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fun fact that's not covered: with a bright enough 720nm LED in the dark, you can see beyond CIE 1931. i use a custom-made ~40w Emisar flashlight. it lies outside the rod cells' spectral response curve and only stimulates the cones, which is ordinarily impossible for light

because it only stimulates the cones and not the rods, it has numerous characteristics that are perceptually fucked and evil. it's the reddest red anyone has ever seen. it turns purple at the edges, stops after 10 feet, looks dim but feels bright, and is weirdly high resolution for a dim light

because the rods should ordinarily always be stimulated when the cones are going off, the brain can tell something is viscerally wrong with the light, but the visual sensation (?) of it is impossible to describe accurately

it doesn't affect night vision *at all*, even compared to ordinary red LEDs. it's blindingly bright when swept across the eyes but leaves no afterimages. you can find your way around with it at night and look up and the milky way is just There.

I showed it to an older fella who'd had artificial lenses implanted and he went "WHOA" and covered his eyes and said it was the most insanely bright red light he'd ever seen

cone cell response to dim light is nonlinear, so combined with inverse-square, it perceptually falls off way WAY faster than light should past a certain point a friend called it my "diabolical flashlight"

the flashlight produces light whose brightness *looks* about equal to something from a crappy AA flashlight but gets hot enough to burn me if I leave it freestanding on turbo mode without the cooling it gets from being firmly held in the hand it also makes a very good handwarmer on night hikes