The most common form of graphics monitor employing a CRT is the raster scan display, based on television technology. In a raster scan system, the electron beam is swept across the screen, one row at a time from top to bottom. As the electron beam moves across each row, the beam intensity is turned on and off to create a pattern of illuminated spots. Picture definition is stored in a memory area called the refresh buffer or frame buffer.
This memory area holds intensity values for all the screen points. Stored intensity values are then retrieved from the refresh buffer and pointed on the screen one row at a time. Each screen point is referred to as a pixel.
When operated as a random scan display unit, the CRT has the electron beam directed only to the parts of the screen where a picture is to be drawn. Random scan monitors draw a picture one line at a time and for this reason are also known as vector displays. The component lines of a picture can be drawn and refreshed by a random scan system. A pen plotter operates in a similar way and is an example of a random scan, hard copy device.