Thread by @fioraesoterica

i spent 30 minutes last night the atelier, eyes dark-adjusted, with nothing but 395nm and 425nm blacklights to play with and it was a real interesting experience. i highly recommend it for screwing with your visual perception sometime.

i can't post pictures because none of it can be meaningfully photographed. (more on this later)

the first interesting effect, at 425nm, with all other lights off, i becomes clear the light isn't actually blue (which is what it looks like at first glance), it's indigoish violet

it's a very much out-of-gamut color, something with a qualitative feel to it different from anything ordinary.

but it gets more interesting.

425nm is near the edge of human cone perception. note this is a log chart; the falloff is really rapid between ~475nm and 425nm.

according to traditional lumen/wavelength curves, a 425nm light is something like *100 times* less bright to the eye than a green light of the same power.

but.

look at the "rods" line. near 425nm, it's still within a factor of 5-10 of peak sensitivity.

the practical effect of this is really cool. a seemingly tiny 425nm light has ENORMOUS throw in a dark room, and can light up the *entire room* bright enough that your rods can see it.

but the room stays black and white, not blue or violet, because it's still too dim for cones!

so you have effectively an indigo light that turns everything black and white.

obviously this is impossible to take a meaningful photo of with an ordinary camera, since it's specifically utilizing the behavior of rods in the eye! you have to see it for yourself.

now let's go to 395nm.

this is Actually UV, though on the visible edge. we're also back to a place where the sensitivity gap between rods and cones is closer to where it is at green, so the black and white effect is less prominent.

and of course, like a traditional blacklight, it makes tons of things fluoresce! the entire room changes color! all kinds of neon-colored objects light up due to the UV-reactive dyes in them, ranging from clothes to scissors.

this is a classic elementary school science class thing (as well as a "laser tag lobby" thing), so probably not new to a lot of you. it's still cool to do, especially in an environment with lots of items that behave differently from how you might expect!

there's one benefit to using a 395nm light though, as opposed to 365nm or a more traditional blacklight range (300-350). you can still see it!

if you point it at a wall (hopefully not painted with a fluorescent paint), you can see its true color.

and its true color is something really special.

it's a deep, pure violet. a true violet, not red mixed with blue. yes, there is a significant qualitative difference. no, it probably won't look right on your camera.

it's a color you may have never seen before in your whole life.

at 395nm, your cones are approximately ~500x less sensitive than at green, so a 5 watt flashlight will just barely light up a wall in a dark room enough to see it. outside of this environment, such a color is nearly invisible.

i should try this sometime with the inverse: infrared.

here's a short blog post i found that covers some of the weird effects of spectral colors at the edge of perception: lightcolorvision.wordpress.com/2014/06/22/wav…

some interesting effects described here:

1. between 400nm to 350nm, colors actually go from violet back to blue because the red cone loses its last bit of sensitivity before the blue cone does.

2. "pure red" is actually a nonspectral color, because all "red" colors also hit the green receptor. "the most pure red hue" is approximately 7 parts red, 1 part yellow, 1 part blue, to cancel out the green! this is a less saturated color, but with a hue maximally close to "red".

3. the eye is not very sensitive at 700nm; it falls off even more rapidly than at UV ranges.

this leads to an effect called "infrared color reversal". as you pass 700nm, colors get *less red* because the red receptor's sensitivity falls off faster than green (Brindley 1955)

note that very little of the cool stuff in this thread is elucidated by ANY of the charts you'll typically find for "human eye spectral response". the ends are often extrapolated or completely omitted (or invisible under a linear scale), and all nonlinear effects omitted.

I only wish I could post pictures.

also if its not clear literally everything here can be gotten off amazon for like $10