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Originally Posted by NowhereMan 1966
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I think I read somewhere the roundies used a rare Earth element to produce reds in better quality, yittrium, IIRC but they switched over to something else when the rectangular sets came out and the phosphor leaned more towards a red/orange.
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Phosphor history has been discussed in several threads here, but rather than searching for all that (you can do it), let me summarize:
Earliest regularly used phosphor set - 15GP22 - had a very inefficient red, a medium efficiency green (P1 oscilloscope green, in other words, the NTSC spec, not as yellow as current phosphors), and the NTSC blue, which was more cyan and less deep violet blue than today's phosphors. RCA and NTSC would have liked a deeper blue, but there was a problem with copper contamination that prevented using a deep sulfide blue - copper would turn it green.
The first really major change in phosphors was the all-sulfide tubes in the early 60's. These are the ones you see with a greenish face. This included a deeper-blue zinc sulfide phosphor (silver activated) since the copper problems had been solved, and a zinc sulfide green (copper activated) that was yellower than "NTSC green" and therefore brighter, and a zinc cadmium-sulfide red that was much brighter than the old red, but was slightly more orange at low current and still more orange at high current.
The next innovations were rare earth reds, which came very close to NTSC red. Note that NTSC red is actually somewhat on the orange side. For example, traffic signal red is outside the NTSC gamut. First was a vanadate red, then europium oxy-sulfide, which became the defacto red for a long time. From this point on, the differences in color renditions in different sets had more to do with different designs of the color matrix to compensate for the yellower green and deeper blue than it had to do with differences in phosphor primary colors. (There were some tubes that added a bit of cadmium to the green, making it even more yellowish and still brighter. In the 70's cadmium was removed due to environmental concerns.)
PAL, and later HDTV, came in after this phosphor set was established, so the color matrixing could be done at the camera on linear signals (where it should be), instead of the approximations in the receiver that NTSC set makers had been fooling with for years. This phosphor set was standardized in several very similar color specs for different applications until they were combined into the specs now used for HDTV and still camera sRGB.
Flat panels are now being designed (and a few produced) that can produce a greater color gamut, some equaling the old NTSC gamut or more, and specs are being developed for extended color gamut signals to drive these displays correctly.
Edit: Yes, yttrium was somewhere in the red phosphor work too, I've forgotten the exact sequence.