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2.2.2 A/D CONVERSION PROCESSING
The analog signal from the contact image sensor is converted into a digital signal
that represents 64 grayscale steps. This process is carried out in the halftone
processing chip in the MPU.
Shading Distortion Correction
The image data from one main scan line does not exactly represent the line from
the original image, because of the following reasons:
1) Loss of brightness towards the ends of the exposure lamp.
2) Variations in sensitivity among elements of the contact image sensor
3) Distortions of the light path
Such distortions in the image data are corrected when they are converted into
digital data.
Before scanning the document, the scanner reads the white plate on the back of
the original scale. The output of each contact image sensor element is changed to
a 6-bit digital value and is stored in the shading distortion memory.
To change the analog shading distortion signals to digital data, a scale of 64 steps
is made between the whitest level when the white plate is scanned and 50% of the
whitest level. Using this scale, the analog signal is changed to 6-bit digital data.
While an original is scanned, the 6-bit shading distortion value for each pixel is
sent, in series, from memory to the D/A converter, synchronizing with the image
signal being sent to the A/D converter. The D/A converter changes the distortion
value to an electrical current. The current is converted to the voltage to be used as
high reference data for A/D conversion. In this way, the high reference voltage for
A/D conversion is changed sequentially for each pixel depending on the shading
distortion data for that pixel.
SM
CIS
2-11
IMAGE-PROCESSING
CIS
C231D510.WMF
C231
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