Wayne Bretl

The deep shortwave violet wavelengths stimulate the S (short wavelength) cones of the eye very strongly, and the L (long wavelength) cones much less, and the M (mid wavelength) cones hardly at all. This results in a sensation of intensly colorful purplish blue with very little accompanying brightness. The combination of cyan and magenta dyes in a photographic process (or the blue primary and a bit of red in a CRT) excites the M cones more than the deep violet light - so, while you can reproduce the violet hue, you cannot reproduce the combination of high colorfulness and low brightness, except with a system that uses a very narrow-band violet primary light in place of the usual blue primary. This is not a good idea, because such a source will be down the edge of the eye's response curve, and will result in unacceptable observer variations in colors mixed from combinations of this true violet primary with red and green. It is much more reasonable for three color imaging systems to use a less-violet blue, and approximate violet with a purple of the same hue. of course, if the original violet-hued color is not fully saturated, and lies within the RGB triangle, then a common three color system should be able to reproduce it. If you would like to see a case of true violet, you can go to the neon-tube ceiling sculpture in the tunnel connecting United terminals B and C at O'Hare airport. This artwork has a true spectral series of different colors, with the last ones being in the far violet, to the point where your eye cannot focus them properly, and the violet glow seems to extend outside the tube. Photographs of these tubes come out variously blue or purplish blue, but are either too bright, too desaturated, or both, to replicate the eyeball sensation. Whether a given camera can correctly sense violet and other colors outside the reproducible triangle is an entirely separate question. I will be presenting a paper on this subject at the SMPTE conference in October.