For a true-to-original archiving and further processing (e.g. OCR reading) especially of historical books and originals the realized resolution of a book scanner is a main aspect.
The technical statements of the providers concerning the resolution are mostly specified in ppi (pixel per inch) and refer to the used scanning sensor. Due to the fact that the realized resolution is additionally influenced by other components like optics and mechanics of the book scanner it makes sense to verify the achieved resolution with the mapped test pattern which identifies how many line pairs per millimeter the book scanner could separate. After scanning the test pattern you can easily figure out how many line pairs the book scanner could separate by looking at the enlargement on your monitor. The conversion into ppi could easily be done by using the following formula:
Value x 2 x 25,4 (⇒ inch) = ppi