[old_tv_nut]

Another thing stated near the beginning and end of the video needs correction, that the blue skies in the early sets were too purple. I wonder if this is a comment picked up from the newspaper review by the AI. This could have occurred on a particular broadcast viewed on a particular misadjusted early set, but definitely was not a feature of the phosphors.

The 15GP22 and early 21AXP22 tubes used the NTSC blue phosphor, which was more cyan and less deep violet blue than RCA originally wanted to get maximum deep blue and magenta saturation. RCA had trouble using the sulfide blue they wanted because copper contamination would turn the blue phosphor to green, so they settled on a different phosphor that did not suffer from copper contamination.

Later, at a time I have never found documented, when the copper problem was fixed, the 21 inch blue phosphor was changed to the more violet-blue zinc sulfide (with silver activator). This is the point at which the phosphor choice would make blues more purple and also yellows more greenish. RCA had chassis for a few years in which the color demodulators were not redesigned for the changed phosphor. Eventually, some time after the green phosphor was changed from P1 to the yellower zinc sulfide (deliberately doped with copper!), the demodulator gains and angles were changed to partly compensate for the colors of the new phosphors, particularly the green but also the blue.

Zinc sulfide with silver blue has been used for CRTs ever since, all the way through computer displays and HDTV, and is the blue primary for sRGB color space in jpg images.