That is exactly what it is. Move the mask light out of the visible spectrum so that the masking operation does not interfere with any colors.

The sodium vapor light process was the best tech in the 1950s, Sodium vapor lights were used because they deliver a very pure single wavelength light. But we can do better now. leds natively illuminate with a single wavelength(and we have to put a lot of engineering into them not doing this) and we have cameras that can view frequencies that the eye cannot. put this together and in theory you can do the single frequency illuminated backing sheet mask(green screen) with a frequency that is not visible to the human eye and therefore does not interfere with any of the colors in the final shot.