Raster

A raster display is one which produces a rectangular array of parallel, closely-spaced lines of output (the rasters). It takes as its input signal a scan across the screen in precisely the form needed to produce that output in real time, i.e. it consists of a sequence of horizontal line segments, arranged from the top to the bottom of the display (with a short, constant vertical distance between them).

After scanning across the display on one raster, the means for producing the display of each line segment must then return to the other edge where it started, during which time nothing is shown on the display (it is said to be 'blanked'); this is known as the 'horizontal retrace interval'. Likewise, when it gets to the bottom of the display, it must return to the top, which is called the 'vertical retrace'.

This technique for displaying an image was first used in televisions using CRTs, where the electron beam was scanned across the screen which formed the front face of the CRT, with the input signal being instantaneously used to control the intensity (i.e. brightness) of the beam as it scanned. This method was later adopted by video terminals and other video displays; a similar technique is used by laser printers. The general form of the interface (a series of line segments) is used in other display technologies.