Scan resolution (DPI)
Some important definitions are clarified here. They comprise the vital basics of image reproduction theory.

Input resolution
Resolution is a common term that refers to the number of finest elements in an image, or pixels - 'pixel' is a word made from the combination of the words picture and element - that a sensor such as a scanner, can record or distinguish. The unit of measure is in general dpi (dots per inch) or dpcm (dots per centimeter). The higher the resolution, the greater the number of pixels that can be scanned.

Optical resolution / interpolated resolution
Greyscales are of very high importance for scanning technology because in order to reproduce a greyscale image, the scanner has to sense every single image point with a certain depth of data in order to accurately reproduce the many different levels of grey. A good scanner should be able to distinguish 256 levels of grey (8-bit) but, as we will see on the next page, this is not enough. The scanner should distinguish more than 256 levels of grey internally.