Toshiba 4511 General Description of CCD Control, Opto-electronic conversion, Shading correction

I = k x

6.5 General Description of CCD Control

6.5.1 Opto-electronic conversion

A CCD (Charge-Coupled Device) is used to produce electrical signal corresponding to the reflected light amount from the original. 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 includes a CCD which has 7,450 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 (-) 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 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 clock		Light energy
Transfer section						layer N
						layer P
	1 2	3 4	7447 7448		7449 7450
Light receiving section						Details of light receiving element

Transfer section
Shift register	Transfer clock
	Fig. 6-501

6.5.2 Shading correction

Signal voltages read by the CCD have the following characteristics:

(1)Light source has a 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 7,450 elements varies in opto-electronic conversion efficiency.

These variation need to be corrected and this correction is referred to as shading correction. Shading correction is performed by applying normalization process using the following formula on the black and white data obtained in advance to correct lighting variance and element variation of the image data.

(S — K)

(W — K)

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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November 2003 © TOSHIBA TEC 6 - 11 e-STUDIO3511/4511 SCANNER