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SCANNING SECTION
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6.5 CCD Control
6.5.1 Opto-electronic conversion
A CCD (charge-coupled device) is used to produce an electrical signal corresponding to the reflected
light amount 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 microns square.
This model is equipped with a CCD which has 7,500 light-receiving elements.
Each element of the light-receiving section consists of the semiconductive layers P and N. When the
light irradiates the element, the light energy produces a (-) charge in the layer P; the amount of the
charge produced is proportional to the energy and irradiating time. The charges produced in the light-
receiving section are then sent to the transfer section where they are shifted by the transfer clock from
left to right as shown in the figure below, and are finally output from the CCD. At this time, to increase
the transfer speed of the CCD, image signals are separated and output in parallel via two channels.
Fig. 6-9
6.5.2 Shading correction
Signal voltages read by the CCD have the following characteristics.
1. Light source has variation in its light distribution.
2. Since the light beam reflected from the original is converged using a lens, the light path is the
shortest at the center of the CCD and the longest at ends. This causes difference in the amount of
light reaching the CCD (i.e. the light amount is maximum at the CCD center, gradually decreases
toward ends).
3. Each of the 7,500 elements varies in the opto-electronic conversion efficiency.
These variations need to be corrected and this correction is referred to as shading correction. Based
on the black and white data obtained in advance, a normalization process using the following
formula is applied to the raw image data to correct lighting variance and element variation of the
image data.
Transfer clock
Transfer section
Transfer section
Light-receiving section
Light energy
Layer N
Layer P
Transfer clock
Shift register
Details of light-receiving section
(S-K)
(W-K)
I = k
x
k: Coefficient
S: Image data before correction
K: Black data (stored in "Black" memory)
W: White data (stored in "White" memory)
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