Document reader with light source control

A document reader has a light source which is turned on upon energization, a read unit for reading an image of a document illuminated by the light source and a control circuit for controlling the energization of the light source. The read unit reads an intensity of a light reflected by a reference density pattern arranged externally of a document area prior to reading of the document and controls the energization of the light source in accordance with the detected light intensity.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a document reader for converting image 
information to an electric signal by scanning a document by an image 
pick-up device such as a charge storage type of photo-electric converter. 
2. Description of the Prior Art 
A prior art document reader of the type described above is shown in FIG. 1, 
in which numeral 1 denotes a fluorescent lamp drive circuit which is fed 
from an A.C. power supply, numeral 2 denotes a document illumination 
fluorescent lamp energized by the drive circuit 1, numeral 3 denotes a 
document illuminated by a light from the fluorescent lamp 2, numeral 4 
denotes a lens system for focusing a reflected light including pixel 
information on the document 3 illuminated by the fluorescent lamp 2 onto a 
photosensing plane of an image pick-up device 5, numeral 6 denotes an 
image signal amplifier for amplifying an image signal produced by the 
photo-electric conversion by the image pick-up device 5, numeral 7 denotes 
an A/D converter for converting the image signal to a digital image 
signal, and numeral 8 denotes an automatic gain control circuit for 
compensating a variation of an output level of the digital image signal 
from the A/D converter 7. The level of the photo-electrically converted 
digital signal is kept constant by the automatic gain control circuit 8 
and an image signal is sequentially taken out therefrom. 
When the pixel information of the document 3 is photo-electrically 
converted, a signal output level changes depending on a luminance of the 
fluorescent lamp 2 and a background density of the document, and 
significantly varies depending on an amplification of the amplifier 6. The 
automatic gain control circuit 8 is used to compensate for the variation 
of the signal output level. For example, the background density of the 
document bearing the pixel information, that is, a white background area 
of the document is detected and the output signal level is kept constant 
based on the signal level of the white background area. An ordinary 
document includes the white area in each of scan lines and the signal 
level is maximum in the white area. Thus, the variation of the image 
signal output level is compensated by keeping the maximum value of the 
photo-electrically converted signal at a constant level. 
However, when a document including a half-tone image in an entire area such 
as a photograph is to be read, a reference background area (white area) 
which an ordinary document includes is not present and a maximum level of 
the photo-electrically converted signal may change from scan to scan. 
Accordingly, if the level of the photo-electrically converted signal is 
controlled by the prior art method, the density of the reproduced image 
varies locally and an exact half-tone image is not reproduced. 
Accordingly, for the half-tone image such as the photograph, it is 
necessary to keep the amplification factor of the photo-electrically 
converted signal at a constant level during the reading of the document. 
If a surrounding temperature or a power supply voltage changes or the 
luminance of the fluorescent lamp changes by degradation during this 
period, the level of the photo-electrically converted signal varies and 
the image is not exactly reproduced. 
Further, the reading operation of the image pick-up device is influenced by 
the light adjustment operation of the light source and a nonuniform image 
may be reproduced. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a document reader which 
can exactly read a document. 
It is another object of the present invention to provide a document reader 
which can constantly set a luminance of a light source for reading a 
document at a constant level. 
It is another object of the present invention to provide a document reader 
which can control a luminance with a simple circuit configuration without 
using a sensor for sensing the luminance of a document illumination light 
source. 
It is a further object of the present invention to provide a document 
reader which reads a document when a read condition is met. 
It is a still further object of the present invention to provide a document 
reader which eliminates a problem of image reading due to flickering of a 
light source. 
The above and other objects and advantages of the present invention will be 
apparent from the following description of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 2 diagrammatically shows a schematic sectional view of a document 
image reading unit R in one embodiment of the present invention. A 
document mount GD made of glass is arranged at a top of the reading unit R 
and a user mounts a document to be reproduced on the document mount GD. 
Arranged under the document mount GD are fluorescent lamps L1 and L2 for 
illuminating the document, reflection mirrors RM1 and RM2 arranged such 
that the lights emitted from the fluorescent lamps L1 and L2 efficiently 
illuminate the document surface mounted on the document mount GD, first 
and second plane mirrors PM1 and PM2 for scanning (sub-scan) the document 
and a one-dimensional charge coupled device (CCD) for reading light 
transmitted through optical lens OPL which focuses the optical image of 
the document. 
The light sources L1 and L2, the reflection mirrors RM1 and RM2 and the 
first plane mirror PM1 are supported in union by a support ST which is 
fixed to a carriage CA1. The carriage CA1 is reciprocated along a guide 
rail GL from left to right (forward direction) and from right to left by a 
known drive means. The second plane mirror PM2 is moved along the guide 
rail GL by a carriage CA2 in the same direction as the first plane mirror 
PM1 at one half speed of that of the first plane mirror PM1. At the end of 
the forward movement, the plane mirrors PM1 and PM2 have been moved to 
positions PM1' and PM2' shown by broken lines. An optical path length from 
the document mount GD to the image pick-up device CCD through the plane 
mirrors PM1 and PM2 and the lens OPL is always kept constant. 
A main scan direction of the image pick-up device CCD is normal to the 
plane of the drawing. If the signals from the photo-sensing elements of 
the image pick-up device CCD are sequentially read out in a proper order 
during the forward movement of the plane mirrors PM1 and PM2, a sequential 
signal which raster-scan the document is produced. 
A solid line position of the support ST indicates a home position at the 
start of document reading. When the support ST is at the home position ST, 
a white area WB arranged externally of a document mount area of the 
document mount GD is scanned in order to produce a reference read signal. 
FIG. 3 shows one embodiment of a document reader of the present invention, 
in which the like elements to those of FIG. 1 are designated by the like 
numerals. Numeral 9 denotes a light adjustment circuit which controls a 
power supplied to the fluorescent lamp 2 by controlling a duty factor of a 
high frequency power supply, numeral 10 denotes a white area arranged 
externally of the document mount area of the document mount for the 
document 3 in order to produce a reference read signal, and numeral 11 
denotes a comparator which compares a 6-bit image signal from the white 
area 10, which is supplied from an A/D converter 7 by a clock pulse .phi. 
with a preset 6-bit white reference pixel signal. It produces a compare 
output when the input reference pixel signal is larger than the white 
reference pixel signal. The compare output is supplied to an up-down 
control input terminal U/D of an up-down counter 12, which counts clock 
pulses applied to a terminal CLK in a direction determined by the input 
signal to the input terminal U/D in synchronism with a horizontal 
synchronizing signal HS applied at every beginning of one line of image 
reading, and produces a count output at an output terminal Q. Numeral 13 
denotes a clock counter which counts the clock pulses applied to the 
terminal CLK in synchronism with the horizontal synchronizing signal HS 
which instructs the start of storage of the image pick-up device 5 for 
each line, and produces a count output at an output terminal Q. The count 
outputs from the counters 12 and 13 are supplied to a comparator 14 which 
produces a compare output when both count outputs are equal. The compare 
output is supplied to a reset input terminal R of a flip-flop 15 and the 
horizontal synchronizing signal HS is supplied to a set input terminal S 
of the flip-flop 15, which produces a control signal at an output terminal 
Q in synchronism with the horizontal synchronizing signal HS. The control 
signal is then supplied to the light adjustment circuit 9. 
The circuits 11, 12, 13, 14 and 15 form an automatic gain control circuit 
16 for the light adjustment circuit 9. 
By the above circuits, a level change of the photo-electrically converted 
signal of the white area 10 due to a luminance change of the fluorescent 
lamp 2 is detected by the comparison with the white reference pixel signal 
of the pixel under consideration in one line by the comparator 11. In 
accordance with the compare output, the light adjustment circuit 9 changes 
the duty factor of the high frequency power supply to be applied to the 
fluorescent lamp 2 to control the luminance of the fluorescent lamp 2. 
This is more specifically described below. 
When the flip-flop 15 is set by the horizontal synchronizing signal HS, the 
control signal output Q is kept at H level until the count outputs of the 
counters 12 and 13 coincide, that is, until the compare output signal from 
the comparator 14 assumes the H level, and the output Q is supplied to the 
light adjustment circuit 9. As a result, the fluorescent lamp 2 is turned 
on and the image reading by the image pick-up device 5 is started. The 
image signal stored in the image pick-up device 5 is read out by a scan 
clock not shown, supplied to the A/D converter 7 through the amplifier 6 
and converted to the digital image signal. 
A duty factor of the fluorescent lamp 2 in one scan period is illustrated 
in FIGS. 4(A)-(C). FIG. 4(A) shows the reference clock pulse train, FIG. 
4(B) shows the horizontal synchronizing signal HS and FIG. 4(C) shows the 
output Q of the flip-flop 15. The fluorescent lamp 2 is turned on during a 
period of m clocks in one scan period starting from the fall of horizontal 
synchronizing signal HS and turned off during a period of n clocks. 
Accordingly, the duty factor of the fluorescent lamp 2 is given by 
EQU duty factor=m/(m+n).times.100 (%) 
Let us assume that the level of the white reference pixel signal applied to 
the terminal B of the comparator 11 is preset to (000001) and a pixel 
level read from the white area when the number of clocks is m, which is 
applied to the terminal A of the comparator 11, is (000010), that is, the 
luminance is under. Those levels are compared by the comparator 11. Since 
A&gt;B, the comparator 11 produces the H-level output which is supplied to 
the up-down counter 12, which counts up the clock signal CLK so that the 
count reaches m+1. This count m+1 is set to the up-down counter 12 as the 
turn-on time of the fluorescent lamp in the next scan period. At the start 
of the next scan, the counter 13 starts to count the clock in synchronism 
with the horizontal synchronizing signal HS. The setting of the up-down 
counter 12 and the count (output Q) of the counter 13 are compared by the 
comparator 14. The output of the comparator 14 is at the H-level until the 
output Q of the counter 13 reaches m+1, and output Q of the flip-flop 15 
is at the H-level from the fall of the horizontal synchronizing signal HS 
until the output of the comparator 14 assumes the L-level. The light 
adjustment circuit 9 turns on the fluorescent lamp 2 during the input of 
the H-level output Q. 
When the output Q of the counter 13 reaches m+1, it coincides with the 
setting of the up-down counter 12 and the comparator 14 produces the 
L-level output and the flip-flop 15 produces the L-level output. As a 
result, the light adjustment circuit 9 turns off the fluorescent lamp 2. 
The charge stored in the image pick-up device 5 is again compared by the 
comparator 11, and if it is still smaller than the white reference signal, 
the up-down counter 12 is set to the count-up mode so that the count is 
incremented to m+2. As a result, the next illumination time is extended by 
one count period. 
The above operation is repeated so that the turn-on time of the fluorescent 
lamp 2 in one scan period in incremented until the pixel level read from 
the white area 10 coincides with the white reference level, that is, until 
the comparator 11 produces the L-level output. In this manner, the duty 
factor of the fluorescent lamp 2 is controlled by utilizing the white area 
10 so that the photo-electrically converted signal of a constant level is 
produced. 
If the charge stored in the image pick-up device 5 is larger than the white 
reference signal, the up-down counter 12 is set to a count-down mode to 
decrement the count so that the turn-on time in the next scan is reduced. 
After the luminance has been set to the desired level through a certain 
number of times of scan of the white area 10, the reading of the document 
is started. 
While the white area is used to produce the reference pixel signal in the 
present embodiment, it is not limited to the white area but a half-tone 
area may be used to produce the reference pixel signal. By changing the 
preset level of the reference pixel signal of the comparator 11 depending 
on the density of the half-tone area, the same luminance control as that 
for the white area is attained. By providing the white and half-tone areas 
and selectively using them in accordance with a particular document, an 
image which is closer to the document pixel level can be reproduced. The 
document illumination light source is not limited to the fluorescent lamp 
but it may be another discharge tube such as a halogen lamp or any other 
light source. 
The reference pixel signal may be an average of the pixel signals read in 
one scan period of the white area. 
As described above, in the present embodiment, the preset reference density 
level and the pixel level read from the corresponding density area are 
compared and the duty factor of the turn-on signal supplied to the light 
adjustment circuit is controlled by the compare result to control the 
luminance of the illumination light source. Accordingly, the constant 
level of photo-electrically converted signal is always produced and an 
excellent quality of image is reproduced. 
FIG. 13 shows another embodiment of the automatic gain control circuit 16 
of FIG. 3. When a light adjustment start signal for the fluorescent lamp 
is applied in the read operation of the document, flip-flops 26 and 29 are 
cleared and an up-down counter 22 is set to a count mode to count the 
horizontal synchronizing signal HS. The light adjustment start signal also 
acts as a charge store start signal for the image pick-up device 5. 
A count n corresponding to a normal turn-on time is preset to the up-down 
counter 22. A latch 23 supplies the count n preset in the up-down counter 
23 to an input terminal B of a comparator 24 in synchronism with a clock 
from an AND gate 27 a count in a counter 25 which starts to count clocks 
in synchronism with the horizontal synchronizing signal HS is applied to 
an input terminal A of the comparator 24, which compares the count A with 
the count from the latch 23 and supplies the H-level output to a flip-flop 
26 when the count to the latch 23 is larger (A&lt;B). The flip-flop 26 
supplies the H-level signal (output Q) to the light adjusting circuit 9 in 
synchronism with the clock input when the H-level signal is applied 
thereto. 
On the other hand, when the count of the counter 25 is equal to the count 
of latch 23, the comparator 4 produces the L-level output at a terminal 
A&lt;B and the output Q of the flip-flop 26 synchronized with the clock 
assumes the L-level. 
If the charge stored in the image pick-up device 5 does not reach the white 
reference signal level in the turn-on time, the signal at the output 
terminal A&gt;B of the comparator 21 is at the H-level and the up-down 
counter 22 is set to the count-up mode so that it is incremented by one 
count (to count n+1) by the input of the horizontal synchronizing signal 
HS. The incremented count is supplied to the input terminal B of the 
comparator 24 through the latch 23 and it is compared with the count 
(output Q) of the counter 25 to control the turn-on time in one scan 
period. 
The above operation is repeated until the signal read from the white area 
coincides with the white reference signal. 
When the comparator 21 determines that the output of the A/D converter 20 
and the white reference signal are equal, the comparator 21 produces the 
L-level output at the output terminal A&gt;B and the H-level signal at the 
output terminal A=B. As a result, the flip-flop 29 produces the read start 
signal in synchronism with the clock from the AND gate 27 and deactivates 
the AND gate 27 through an inverter 28. Accordingly, the clock is not 
applied to the latch 23 and the present lach output is maintained. The 
count held in the latch 23 represents the number of clocks corresponding 
to an optimum turn-on time of the fluorescent lamp in one scan period. The 
read start signal from the flip-flop 29 is supplied to an exposure scan 
control unit such as a microcomputer not shown to start the exposure scan 
of the document. Accordingly, the document is exposed and scanned with the 
proper turn-on time. 
When the input from the A/D converter 20 is larger than the white reference 
signal, the comparator 21 produces the L-level signal at the output 
terminal A&gt;B so that the up-down counter 22 is set to the count-down mode 
to decrement the count in synchronism with the horizontal synchronizing 
signal HS. As a result, the count to the input terminal B of the 
comparator 24 is incremented and the turn-on time of the fluorescent lamp 
in one scan period is reduced. 
When a new document is to be scanned, the light adjustment start signal is 
again applied and the flip-flops 26 and 29 are cleared and the up-down 
counter 22 starts new counting. After the determination of the optimum 
turn-on time by utilizing the white area, the document is exposed. Since 
the optimum amount of exposure is set for each document, the image can be 
read in high quality. 
FIG. 6 shows another embodiment of the present invention. It shows a 
document read unit and a write unit in a laser beam printer. The like 
elements to those shown in FIG. 2 are designated by the like numerals. In 
the document read unit 30, numeral 10 denotes a white area arranged in an 
effective read area of the image pick-up device 5 such as on a document 
holder for producing a read signal, numeral 32 denotes a microcomputer 
which compares the read signal with a reference signal and carries out a 
predetermined arithmetic operatioh, numeral 33 denotes a counter for 
counting the clock bulse CK, numeral 34 denotes a comparator which 
receives the count from the counter 33 at an input terminal A and the 
operation result from the microcomputer 32 at an input terminal B and 
compares those inputs, and numeral 35 denotes a flip-flop which receives 
the compare output from the comparator 34 at an input terminal D and 
produces a control signal at an output terminal Q and supplies it to the 
light adjustment circuit 9. In the write unit 40, numeral 41 denotes a 
position detector for a laser beam, numeral 42 denotes a photosensitive 
material for forming an electrostatic latent image, numeral 43 denotes a 
laser oscillator, numeral 44 denotes a laser beam emitted from the laser 
oscillator 43, numeral 45 denotes a rotary polygon mirror for scanning the 
laser beam 44 onto the photosensitive material 42, and numeral 46 denotes 
a photo-electric converter which converts the light detection signal to an 
electrical signal and supplies it to the read unit 30. 
With this arrangement, the level of the photo-electrically converted signal 
read from the white area illuminated by the fluorescent lamp 2 is compared 
with the reference signal and the turn-on time of the fluorescent lamp 
which illuminates the document is controlled and the turn-on cycle of the 
fluorescent lamp 2 is synchronized to the one line scan period of the 
write unit to control the amount of exposure to the document 3. This 
operation is described below in detail. 
An initial value n (n&gt;0) is set to an output port PO of the microcomputer 
32 and an initial value 0 is set to the counter 33. A reset signal RES is 
applied to the microcomputer 32. As a result, the operation is started 
under the control of the microcomputer 32 in accordance with programmed 
instructions as shown in FIG. 7. The comparator 34 compares the values (A) 
and (B) supplied to the input terminals A and B. Since (A)=0 and (B)=n, 
the comparator 34 determines (A)&lt;(B) and produces the H-level output. As a 
result, the fluorescent lamp 2 is turned on under the control of the drive 
circuit until the count of the counter 33 reaches n. 
The light reflected by the white area 10 is focused onto the image pick-up 
device 5 through the focusing lens 4 and the photo-electrically converted 
signal is amplified by the amplifier 6. The signal is then converted to 
the digital signal by the A/D converter 7 and it is supplied to the 
microcomputer 32. The white reference signal level L is set to (0001) by 
the program and the image signal level L' (FL DATA) read from the white 
area 10 is compared with the reference signal level (0001) as shown in 
FIG. 7. 
If the image signal level L' is, for example, (0010), the comparator 34 
determines that the luminance is smaller and the preset count n at the 
output port PO is incremented by one to a count (n+1), which is supplied 
to the comparator 34. Thus, the comparator 34 produces the L-level output 
when the value (A) supplied to the input terminal A, that is, the count of 
the clock pulse CK reaches (n+1). As a result, the turn-on time of the 
fluorescent lamp 2 is extended by one clock period longer than the 
previous scan. Thus, the amount of illumination to the white area 10 by 
the fluorescent lamp 2 is increased. The above operation is repeated to 
increment the value at the output port PO of the microcomputer 32 by one 
at a time for each scan until the image signal level L' reaches (0001). 
On the other hand, if the luminance is larger, that is, if the image signal 
level L' is (0000), a count (n-1) which is one less than the initial value 
n is supplied to the comparator 34. As a result, the turn-on time of the 
fluorescent lamp 2 is reduced and the amount of illumination by the 
fluorescent lamp 2 is reduced. 
The operation of the write unit 40 which produces the horizontal 
synchronizing signal H.sub.SYNC is now explained. FIG. 8 shows the 
waveforms of H.sub.SYNC, the output of the comparator 34 and the output of 
the light adjustment circuit 9. The write unit 40 is a light scanner which 
uses the laser as the recording light source and irradiates the laser beam 
to the photosensitive material through the optical system such as a light 
deflector to form a beam spot. The image signal from the read unit 30 is 
supplied to the laser oscillator 43 where it is converted to the laser 
beam 44 in accordance with the image signal. The laser beam 44 is 
reflected by the rotary polygon mirror 45 so that the beam spot is formed 
on the photosensitive material 42. The beam spot passes through the laser 
beam detector 41 for each line scan and the light signal detected by the 
laser beam detector 41 is converted to a trigger pulse by the 
photo-electric converter 46, which supplies it to the input terminal LD of 
the counter 33 of the read unit 30 as the horizontal synchronizing signal 
H.sub.SYNC and to the image pick-up device 5 as the charge storage start 
signal. The counter 33 is reset to zero in synchronism with the signal 
H.sub.SYNC and starts to count the clock pulse CK from count 0. Thus, the 
comparison of the count of the counter 33 and the count at the output port 
PO of the microcomputer 32 for each scan is made at the period of the 
signal H.sub.SYNC. 
The above operation is repeated to change the duty factor of the high 
frequency voltage source supplied to the fluorescent lamp 2 to control the 
turn-on time by the count at the output port PO and the turn-on cycle is 
synchronized to the period of the horizontal synchronizing signal 
H.sub.SYNC so that the white pixel signal level L' coincides with the 
reference signal level L. Thereafter, the document is exposed. 
While the light adjustment is made by the combination of the microcomputer 
and the peripheral elements in the above embodiment, the adjustment may be 
made by only the microcomputer. A configuration therefor is shown in FIG. 
9, in which the like elements to those shown in FIG. 6 are designated by 
the like numerals. The output of the microcomputer is directly supplied to 
the light adjustment circuit 9 and the horizontal synchronizing signal 
H.sub.SYNC from the write unit 40 is supplied to the microcomputer 32. The 
operations carried out by the counter 33, the comparator 34 and the 
flip-flop 35 in FIG. 6 are programmed and stored in a ROM of the 
microcomputer 32 to control the turn-on time at the period of the 
horizontal synchronizing signal H.sub.SYNC. 
FIG. 10 shows the program stored in the ROM. At the start of the exposure, 
counters CNT1 and CNT2 in a RAM of the microcomputer 32 are set to m. When 
the horizontal synchronizing signal H.sub.SYNC is applied from the write 
unit 40, the port PO is set to the H-level. The counter CNT1 is 
decremented by one at a time in synchronism with the count input, and when 
the count reaches zero, the port PO is set to the L-level. Thus, the port 
PO is held at the H-level for the period corresponding to the count set in 
the counter CNT1 and the H-level signal is applied to the light adjustment 
circuit 9, which supplies the high frequency voltage to the fluorescent 
lamp when the input signal thereto is at the H-level. In this manner, the 
turn-on time in one scan period is determined. 
After the termination of the output at the port PO, the reference signal 
(e.g. 0001) is subtracted from the data supplied from the A/D converter 7. 
If the resulting difference is larger than zero, it is determined that the 
turn-on time is shorter and the counter CNT2 is incremented by one. If the 
difference is smaller than zero or equal to zero, the counter CNT2 is 
decremented by one. The result is used as the turn-on time in the next 
scan. 
The above operation is repeated to determine the turn-on time of the 
fluorescent lamp in one scan period. The document is exposed with the 
resulting turn-on time. 
As described hereinabove, according to the present invention, the reference 
color area is provided at a portion of the effective read area and the 
amount of illumination of the document is controlled such that the 
reference color pixel signal level derived from the reference color area 
coincides with the preset reference signal level. Accordingly, the 
constant level of photoelectrically converted signal can always be 
produced and the image is exactly reproduced even for the document having 
half-tone image on the entire area such as photograph.