Shading correction device

A method and apparatus for correcting the photoelectric converter of a document reading unit for non-linearities produced by the optics of the system. A test document having no image data thereon is scanned to produce a shade correction signal which is subjected to A/D conversion and stored in a memory. In binary-encoding the picture data of the original, a switch is operated according to the data stored in the memory so as to change a threshold level for the picture signal, whereby digital picture data whose shading has been corrected is obtained. In the case where the picture data of the original are represented by an analog signal, the picture data are also subjected to A/D conversion, and division is carried out with the data stored in the memory being employed as dividends. From the results of such a division, corrected multi-level picture data is obtained.

BACKGROUND OF THE INVENTION 
This invention relates to a shading correcting device which is used in a 
reading device having a solid-state image pickup element. 
Solid-state image pickup elements are used extensively in reading and 
recording devices such as facsimile devices or other copying machines. The 
image pickup element reads the picture datum on an original and converts 
the read information into a series of electrical signals. 
FIG. 1 shows one example of such a reading device. An original 2 is placed 
on a transparent platen 1 in such a manner that the surface of the 
original to be read faces downwardly. A fluorescent lamp 3 is provided 
below the platen 1 in such a manner that light therefrom is extended in 
the main scanning direction of the original 2. Light from the fluorescent 
lamp 3 is applied to the original 2, and light reflected from the original 
2 is applied through a lens 4 to a solid-state image pickup element 5 
which forms the optical image. The image pickup element is, for instance, 
a one-dimensional image pickup element which utilizes a CCD. As the 
original 2 is moved in the auxiliary scanning direction, the picture datum 
is read in a raster scan pattern. 
With the above-described recording device, even when an original is uniform 
in density over a line (as in the case of a substantially blank original), 
the photo-electric conversion output of the solid-state image pickup 
element 5 is not uniform. One reason for this phenomena is that the light 
source is not uniform in luminance distribution. As shown in FIG. 2, when 
a fluorescent lamp 3 is used as the light source, the rays 6 therefrom are 
concentrated at the center of a reading line. Accordingly, the illuminance 
is highest at the center of the original, and it decreases towards either 
end of the original. Thus, the photo-electric conversion output is not 
uniform. Other reasons for the non-uniform photo-electric output are that 
the quantity of light at the periphery of the lens 4 is small due to the 
cosine biquadrate rule, and the solid-state image pickup element 5 is not 
uniform in sensitivity. 
If the photo-electric conversion output of the solid-state image pickup 
element 5 is not uniform, a subsequent signal processing operation such as 
a binary-encoding of the analog picture signals is adversely affected, 
resulting in a reduction in the quality of picture. This phenomena will be 
described in greater detail with reference to FIG. 3. In the description 
to follow, it is assumed that an original's reading line has picture data 
7 (black-and-white data) as shown in FIG. 3A. In this case, the 
solid-state image pickup element provides a non-uniform photo-electric 
conversion output 8 as shown in FIG. 3B. The output 8 is binary-encoded by 
comparison with a preset threshold level l.sub.1. In this case, in the 
central portion of a line, a signal level corresponding to black picture 
data (hereinafter referred to as a "black level", when applicable) may be 
binary-encoded erroneously as white picture data. In addition, in the 
vicinity of each end of a given picture line, a signal level corresponding 
to white picture data (hereinafter referred to as a "white level", when 
applicable) may be binary-encoded erroneously as black picture data. 
Accordingly, when a threshold level l.sub.1 is provided as shown in FIG. 
3B, the generated binary-encoded picture signal 9 is considerably 
deteriorated when compared to the original picture data. 
In order to prevent the picture data from being deteriorated in the 
binary-encoding operation, a shading correcting device is known in the art 
in which a threshold level is set by an A/D converter and a D/A converter. 
With reference to FIG. 4A, a white (blank) line is first read by the 
solid-state image pickup element so that a photo-electric conversion 
output (a shading waveform) 11 over a line is obtained. Then, the output 
11 is converted into digital data by the A/D converter, and the digital 
data is stored in a memory. Thereafter (i.e., when the picture signals are 
actually read), the digital datum are converted into analog datum by the 
D/A converter. According to the analog datum, a threshold level l.sub.2 
similar to the shading waveform 11 is set as shown in FIG. 4B, so that the 
picture signal 12 is subjected to binary-encoding. Accordingly, the white 
and black levels of the picture data are binary-encoded correctly, as a 
result of which a digital picture signal 13 high in quality can be 
obtained as shown in FIG. 4C. 
The above-described conventional shading correcting device is 
disadvantageous in that it necessitates the use of a D/A converter for 
converting digital signals which are obtained through A/D conversion into 
analog signals. 
SUMMARY OF THE INVENTION 
In view of the foregoing, an object of this invention is to provide a 
shading correcting device which can satisfactorily correct shading without 
using a D/A converter. 
The foregoing and other objects of the invention are achieved by providing 
a waveform for shading correction which is obtained during a preliminary 
scanning of an original and is subjected to A/D conversion and stored in a 
memory. In binary-encoding the picture data of the original, a switch is 
operated according to the data stored in the memory so as to change a 
threshold level for the picture signal, whereby digital picture data whose 
shading has been corrected is obtained. In the case where the picture 
datum on the original is represented by an analog data (multilevel), the 
picture datum is also subjected to A/D conversion, and division is carried 
out with the datum stored in the memory being employed as dividends. From 
the results of such a division, corrected multilevel picture data is 
obtained.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 5 shows a first embodiment of a shading correcting device according to 
the invention. In the device, an amplifier 21 amplifies picture datum 
which has been subjected to photo-electric conversion by a solid-state 
image pickup element 5. A peak hold circuit 22 for holding the peak value 
of the picture datum and an n-value comparison circuit 23 for subjecting 
the picture datum to an n-value comparison by referring to the peak value 
held in peak hold circuit 22 are both provided on the output side of 
amplifier 21. A digital picture signal (shading correcting waveform) in a 
preliminary scanning step prior to an original scanning step is stored in 
a random access memory (RAM) 24. In a step of scanning picture data, a 
switching circuit 25 is controlled by the contents of the random access 
memory 24 so that the threshold level is changed and corrected digital 
picture datum is produced thereby. 
The operation of the above-described circuit will now be described in more 
detail. An original 2 is placed on a platen 1 which can reciprocate 
freely. Immediately before the original is to be scanned, the platen 1 is 
held at its start position. Under this condition, a correcting board 28 
which is arranged at the left end of the original 2 in FIG. 5 is on the 
optical axis of a lens 4. When the scanning operation for reading the 
original is started, the platen 1 is moved in the direction of the arrow 
(or in the auxiliary scanning direction) so that the solid-state image 
pickup element 5 begins to output picture datum. 
Picture datum 30 are produced by scanning the first several lines of the 
correcting board 28. The correcting board 28 is an elongated board which 
is equal in length to the scanning width of the original, and its lower 
surface (or a reading surface) is coated with white paint which is equal 
in brightness to the white areas of the original. By scanning a white 
(blank) line, an intensity distribution (i.e., the datum 30) will be 
produced which corresponds to the shading waveform inherent in the system 
optics, as shown in FIG. 6A. The picture datum 30 are processed through 
the amplifier 21, which outputs amplified picture datum 31. The picture 
datum 31 are then applied to the peak hold circuit 22 such that the peak 
value thereof is detected and held. FIG. 6A shows the shading waveform 
provided when the correcting board 28 is read, having a peak value P.sub.1 
which is detected by hold circuit 22. The peak value P.sub.1 is supplied 
to the n-value comparison circuit 23. 
FIG. 7 shows the n-value comparison circuit 23 and the switching circuit 25 
in greater detail. The peak value P.sub.1 is applied to a reference 
voltage generating circuit 231, where a reference voltage V.sub.1 is 
maintained until one original has been scanned. The reference voltage 
V.sub.1 corresponds to the peak value P.sub.1. 
The reference voltage V.sub.1 is applied to one end of a series circuit 232 
of (n+1) resistors R.sub.O through R.sub.n, the other end of which is 
grounded. N different comparison reference voltages V.sub.11 through 
V.sub.1n, are provided by the series circuit 232 at the connecting points 
of the resistors R.sub.O through R.sub.n, respectively. The resistor 
R.sub.O is provided in order to make the comparison reference voltage 
V.sub.11 slightly lower than the reference voltage V.sub.1 for the purpose 
of binary-encoding. The resistors R.sub.O through R.sub.n may be equal to 
or different from one another in resistance. The comparison reference 
voltages V.sub.11 through V.sub.1n thus provided are applied to the 
reference voltage input terminals of comparators 233.sub.1 through 
233.sub.n, respectively. 
After the peak value is detected by the peak hold circuit 22, the amplified 
picture datum 31 produced by the scanning of one line of correcting board 
28 are supplied to the remaining input terminals of the comparators 
233.sub.1 through 233.sub.n. In the comparators 233.sub.1 through 
233.sub.n, the analog picture data 31 are compared to the comparison 
reference voltages V.sub.11 through V.sub.1n, as shown in FIG. 6B, and the 
results of the comparison are supplied as a correction signal 33 (as shown 
in FIG. 6C) to an encoder 234. The generated correction signal 33 is the 
binary-encoded version of the shading waveform. 
The encoder 234 receives a clock signal 34 and samples the correction 
signal 33 for one line with a predetermined period. The signals (33) thus 
sampled are encoded into n different codes according to their levels. The 
encoded signals are then written into respective storage areas of the 
random access memory (or RAM) 24. Thus, the original has been 
preliminarily scanned for shading correction. 
When the platen 1 is further moved in the direction of the arrow and 
scanning of the original begins, the amplified analog picture datum 36 (as 
shown in FIG. 6D) which are outputted by the amplifier 21 are applied to 
the n-value comparison circuit 23. The random access memory 24, receiving 
a line synchronizing signal 37 which is produced in synchronization with 
the production of analog picture datum 36 of each line, outputs codes 38 
successively at respective positions on one line in synchronization with 
the synchronizing signals 37. In other words, for a given position along a 
scanned line of the original, the RAM 24 will output a particular output 
code 38. 
The switching circuit 25 is an electronic switch having n contacts 25.sub.1 
through 25.sub.n. The switching circuit 25 receives the code 38. The 
armature of the switching circuit 25 is tripped according to the content 
of the code 38. The contacts 25.sub.1 through 25.sub.n are connected to 
the outputs of the comparators 233.sub.1 through 233.sub.n, respectively. 
Accordingly, among the picture datum 36 binary-encoded by comparison to 
the reference comparison voltages V.sub.11 through V.sub.1n, an optimum 
data 39 to which shading correction has been given is outputted by the 
switching circuit 25. In other words, the outputted code 38 switches the 
switching circuit 25 to the output of one of the comparators 233.sub.1 
-233.sub.n having the threshold level corresponding to the given position 
of the scanned line. For example, with reference to FIG. 6E, when the 
synchronizing signal indicates that the midpoint of a particular scanning 
line is being read, the output code 38 extracted from RAM will switch the 
switching circuit 25 to the output of comparator 233.sub.1, which compares 
the picture data 36 read at the midpoint of the scanning line to the 
highest reference voltage V.sub.11. In FIG. 6E, the dotted line indicates 
the threshold levels selectively compared to the picture datum 36, and 
FIG. 6F shows the resultant digital picture data 39. Thus, it can be 
understood that the first embodiment of the invention produces 
satisfactory shading correction for each line of the original which is 
subjected to binary-encoding. 
FIG. 8 shows a second embodiment of the shading correcting device according 
to the invention. In FIG. 8, those components which have been previously 
described with reference to FIG. 5 are designated by the same reference 
numerals. In the device also, the correcting board 28 is read immediately 
before the original (2) is scanned. The analog picture datum 31 of one 
line of the correcting board 28 is applied to a sample and hold circuit 
51. The circuit 51 operates to sample the datum 31 with a predetermined 
sampling period and to temporarily hold the sampled datum. The analog 
signals 52 which are provided by sampling are applied to an A/D converter 
23'. 
The A/D converter 23' produces 8-bit binary signals as a function of the 
brightness levels of the analog signals 52. The binary signals are 
outputted on first signal lines 53 as shown in FIG. 9. The binary signals 
supplied to signal lines 53 are similar in nature to the outputs of the 
encoder 234 in the first embodiment of the invention. That is, the signal 
processing which is carried out from the start of the reading operation 
until this step may be the same as that of the first embodiment as long as 
the outputs of the encoder 234 are digital data corresponding to the 
analog levels of the picture data 31. 
The first signal lines 53 branch into two groups of signal lines, so that 
the binary signals outputted by the A/D converter 23' are applied to both 
the random access memory 24 and a divider 54. In the preliminary scanning 
step, the binary signals are written in the random access memory 24. Thus, 
the step of preparation for shading correction has been accomplished. 
When the platen is further moved to start scanning the original, the analog 
picture datum 36 which are outputted by the amplifier at this time are 
supplied to the sample and hold circuit 51, where they are sampled with a 
period corresponding to the main scanning density. The analog signals 52 
obtained through the sampling are converted into 8-bit binary signals by 
the A/D converter and the binary signals are supplied to the first signal 
lines 53. 
In this operation, the random access memory 24 reads out the binary signals 
in synchronization with the line synchronizing signal 37. The binary 
signals are supplied through second signal lines 55 to the divider 54. In 
the divider 54, the 8-bit binary signal supplied through the second signal 
lines 55 is divided into the 8-bit binary signal supplied through the 
first signal lines 53. 
FIG. 10 is a diagram for a description of the shading correction according 
to the second embodiment of the invention. In FIG. 10, the curve 56 
indicated by the dotted line is the analog data of the 8-bit binary 
signals supplied over the second signal lines 55 corresponding to a 
waveform for shading correction. Further in FIG. 10, the curve 57 
indicated by the solid line is the analog data of the 8-bit binary signals 
supplied over the first signal lines 53 corresponding to the signal levels 
of the picture data 36 of the original 2. It is assumed that the peak 
level (white) of the curve 56 is "1", and the zero level (black) is "0". 
With respect to optional points S.sub.1 through S.sub.6 on the curve 57, 
division will be carried out by the divider 54. 
For the points S.sub.1 and S.sub.4 which are on the curve 56, the result of 
division is "1". More specifically, with respect to the point S.sub.1, 
0.625/0.625=1, and with respect to the points S.sub.4 b 1/1=1. Thus, it 
can be detected from the results of division that, at such points, the 
picture data are "whit". 
The results of division for the other points are as follows: 
S.sub.2 ; 0.375/0.825=0.4545 
S.sub.3 ; 0.575/0.95=0.6053 
S.sub.5 ; 0.5/1=0.5 
S.sub.6 ; 0.25/0.55=0.4545 
It can be understood from the foregoing ratios of the dividends to the 
divisors that at these points the half-tones of the original are 
represented as numerical values. 
The divider 54 outputs the results of the division as digital picture data 
59 according to the required degree of gradation. For instance, when the 
gradation required is eight (8=2.sup.3) steps, the three highest bits of 
the result of division are outputted, and when the gradation required is 
sixteen (16=2.sup.4), the four highest bits are outputted. It goes without 
saying that gradation in more steps can be indicated. The above-described 
operation is carried out for every line, and picture data is read with 
satisfactory shading correction. 
As is apparent from the above description, the device according to the 
invention can read picture data with sufficiently high accuracy without 
using a D/A converter and is simple in circuitry and high in reliability. 
In the above-described embodiments, the shading correcting data is obtained 
by scanning the correcting board; however, the same data can be obtained, 
for instance by scanning a white end portion of the original.