triad

A triad is a cluster of three phosphor dots on a cathode ray tube display, arranged in close proximity and emitting red, green, and blue light respectively. These three dots work together as the smallest addressable color unit on the screen; the electron beam excites all three in varying intensities to produce any color in the visible spectrum through additive color mixing.

The physical arrangement of triads varies by display design. In shadow mask CRTs, triads are arranged in a regular grid pattern, typically spaced 0.25 to 0.31 millimeters apart (measured center-to-center). The shadow mask itself, a thin perforated metal sheet positioned between the three electron guns and the phosphor layer, ensures each gun strikes only its corresponding color dot. Aperture grille designs use vertical wires instead of a mask but maintain the same triad principle.

The term comes directly from the three-color composition: the Greek prefix tri meaning three. Alternative terminology includes dot triad, color dot triad, or simply pixel in modern parlance, though pixel originally referred to the entire addressable element rather than its sub-components.

Display resolution and sharpness depend critically on triad density. A typical 19-inch CRT with 0.28 mm dot pitch yields approximately 1280 by 1024 pixels. Finer pitch produces sharper images but requires tighter manufacturing tolerances and higher deflection voltages. Convergence errors, where the three beams fail to strike their intended dots, cause visible color fringing and degrade image quality.

Triads became the dominant color architecture for CRT displays from the 1950s onward because the three-color model matched human color perception and enabled practical high-volume manufacturing. The technology remained standard until LCD and plasma displays displaced CRTs in the early 2000s, though the RGB triad concept persists in modern display technologies.