Toshiba e-studio550 Service Manual page 94

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 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) 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.
(S — K)
I = k ×
(W — K)
k : Coefficient
S : Image data before correction
K : Black data (stored in "black" memory)
W : White data (stored in "white" memory)
e-STUDIO550/650/810 SCANNING SECTION
1 2 3 4
[Principle of the opto-electronic conversion by CCD]
Transfer clock
7497 7498 7499 7500
Transfer clock
6 - 10
Light energy
layer N
layer P
Details of light receiving element
JUNE 2002 © TOSHIBA TEC