I.R.I.S. Readiris 14 for Windows User Guide - Manual (Page 59)

Section 6: Scanning and Opening Documents
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Contrast
When scanning low-quality documents, adjusting the Contrast
level is sometimes required to obtain an optimal scan result.
Move the Contrast slider to the right to increase the contrast
between the background and the text. Move it to the left to reduce
the contrast.
Note: when the option Auto-exposure is selected, the Contrast
slider is unavailable. The contrast settings are then adjusted
automatically.
Brightness
When you are scanning dark, blurry images, or extremely light
images, it may be necessary to adjust the Brightness settings.
Move the Brightness slider to the right to increase the brightness of
your scans. Move the slider to the left to reduce the brightness.
Note: when the option Auto-exposure is selected, the Brightness
slider is unavailable. The brightness settings are then adjusted
automatically.