Geer tube

The Geer tube was an early single-tube color television cathode ray tube, developed by Willard Geer. The Geer tube used a pattern of small phosphor-covered three-sided pyramids on the inside of the CRT faceplate to mix separate red, green, and blue signals from three electron guns. The Geer tube had a number of disadvantages and was never used commercially due to the much better images generated by RCA's shadow mask system. Nevertheless, Geer's patent was awarded first, and RCA purchased an option on it in case their own developments didn't pan out. Color television had been studied even before commercial broadcasting became common, but it was only in the late 1940s that the problem was seriously considered. At the time, a number of systems were being proposed that used separate red, green, and blue signals (RGB), broadcast in succession. Most experimental systems broadcast entire frames in sequence, with a colored filter (or "gel") that rotated in front of an otherwise conventional black and white television tube. Each frame encoded one color of the picture, and the wheel spun in sync with the signal so the correct gel was in front of the screen when that colored frame was being displayed. Because they broadcast separate signals for the different colors, all of these systems were incompatible with existing black-and-white sets. Another problem was that the mechanical filter made them flicker unless very high refresh rates were used. RCA worked along different lines entirely, using a luminance-chrominance system. This system did not directly encode or transmit the RGB signals; instead, it combined these colors into one overall brightness figure, the "luminance". Luminance closely matched the black-and-white signal of existing broadcasts, allowing it to be displayed on black-and-white televisions. This was a major advantage over the mechanical systems being proposed by other groups.