A "raster" is a bit of jargon which was conceived at the same time as the cathode ray tube display.. A CRT shows images to the human eye by scanning a narrow beam of electrons across a screen of electrically fluorescent material. This is sometimes done in a vector fashion, from one arbitrary point on the phosphors to another, to produce glowing line segments. More often, as with early television, the beam was scanned left to right and down with each line, to cover the whole screen to some degree, and it was modulated to show varying degrees of brightness over the plane. A single left-to-right scan was called a raster for half a century.