This is the type of file produced when a document is scanned digitally. Raster files (also known as "Bitmaps" are the type of files created by scanners or fax machines and are produced in a similar way to the process used when presenting a picture to a monitor. Raster files are formed when scanning a document in a very narrow line, say "one hundredth of an inch wide" across the page. The Scanner reading head detects the black (or colours) separately from the white of the page.

Each individual scan line is further broken up across the page into small, discreet, individual blocks known as pixels say, hundredths of an inch wide. Consequently, blocks of information one "hundredth x one hundredth" of an inch are examined block by block and recorded digitally as either "black" (or a particular colour) or "white".

A hundredth of an inch is quite a large size for scanning technology, which can scan data in pixels much smaller than this. Once digitised, the scanner (or fax) proceeds to the next line and repeats the process. In this way a digital file (a bitmap or raster file) of the whole document is built up by storing information as black or white (or particular colours) in pixels line by line across the whole of the page. For this reason, all raster files store large amounts of data and they are consequently much larger than other types of files e.g. text files.