplifier 20 (1) via a sample and hold circuit 22 (1) and an error 
amplifier 24 (1). The sample and hold circuit 22 (1) is caused to hold the 
instantaneous voltage by a sample and hold signal V.sub.sh. This allows 
the amplification factor of the amplifier 20 (1) to be varied in 
dependence on the output voltage V.sub.err of the error amplifier 24 (1). 
Specifically, the amplification factor increases and decreases with the 
voltage V.sub.err. The error amplifier 24 (1) amplifies a difference 
between the output V.sub.sh0 of the sample and hold circuit 22 (1) and the 
reference voltage V.sub.ref which is generated by a reference voltage 
generator 26. 
Variable-gain amplifiers 20 (2), 20 (3) and 20 (4) are associated with the 
other CCD chips CCD2, CCD3 and CCD4, respectively. The gains of the 
amplifiers 20 (2), 20 (3) and 20 (4) are variable in association with the 
outputs of error amplifiers 24 (2), 24 (3) and 24 (4), respectively. While 
the CCD chips CCD2 to CCD4 individually produce outputs V.sub.so2 to 
V.sub.so4, the amplifiers 20 (2) to 20 (4) individually produce outputs 
V.sub.out2 to V.sub.out4. Sample and hold circuits 22 (2), 22 (3) and 22 
(4) are associated with the error amplifiers 24 (2), 24 (3) and 24 (4), 
respectively. 
In the illustrative embodiment, a pilot signal generator 28 generates a 
pilot signal V.sub.pl0 in response to a pilot generate signal V.sub.pl and 
based on the output V.sub.os1 of the CCD chip CCD1. The pilot signal 
V.sub.pl0 is added to the individual outputs V.sub.os2, V.sub.os3 and 
V.sub.os4 by adders 30, 32 and 34, respectively, at the input side of the 
variable-gain amplifiers 20 (2) to 20 (4). 
The operation of the image reader having the above construction will be 
described with reference to FIGS. 4A to 4I. The output V.sub.os1 of the 
CCD chip CCD1 constantly involves a reference white voltage which is 
associated with the reference white plate 18. The sample and hold circuit 
22 (1) holds the reference white voltage in response to the sample and 
hold signal V.sub.sh to thereby produce a reference white signal V.sub.w 
(FIG. 4H). The reference white signal V.sub.w is a signal held by the 
sample and hold signal V.sub.sh and is outputted as one form of the sample 
and hold signal V.sub.sh0 (FIG. 4E). The pilot signal generator 28 
generates the pilot signal V.sub.pl0 (FIG. 4I) in response to the pilot 
generate signal V.sub.pl (FIG. 4F) and prior to the appearance of the 
reference white signal V.sub.w which forms a part of the output V.sub.os1. 
The pilot signal V.sub.pl0 is added to the outputs V.sub.os2, V.sub.os3 
and V.sub.os4 (FIGS. 4B to 4D) of the CCD chips CCD2, CCD3 and CCD4 by the 
adders 30, 32 and 34, as stated earlier. The added pilot signal V.sub.pl0 
is indicated by phantom lines in FIGS. 4B to 4D. Since the light shield 
portion is used for a black reference, the pilot signal V.sub.pl0 may be 
added to the outputs V.sub.os2 to V.sub.os4 at any desired timing at which 
the image signal is absent. The added pilot signal V.sub.pl0 is sampled 
and held by the individual sample and hold circuits 22 (2) to 22 (4). It 
is to be noted that, although the added pilot signal V.sub.pl0 occurs at 
the same timing as the pilot generate signal V.sub.pl and is directly 
implemented by the latter in practice, the former is shown as being 
independent of the latter for facilitating an understanding of the 
operation. The voltages held by the sample and hold circuits 22 (2) to 22 
(4) are compared with the reference voltage V.sub.ref by the associated 
error amplifiers 24 (2) to 24 (4). The outputs of the error amplifiers 24 
(2) to 24 (4) are then individually fed to the variable-gain amplifiers 20 
(2) to 20 (4), thereby controlling the gains of the amplifiers 20 (2) to 
20 (4) adequately. 
As stated above, when the output of the CCD chip CCD1 is varied due to a 
change in the quantity of light issuing from the lamp 14, a change in the 
circuit conditions or aging, for example, the variation is cancelled by 
the gain control associated with the reference white signal which is 
derived from the reference white plate 18. As regards the other CCD chips 
CCD2 to CCD4 which lack the reference white plate 18, the pilot signal 
V.sub.pl0 associated with the reference white signal is added to their 
outputs so as to implement the gain control in the same manner as with the 
CCD chip CCD1. This eliminates a difference in level between the chips 
CCD1 to CCD4 and thereby a density difference between the CCD chip 
outputs. 
Assume a standard condition wherein the reference voltage V.sub.ref is 1 
volt and the reference white signal V.sub.w is 0.99 volt, for example. 
Then, the error output voltage V.sub.err of the error amplifier 24 (1) is 
produced by: 
EQU V.sub.err =A.(V.sub.ref -V.sub.w) 
where A is the amplification factor of the error amplifier 24 (1). 
Assuming that A is "100 (times)" and the error voltage V.sub.err is 
100.(1-0.99)=1 volt. The voltage V.sub.err and the amplification factor of 
the variable-gain amplifier 20 (1) are proportional to each other, as 
shown in FIG. 5. Hence, the error voltage of 1 volt is "1" in terms of 
amplification factor. Assume that the reference white signal V.sub.w is 
lowered to 0.98 volt, for example, due to a decrease in the quantity of 
light issuing from the lamp 14. Then, the gain control system operates in 
such a manner as to satisfy the above equation, so that the gain of the 
variable-gain amplifier 20 (1) is stabilized at approximately "1.01 
(times)", i.e. 0.99/0.98.perspectiveto.1.01. That is, the error voltage 
V.sub.err is 1.01 volts. Calculating the reference white signal V.sub.w 
backward, 
EQU 0.01=100.(1-V.sub.w).fwdarw.=1-(0.01/100).perspectiveto.0.99 (volt) 
Thus, the standard reference white signal V.sub.w is obtained. 
This is true with the other CCD chips CCD2 to CCD4 also. Specifically, the 
above-stated gain control is effected with the outputs V.sub.os2 to 
V.sub.os4 by detecting the pilot signal V.sub.pl0, preventing any change 
in the quantity of light from influencing the image signal. Further, even 
a change in the gain of the circuit associated with any of the CCD chips 
which is ascribable to a change in power source voltage or similar cause 
is absorbed. 
In summary, it will be seen that the present invention provides a 
fragmentary sensor type image reader for an image forming apparatus which, 
with a single reference white plate, eliminates a level difference and 
therefore a density difference between individual sensor chips while 
absorbing the influence of a change in the quantity of light issuing from 
a light source or the like on an image signal, i.e. image quality. 
Various modifications will become possible for those skilled in the art 
after receiving the teachings of the present disclosure without departing 
from the scope thereof.