Ricoh C226 Service Manual page 60

Original Background Correction
When an original is scanned, the whitest level of the original background is
stored, and that level is used as the white peak level for A/D conversion. The
grayscale is made based on the white peak level of the original. As a result,
dark background does not appear on the printout.
If the original background correction is disabled, the whitest level when the
white plate is scanned is used for the high reference voltage.
Peak Hold
The peak hold circuit holds the voltage for the white peak level. Before
scanning an original, it holds the white peak voltage from the white plate to
shading distortion data. When the original is scanned, it stores the white peak
level of the original for the original background correction.
NOTE: The white peak level is checked 5 mm from the leading edge of the
original set on the exposure glass (and from the central 147 mm
width). If the original leading edge is not flush with the original scale
and the platen cover stays open, insufficient voltage will be input as
the white peak level. If insufficient voltage is detected, a fixed voltage
is used as the white peak level to avoid a faint image copy.
SM
OPTICS/IMAGE PROCESSING
2-11 C226