I.R.I.S. Readiris 14 for Windows User Guide - Manual (Page 76)

Section 7: Adjusting the Image Quality
82
Brightness
Move the slider to increase or decrease the brightness of your
documents.
Example 1: image is too dark
The image below is too dark and results in a plain black
image. There is no text to be recognized.
In this case you need to brighten the image in order for the
text to be displayed.
Example 2: image is too bright
The image below is too light and results in broken text
characters. The text is hardy legible.