Trying to do some color matching I purchased a 450 nm laser. I expected monochromatic light of this laser to have similar properties to those of all others I've already played with — 808, 640, 520, 405 nm — in that they all cause an unambiguous color sensation.
But when I shined it onto my wall (having removed the lens), I was surprised to see something unusual. In the center, where the intensity is high, it looks as "very blue" — much like those 465 nm blue indicator LEDs we can see everywhere today, only more saturated. But on the sides, where the intensity falls off, the color looks like violet! When I shine this laser on a ceiling obscured by a wall, I see its reflected light as violet. When I move the spot so that the reflected light becomes more intense, I begin to see it as blue again in the intense areas, and still violet in dimmer ones.
I've checked with a spectrometer, and there doesn't appear to be any fluorescence in the light spot to confuse me.
Interestingly, if I increase amount of ambient lighting (white LED lamps + CCFL ones), sensation of blue once again transforms to violet. Also, the "very blue" intense spot looks violet in bright sunlight, although I can make it blue again by focusing into a smaller spot to make it brighter. In a fully dark (apart from the laser) room I still do notice the violet areas on the sides of the spot.
405 nm light also seems a bit silvery-whitish when its intensity is high, unlike at low intensity. Maybe it's the same phenomenon, which I simply didn't notice before because the tint is not that blue in this case.
In both cases of 450 nm and 405 nm the additional color on high intensities is still "shiny" due to the speckles specific to high monochromaticity, so this indeed doesn't look like the result of fluorescence of the objects I shine the light to.
I've asked several people whether they see it the same, and they answered affirmative.
I suppose it's not related to color balance, because I only changed the intensity of ambient light, not the tint to observe the changes in color. Especially it shouldn't be due to color balance since I can simultaneously see different colors in the areas with different intensities.
Interestingly, while I thought the "very blue" color to be the main color of 450 nm, CIE 1931 XYZ value for it, converted to sRGB, appears to be (if we desaturate and normalize to fit in sRGB range) (0.43,0,1), which is purple, not blue.
So, what is going on here? Is it a well-known phenomenon? Could it be due to some fluorescence of the retina itself rather than the objects lit by the laser?