General Description Of Ccd Control; Opto-Electronic Conversion; Shading Correction - Toshiba e-STUDIO 200L Service Manual

6.5

General Description of CCD Control

6.5.1

Opto-electronic conversion

A CCD (Charge-Coupled Device) is used to produce an electrical signal corresponding to the amount of
light reflected from the original. The CCD is a one-chip opto-electronic conversion device, comprised of
several thousand light-receiving elements arranged in a line, each one of them is a few micron
square.This equipment has a CCD which has 7,400 light-receiving elements.
Each element of the light-receiving section consists of semiconductive layers P and N. When the light
irradiates the element, light energy produces a minus (-) charge in the layer P; the amount of the
charge produced is proportional to the energy and irradiating time. The charge produced in the light-
receiving section is then sent to the transfer section where it is shifted by transfer clock from left to right
as shown in the figure below, and is finally output from the CCD. At this time, to increase the transfer
speed of the CCD, image signals in the even-number and odd-number elements are separated and
output in parallel via two channels.
Transfer section
Light-receiving section
Transfer section
Shift register
6.5.2

Shading correction

Signal voltages read by the CCD have the following characteristics:
1) The light source has a variation in its light distribution.
2) Since the light beams reflected from the original are converged using a lens, the light path is the
shortest at the center of the CCD and the longest at the ends. This causes a difference in the
amount of light reaching the CCD (i.e. the light amount is maximum at the center of CCD, gradually
decreases toward the ends).
3) Each of 7,400 elements varies in opto-electronic conversion efficiency.
These differences need to be corrected and this correction is referred to as a shading correction. The
shading correction is performed by applying a normalization process using the following formula on the
black data and white data obtained in advance to correct the lighting variance and element variation of
the image data.
( S - K )
I=k
x
( W - K )
k : Coefficient
S : Image data before correction
K : Black data (stored in "black" memory)
W : White data (stored in "white" memory)
June 2004 © TOSHIBA TEC
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Fig. 6-13
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Transfer clock Light energy
7399 7400
Details of light-receiving section
Transfer clock
e-STUDIO200L/202L/230/232/280/282 SCANNER
Layer N
Layer P
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