Sharp AR-650 Manual de servicio descargar pdf (Pagina 116)

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7.6 Outline of Control

7.6.1 Photo-voltaic conversion

The purpose of photo-voltaic conversion is to form electrical signals corresponding to the intensity of light

reflected from the original. A CCD is used for photo-voltaic conversion. CCDs are single-chip photo-

voltaic conversion device in which several thousands of photosensing elements several microns square

are arranged in a single line. This copier uses a CCD having 7,500 of these photosensing elements.

The photosensor comprises a P-layer, N-layer semiconductor. Optical energy generates a (–) (minus)

charge on the P-layer proportional to the optical energy and irradiation time. The charge generated at the

photosensor is transferred to the transfer area, shifted from center to right and left in the figure below

according to the transfer clock, and is output from the CCD. During this operation, in order to increase the

image transmission speed from CCD, even-numbered and odd-numbered image signals are separated

into left and right signals, and 4 channels are output in parallel.

Fig. 7.6-1 Principle of CCD Photo-Voltaic Conversion

Transfer area

Photosensor area

Transfer area

Shift register

Transfer block

Transfer clock

Optical energy

P-layer

N-layer

Detailed photosensor

7.6.2 Shading compensation

The following problems affect the voltage values obtained by photo-voltaic conversion by the CCD:

Variation in light distribution from the light source.

Light intensities vary at the CCD elements. The elements farthest from the center do not receive as

much light as those elements in the center of the CCD.

The photo-voltaic capacities of each of the 7,500 CCD elements are uneven.

Consequently, these problems must be compensated. This is called “shading compensation.” Shad-

ing compensation involves normalizing optical energy according to the following equation based on

already known scanned black data and white data, and compensating for uneven illuminance of the

image data and device unevenness.

(S – K)

I = k × —————

(W – K)

where,

k: Coefficient

S: Image data before compensation

K: Black data (in black memory)

W: White data (in white memory)

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Sharp AR-650 Manual de servicio Pagina 116

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