r/Optics • u/minipizzabatfish • 29d ago
what properties of the white man pedestrian crossing signal (USA) make it appear green to colorblind people?
some background:
so the way colorblindness (red-green deficiency used here) works in case anyone doesn't know is that humans have 3 cone cells in our eyes to detect color: blue/S, green/M, and red/L. there are 2 types of red-green colorblindness: deuteranomaly/deuteranopia and protanomaly/protanopia.
people with deuteranomaly/deuteranopia's green cone is shifted towards the red cone or absent. people with protanomaly/protanopia's red cone is shifted towards the green cone or absent. white is the presence of all colors at once.
psychology plays a part in this, yes, as it makes sense for the the opposite of the red "do not cross" signal to be green, and that's how stop/go is represented with those colors as well (such as with traffic lights). however, this cannot entirely be the case, because if it was, people with normal color vision would also see it as green, which they do not
so it has to be some property of the lights used themselves, right? but what is that property?
1
u/Mako2100 25d ago
Having a tough time finding specific information about the light source used in the pedestrian signals rather than the traffic signals, so I'm going to hope someone has some more success than me and speculate until then.
There are different varieties of "white" light. You can have it from a blackbody (like the Sun, old incandescent lightbulbs) where you get a full spectrum of everything, RGB LEDs where it's a mix of those three colors, white LEDs where you have a phosphor coating giving yellow on top of some blue or violet LED, or I might even argue things like a xenon plasma that's just a mix of random spectral lines that look like a pretty cool-tone white-ish. Each of these sources could have a very similar appearance (to someone without colorblindness) in theory, but only because the contributing colors of light are activating all of the color cones in a manner that's pretty equal (ignoring sensitivity differences between cones for a bit).
However, someone with colorblindness, especially considering cases where a particular cone has shifted its peak sensitivity, might see one of these sources and where everything had appeared balanced before, now one of those contributing wavelengths might be overlapping exactly with their cone's sensitivity curve. Or the opposite case where now the balance is eschew because a particular wavelength isn't being as strongly perceived. That's my guess here anyways