IMAGE PROCESSING30 March, 1999
Shading Compensation Circuit
(1) Shading compensation
Before scanning each original, the
machine generates a reference white
waveform (also known as "white shading
+
data") by scanning 5 mm of the white
reference plate [A] in the sub-scan
direction (this equals 79 lines at 100%
magnification).
The white shading data is calculated for
each pixel across the main scan. To do
this for a particular pixel, it takes the whiteA269D068.WMF
levels for that pixel on each of the main scan lines taken from the white reference
plate, and calculates a value from these. The white waveform is made by repeating
this process for each pixel across the main scan.
To improve image reproduction for high density areas, the machine also measures
the black shading data. It does this by reading the black video level at the first 4
pixels of the CCD, which should be black because these pixels are masked off.
The average of the 4 pixels is represented as the black shading data for one CCD
scan line.
White1023
Correction
Black
Correction
0
1 line1 line
A269D069.WMF
The video signal for each pixel obtained during image scanning is corrected by the
shading circuit as follows:
(Data scanned for each pixel) - (Black correction data)
x 1023
(White correction data for each pixel) - (Black correction data)
The white shading data are updated before every scanning. The black shading
data are updated every scan line. The white shading data corrects the image data
for irregularities in the CCD and the optics across the main scan. The black
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ACEC
