Click the [Preparation] button to bring up a popover that allows
changes to the brightness, contrast and background level.
Brightness is useful, for instance, when the text on the page
is relatively light. In that case it may be best to lower the
brightness, so that the text becomes darker and clearer. If a page has
a lightish, but uneven background, increasing the contrast often
clears the evenness of the background, yielding better results during
the scanning process. The background level is a value between 1 and
255, that indicates up to what level of lightness pixels are
considered background. Lowering the background level often has the
effect of making the text heavier and fuller, while increasing the
background level tends to make text lighter/thinner. Depending on the
original, adjusting this setting may yield better results.
For some documents you may need to use and interplay between the three settings to achieve optial results. Generally speaking, if the text in the preview canvas looks clear and crisp, the results tend to be good. Of the original is of poor quality, you may want to perform a test run on a single pages, and redo the OCR with adjusted settings until you achieve satisfactory results.