Xerox 450c - Manual (Page 216)

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Color Depth
Specifies the number of bits per picture element
(pixel) at which the scanned image is stored in a file.
Options are black and white and 16 shades of gray.
Contrast
Adjusts the lightness and darkness of the scanned
document. Options are darken, normal, and lighten.
Apply Halftone to Photo
Processes a photographic image into halftone dots.
Use this option when scanning original photographs
and using black-and-white color depth.
6. Click OK. The WorkCentre begins scanning the
document.