I.R.I.S. Readiris 14 for Mac User Guide - Manual (Page 72)

Readiris TM 14 – User Guide
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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 increase the brightness of the image in
order for the text to be displayed, as in the image below.
Example 2: image is too bright
The image below is too light and results in broken text characters.
The text is hardy legible.
In this case you need to decrease the brightness of the image to
obtain satisfactory results, as in the image below.