Canon DR-2580C Service Manual page 43

2. Image Processing in the Computer
1) Shading Correction
Even if the image brightness is consistent,
the values output from the image sensor
are not necessarily consistent because
the sensitivities of each element of the
image sensor and the performance of
each reading system would vary. In the
shading correction, the variations of each
element are compensated. This process-
ing is done for the digital signals after A/D
conversion.
The correction values of each element are
calculated in advance and stored in a
memory.
There are two types of correction values:
black and white correction values. For
black correction value, readout indicated
when an LED does not illuminate, in other
words, when a black image is read (in-
tense black) is set as a target value. For
white correction value, readout of stan-
dard shading plate is set as a target value
(pure white).
The data of each element which have
been converted into digital signals are
compensated in accordance with the
corresponding values.
LEDS turned off
After A/D
conversion
Figure 2-504
COPYRIGHT © 2005 CANON ELECTRONICS INC.
2) Color Correction
Black
correction
value
CANON DR-2580C FIRST EDITION APR. 2005
CHAPTER 2 FUNCTION & OPERATION
Same brightness
White
document
After A/D
conversion
After shading correction
Figure 2-505
This machine prepares a thin long shad-
ing plate near the reading glass instead of
a standard shading plate in order to
calculate correction
correction values can be automatically
calculated during normal operation.
To improve the reproducibility of color
images, the 3-dimensional gamma con-
version is performed on the color data
after the shading
3-dimensional conversion table that suits
the characteristics of this machine is pro-
vided to convert RGB values.
values. Thus,
correction. A
2-17