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

Input resolution
Resolution is a common expression that refers to the number of finest elements of an image or pixels (pixel is an artificial word made of picture and element), which a sensor, for instance a scanner, can record or distinguish. The unit of measure is in general dpi=dots per inch or dpcm=dots per cm. The higher the resolution, the greater the number of pixels can be scanned.

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