Electronic copying machine having a line sensor in which the charge storage time is set based on image signals of the previous scan line

An electronic copying machine has an image line sensor whose charge storage time is controlled individually for each line of the image such that the charge storage time for the next line is determined according to the average detected brightness of the line immediately preceding that next line, based on the image signals thereof.

FIELD OF THE INVENTION 
The present invention relates to an electronic copying machine for making a 
hard copy of a remote surface such as a writing board. 
BACKGROUND OF THE INVENTION 
Electronic copying machines capable of making a hard copy of notes, 
illustrations or the like written on a writing board itself or on a sheet 
placed on or tacked to the writing board are known, wherein an image line 
sensor (hereinafter referred to simply as a line sensor) moves stepwise in 
a transverse direction in the focal plane of a taking lens to scan an 
image formed by the taking lens and to generate at each step image signals 
representative of a line of pixels of the image, based on which a printer 
prints out the image on a thermographic or other type copy paper 
sequentially from line to line. The line sensor has a great number of 
pixels arranged in a line for photoelectrically converting incident light 
into signal charges and storing the signal charges, and a shift register 
for transferring the signal charges as serial image signals. 
Before picking up the image by the line sensor in such a conventional 
electronic copying machine, an average brightness of the whole surface to 
be printed is detected by a photosensor disposed at the front of the 
electronic copying machine; and according to the average brightness the 
charge storage time of the line sensor is adjusted so as to control the 
exposure for making a good copy. 
If, however, an image to be printed has very bright or dark areas, for 
example if an image is bright on the right side and dark on the left side, 
then the charge storage time determined in accordance with the detected 
average brightness of the whole surface would be too long for the bright 
area and too short for the dark area, resulting in a hard copy of poor 
quality. 
OBJECT OF THE INVENTION 
An object of the invention is therefore to provide an electronic copying 
machine in which the charge time of the line sensor can be properly 
determined for any image. 
SUMMARY OF THE INVENTION 
To achieve the above object, an electronic copying machine of the present 
invention is provided with a charge storage time setting means for setting 
the charge storage time of a line sensor individually for each line of an 
image such that the charge storage time for the next line is determined 
based on the average brightness of the line immediately preceding that 
next line.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, in an upper front portion of a housing 2 of an 
electronic copying machine of the invention, there is formed a camera 
section 3. The camera section 3 has an exposure opening 5 in its front 
wall in which a taking lens 6 is secured. As schematically shown by dashed 
lines, a line sensor 8 such as a CCD (charge coupled device) type solid 
state imaging device is disposed in the focal plane of the taking lens 6 
in the camera section 3. The line sensor 8 is driven by a pulse motor 9 to 
slide stepwise along a pair of horizontally parallel guide shafts 7. The 
line sensor 8 photoelectrically converts the image formed in the focal 
plane of the taking lens 6 into image signals from line to line. 
On the front wall of the camera portion 3, there is further provided an 
ultrasonic oscillator 10 of an ultrasonic autofocus device which generates 
ultrasonic waves forwardly upward at an angle of approximately 10.degree. 
relative to the optical axis of the taking lens 23 and receives the 
ultrasonic waves reflected by a subject, e.g. the surface of a writing 
board to measure the distance between the copying machine and the writing 
board. Furthermore, a start button 11 and a finder window 12 are provided 
in the top wall of the camera portion 3. 
In one side wall of the housing 2 is provided a door 15, and a door 16 
which is smaller in size than the door 15. Inside the door 15 is disposed 
a thermographic paper 17 on which the image of the field viewed through 
the finder window 12 can be printed by a thermal printing head 23. The 
thermographic paper 17 is transported by a pair of feed rollers 18 and 19 
to be fed out through a slot formed in the side wall along the lower edge 
of the door 15. The feed rollers 18 and 19 are driven by a pulse motor 21 
through gears (not shown), which also drives a platen 22 through gears 
(not shown). The thermographic paper 17 is transported between the platen 
22 and the thermal printing head 23 while being printed with an image. The 
thermal printing head 23 consists of a great number of aligned resistance 
heating elements 23. These heating elements are grouped in eight groups 
designated 23a-23h in FIG. 1. The door 16 is provided for accessing a 
control portion having a power switch, a print mode switch, a density 
control switch and so forth, none of which is shown. 
Referring now to FIG. 2 showing an embodiment of the circuitry of the 
electronic copying machine of the invention, the line sensor 8 disposed in 
the focal plane of the taking lens 6 consists of an image sensing section 
8a and a shift register 8b. The image sensing section 8a picks up a line 
of pixels of an image to be printed and converts them into signal charges. 
Upon receipt of a transfer signal from a driver 44, the shift register 8b 
begins to transfer the signal charges as a series of analog image signals 
to a charge integration circuit 31 and a binarization circuit 32 in 
synchronism with shift pulses supplied at a constant frequency from the 
driver 44. The charge integration circuit 31 integrates the image signals 
representative of a line of pixels and outputs the integrated value to a 
controller 34 through an A/D converter 33. The binarization circuit 32 
compares the image signals from the shift register 8b with a threshold 
level to provide binary image signals, which are then sent to a 
black-white ratio counter 35 and a shift register 36. The black-white 
ratio counter 35 calculates the ratio of black to white (hereinafter 
referred to as the B-W ratio) within the binary image signals 
representative of a line of pixels and sends the B-W ratio data to the 
controller 34. The shift register 36 converts the serial binary image 
signals into parallel signals and sends them to a latch circuit 37. 
The latch circuit 37 is connected in parallel with eight gate circuits 
38a-38h. These gate circuits 38a-38h have a predetermined number of inputs 
and corresponding outputs which are each independently connected to a 
heating element 230. The heating elements 230 that are connected to a same 
gate circuit 38a-38h constitute a same group 23a-23h, respectively. Each 
heating element group 23a-23h is actuated when the controller 34 outputs a 
print signal to the corresponding gate circuit 38a-38h. 
The controller 34 also controls the line sensor 8, the start button 11, the 
shift register 36, the latch circuit 37, and drivers 42, 43 and 44, in 
accordance with pulse signals supplied from a pulse generator 41. The 
driver 42 is connected to the pulse motor 9 for controlling the stepwise 
scanning movement of the line sensor 8, whereas the driver 43 controls the 
pulse motor 21. The time chart of the controller 34 is shown in FIG. 3. 
Next will be described the operation of the above embodiment with reference 
to FIGS. 3 and 4. To make a hard copy of notes, illustrations or the like 
written on the writing board itself or on a sheet placed on or tacked to 
the writing board, the electronic copying machine of the present invention 
is placed on a table or desk in front of the writing board. The surface of 
the writing board is viewed through the finder window 12 for framing an 
area to be copied and then the start button 11 is depressed. Upon 
depressing the start button 11, a start signal is applied to the 
controller 34 which then begins to count the pulse signals from the pulse 
generator 41. When counting up to a predetermined number, the controller 
34 causes the line sensor 8 to start the charge storage operation. After a 
predetermined time for the charge storage, the controller 34 outputs a 
read-out signal to the driver 44 which then outputs at first a transfer 
signal to the line sensor 8 upon which signal charges stored in the image 
sensing section 8a are transferred to the shift register 8b. 
Then a predetermined number of shift pulses are supplied from the driver 44 
to the shift register 8b as to read out the image signals of the first 
line and transfer them as serial signals to the charge integrating circuit 
31 and the binarization circuit 32. The charge integrating circuit 31 
integrates the image signals of the first line so as to calculate the 
average brightness of the first line and supplies it to the controller 34 
through the A/D converter 33. Based on the average brightness of the first 
line, the controller 34 determines the charge storage time TS.sub.2 for 
the second line. It is to be noted that the charge storage time for the 
first line is a predetermined constant value TS.sub.1. 
The binarization circuit 3 transforms the image signals into binary image 
signals and sends them to the black-white ratio counter 35 and the shift 
register 36. One level of the binary image signals is a white level which 
represents a blank space, whereas the other level is a black level which 
represents a pixel to be printed as a black or colored dot. The 
black-white ratio counter 35 determines the B-W ratio of the binary image 
signals of the first lien and outputs the B-W ratio data to the controller 
34. The shift register 36 is sequentially supplied with shift pulses from 
the controller 34 so that the serial binary image signals from the 
binarization circuit 32 are deserialized and stored in the shift register 
36. When all the binary image signals of the first line are inputted into 
the shift register 36, the controller 34 outputs a latch signal to a latch 
circuit 37, thereby enabling the thermal printing head 23 to print the 
first line. Thereafter when the controller 34 outputs the print signal to 
each of the gate circuits 38a-38h, the heating elements are actuated from 
group to group, and simultaneously the pulse motor 21 is driven to 
transport the copy paper. 
During reading out the image signals of the first line from the line 
sensor, the controller 34 causes the driver 42 to drive the pulse motor 9, 
thereby to move the line sensor 8 to the next scanning line. In this way, 
the line sensor 8 begins to pick up the second line during the printing of 
the first line. 
On the other hand, the controller 34 also predicts the printing time 
TP.sub.1 necessary for printing the first line based on the B-W ratio 
detected by the black-white ratio counter 35 and compares the predicted 
printing time TP.sub.1 with a constant time interval TT at which the 
above-mentioned read-out signal is generated and hence the pickup process 
of one line is repeated. If TP.sub.1 is not longer than TT, even when the 
first line is not completely printed out, the driver 42 is caused to drive 
the pulse motor 9 to move the line sensor 8 by one step during the reading 
out of the image signals of the second line, in the same manner as the 
line sensor 8 is moved by one step during the reading out of the image 
signals of the first line. The image signals of the second line are 
sequentially transferred from the shift register 8b to the charge 
integrating circuit 31 and the binarization circuit 32 in the same manner 
as the image signals of the first line. The charge storage time TS.sub.3 
for the third line is determined based on the average brightness of the 
image signals of the second line. The binarization circuit 32 also 
transforms the image signals of the second line into binary image signals 
which are sent to the shift register 36 and the black-white ratio counter 
35. Each time a shift pulse is supplied to the shift register 36, the 
binary image signals of the first line stored in the shift register 36 are 
replaced one after another by the binary image signals of the second line. 
The black-white ratio counter 35 also calculates the B-W ratio of the 
binary image signals of the second line, based on which the counter 34 
predicts the printing time TP.sub.2 for the second line, which is compared 
with the above-mentioned constant time interval TT. 
When the binary image signals of the second line are entirely stored in the 
shift register 36 and also the printing of the first line is completed, 
the controller 34 outputs a latch signal to the latch circuit 27 to hold 
the binary image signal of the second line in the latch circuit 27. 
Thereafter, when the controller 34 outputs the print signal to each of the 
gate circuits 38a-38h, the heating elements are actuated from group to 
group, and simultaneously the pulse motor 21 is driven to transport the 
copy paper. During the printing of the second line, the line sensor 8 
begins to pick up the third line. In this way, imaging and printing are 
repeated from line to line. 
If the predicted printing time for the Nth line TPn is longer than the time 
interval TT, the controller 34 does not drive the driver 42 and hence the 
pulse motor 9 so that the line sensor 8 stays in the (N+1)th scanning line 
until the Nth line is completely printed. Even in such a case, the line 
sensor 8 still executes the imaging process at the predetermined time 
interval TT, and therefore the line sensor 8 begins to pick up the image 
signals of the (N+1)th line once again during printing the Nth line. That 
is, the image signals of the (N+1)th line are twice picked up. The firstly 
obtained image signals of the (N+1)th line are sent to the charge 
integrating circuit 31 and the binarization circuit 32 in the same manner 
as for the foregoing lines. Based on the average brightness of the (N+1)th 
line detected by the charge integrating circuit 34 and the A/D converter, 
the controller 34 determines the charge storage time for the next imaging 
process, that is, in this case, the charge storage time TS.sub.(n+1) for 
picking up the (N+1)th line once more. Accordingly, the second charge 
storage time for the (N+1)th line is based on the average brightness of 
the same (N+1)th line. 
The firstly detected image signals of the (N+1)th line are also inputted in 
the shift register 36 and the black-white ratio counter 35 through the 
binarization circuit 32. But when the predicted printing time TP is longer 
than the time interval TT, the controller 34 does not output a latch 
signal and a print signal. Therefore, the first image signals of the 
(N+1)th line thus stored in the shift register 36 are not used for 
printing but rather are revised by the secondly detected image signals of 
the (N+1)th line. Also the data calculated by the black-white ratio 
counter 35 based on the first image signals are canceled, and the B-W 
ratio of the (N+1)th line as well as the printing time TP.sub.(n+1) 
therefor are again calculated based on the secondly obtained image 
signals. Thereafter the controller 34 drives the line sensor 8 and the 
heating elements 230 in accordance with these renewed data, in the same 
manner as above. 
Assuming that the image to be printed is scanned in M lines, the line 
sensor 8 picks up the last or Mth line when printing the (M-1)th line. 
After picking up the Mth line, the line sensor 8 terminates the pickup 
operation. Accordingly, there is no need to predict and compare the 
printing time TPm for printing the Mth line with the time interval TT. 
As described above, the heating elements 230 of the thermal printing head 
23 are actuated from group to group upon a print signal being emitted from 
the controller 34 to the corresponding gate circuits 38a-38h. When, for 
example, the gate circuit 38a receives a print signal, then the heating 
element group 23a is actuated and several heating elements thereof are 
energized according to the binary image signals latched in the 
corresponding locations in the latch circuit 37. The sequence of actuation 
of the heating element groups 23a-23h is determined by the controller 34 
depending on the distribution pattern of the black and white level bits in 
a series of binary image signals representative of a line of pixels. For 
example, assuming that two of these heating element groups are actuated at 
a time upon receipt of a printing signal, it is desirable to select a pair 
of heating element groups: one group has a relatively large number of 
heating elements to be energized to print black dots and the other has a 
relatively small number of heating elements to be energized. In this 
manner, it becomes possible to perform printing effectively while using 
only a low current. The distribution pattern can be detected based on the 
B-W ratio data from the black-white ratio counter 35. For this purpose, 
the black-white ratio counter 35 has, for example, a plurality of 
sub-counters each of which corresponds to a heating element group 23a-23h. 
While the heating elements 230 are actuated in the above-described manner, 
the controller 34 supplies the driver 43 with a predetermined number of 
drive pulses upon receipt of which the pulse motor 21 rotates by two steps 
to revolve the platen 22, thereby transporting the thermographic paper 17 
and each line of the image is printed thereon. 
As described so far, according to the present invention, an electronic 
copying machine is provided with a line sensor whose charge storage time 
is controlled individually for each line according to the average 
brightness of the immediately preceding line, so that the charge storage 
time of the line sensor is always properly determined for any image even 
if part of the image has very bright or very dark areas. Thanks to this 
high-accuracy exposure control, the electronic copying machine of the 
present invention can provide a high quality hard copy or print of an 
image of a remote surface. 
The invention has been described in detail with particular reference to a 
certain preferred embodiment thereof, but it will be understood that 
variations and modifications can be effected within the spirit and scope 
of the invention. For example, the thermal printing head may be a thermal 
ink-transfer recording head used with an ink ribbon for printing an image 
on ordinary paper.