Andy,

There are almost too many phosphor types to count today. Most of them are synthetic types and highly efficient. Many are made for highly specialized applications.

The 15GP22 and 21AXP22 of 1954 used sulphide type phosphors that corresponded exactly to the color specifications of the NTSC signal. This produced richly saturated color that is sometimes described as looking like a technicolor movie. Unfortunately, these phosphors were also inefficient (especially the red), producing a low brighness picture best viewed in a dimly lit room.

RCA received complaints about the low brightness and attempted to fix it with the 21CYP22 in 1957. Every trick they could think of to increase brightness was applied to that tube, including a more efficient red phosphor. While they did achieve some success at increasing the brightness, the red phosphor looked more like orange and more complaints were received.

A further attempt to improve the situation was made in 1961 with the 21FBP22 & 21FJP22. Another new, more efficient red phosphor that looked red was used, along with structural changes that equalized beam currents on the 3 guns. The result was better red and more brightness.

The final improvement in the round tubes was the use of a new rare earth phosphor (for red only) in the 21FBP22-A & 21FJP22-A in 1964, resulting in yet another brightness increase and improvement of red.

Many more changes in color picture tubes followed after the introduction of rectangular types, and have continued right up to the present. To date, nothing has been able to beat the cost effectiveness of the cathode ray tube.