Auto Shading - Gestetner A294 Service Manual

IMAGE PROCESSING

2.3.5 AUTO SHADING

Two things happen during auto shading.
Black Level Correction
The black level is zeroed for each scan line of data by reading the dummy
elements at the end of the CCD signal for each scan line, which should be black.
White Level Correction
The data is corrected for variations in white level across the main scan. To do this,
a white reference plate is scanned before each original (book mode) or every 30 s
(ADF mode). This corrects for the following effects on each pixel:
• Loss of brightness at the ends of the exposure lamp and the edges of the
lens
• Variations in sensitivity among the CCD elements
• Distortions in the light path
A294/A295
CÓPIA NÃO CONTROLADA
2-26
CÓPIA NÃO CONTROLADA
A294D645.WMF
SM