raster

A raster is the fundamental scanning pattern used in cathode-ray tube (CRT) displays and related imaging systems. The electron beam sweeps across the phosphor-coated screen in horizontal lines, starting at the top left, moving right to create one line, then jumping back to the left and moving down slightly to create the next line. This repeats until the beam has traced every line from top to bottom, at which point it resets to start again. Each complete pass from top to bottom is called a frame.

The density and speed of raster lines directly determine image resolution and flicker perception. In television, standard NTSC systems used 525 lines per frame, while PAL systems used 625 lines. The beam must complete this entire scan 50 to 60 times per second (depending on regional frequency standards) for the human eye to perceive continuous motion rather than visible flicker. Faster scan rates reduce flicker but require higher bandwidth and faster electronics.

Not all raster systems scan every line in sequence. Interlaced scanning divides the frame into two fields: odd-numbered lines in the first field, even-numbered lines in the second. This reduces bandwidth requirements by half while maintaining apparent resolution and flicker performance. Progressive scanning, by contrast, traces every line in order during each frame, a technique that became standard in computer monitors and modern displays.

This distinction matters in industrial control and display work. Raster displays are fixed-resolution grids where every pixel is addressed independently. Vector displays, by comparison, draw lines and curves directly by moving the electron beam in any direction. Vector systems were used in oscilloscopes and some specialized graphics terminals because they could render sharp lines at any scale, but raster systems won dominance because they handle photographic images naturally and map cleanly to memory-based frame buffers. Today nearly all displays are raster-based, using active matrix LCD or LED arrays instead of electron beams, but the scanning logic and nomenclature persist.

The term comes from the Latin rastrum, meaning rake or comb, a reference to the rake-like pattern of parallel lines. In modern electronics, understanding raster fundamentals remains essential when working with legacy CRT equipment, video standards, and any system that must synchronize image generation with hardware scan timing. Frame synchronization signals, horizontal and vertical sync pulses, and blanking intervals all exist to manage the raster's mechanical or electrical constraints.