Xerox DocuMate 3640 User Manual page 176

Black background scanners:
Some scanners have a black background plate. A black background results in more precise cropping
results and skew detection. However, when scanning thin paper, such as carbon copy paper or ruled
writing paper, which are less than 18lb paper weight, the background may "bleed" through into the
final image and make the image darker than it should be. In that case, try adjusting the brightness
level between 40% and 60%, then adjust the contrast level between 60% and 80%.
Gamma
Gamma is the tone curve and starting point for image enhancement. Move the slider left or right to
darken or lighten the image. As you raise or lower the Gamma value, the values at which Brightness
and Contrast affect the image are changed. It is recommended that you keep the default gamma
value, or adjust this setting before adjusting the other options.
Choosing gamma values
As the gamma value is adjusted lower, the tone curve begins to straighten and the shadows of the
image are expanded, darkening the shadows without overall darkening the image with gray as seen
with lowering the brightness level. This way, the highlights and shadows in the image are maintained
by saturating the pixels with darker levels of the same color rather than adding gray to all colors.
As the gamma value is adjusted higher, the tone curve bends and the highlights of the image are
expanded, lightening the highlights without overall lightening the image with white as seen with
raising the brightness level. This way, the highlights and shadows in the image are maintained by
saturating the pixels with lighter levels of the same color rather than just adding white.
Adjusting the gamma level is based on how you want the image to appear and there is no standard to
adhere to.
Color image gamma examples
The examples below show the original image at the default setting of 1.8 gamma level, then 1.0
gamma and 2.6 gamma. Adjusting the gamma level does not make the image completely black nor
completely white.
Gamma: 1.0
176 ®
Xerox DocuMate
User's Guide
Gamma: 1.8
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3640
Gamma: 2.6