The analog video signals output from the CMOS sensor are corrected so that they will have a specific gain level (gain correction), and the output voltages generated

in the absence of incident light are also corrected so that they it will have a specific offset level (offset correction).

4.3.6.4 CMOS Sensor Output A/D Conversion

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After completion of the above corrections, the analog video signals are converted to digital signals corresponding to individual pixel voltage levels by the A/D

converter.

4.3.6.5 Shading Correction (Outline)

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The CMOS sensor outputs are necessary even for the following reasons even when the density of the original is uniform:

(1) Variation in sensitivity among CMOS sensor pixels

(2) Variation in light intensity of rod lens array

The machine performs shading correction to even out the CMOS sensor output.

There are two types of shading correction: shading adjustment performed in the Service mode and shading correction performed for each job.

4.3.6.6 Shading Adjustment

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The machine measures the density of the standard white plate, and stores the measured density data. It then processes the stored data to use it as the target value for

shading correction.

4.3.6.7 Shading Correction

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The machine performs shading correction for each scan. It measures the density of the standard white plate, and compares the measured value with the target value

stored in the shading correction circuit to use the difference between the two as the shading correction value. The machine uses this shading correction value to

correct the variation among CMOS sensor pixels when scanning the original, thus evening out the image density level.

CMOS

sensor

output

Target

value

Measurement

value

Characteristics

after correction

Characteristics

before correction