Supposedly it doesn’t affect your night vision as much, but why is this the case? Does it have to do with rods and cone cells? If so, looking at the Wikipedia page’s frequency sensitivity chart, it seems like rod cells are most sensitive to bluegreenish (cyan) light rather than red. On an information-per-lumen basis, wouldn’t it make more sense to have a dimmer light that rods more readily pick up and which don’t trigger as many cells?
Or am I misunderstanding how this works?
From my involvement in astronomy, I believe it has something to do with red light not affecting your visual purple, though I don’t remember any of the physiological details of this.
That’s exactly it, thanks!
Wikipedia’s section on biological night vision says:
So it was the opposite of what I guessed… you don’t want to use the blue-green light that they’re most sensitive to because it apparently depletes a limited store of rhodopsin, which the rod cells don’t fully regenerate for 30 minutes.
Interestingly, that same article also quotes a navy study that measures dark adaption after exposure to low levels of red vs white light and found no significant difference at lower light intensities (such as used in a submarine at night). However, this was done back in the 80s and they didn’t have a way to accurately measure the brightness of red lights and so they used human observers to try to match the apparent brightness of the calibrated white light.
But if we’re less sensitive to red light, wouldn’t that mean they had to artificially brighten it to lumen levels exceeding that of the white light so that the apparent brightness would be the same?
In astronomy the idea was that you used red light when looking at a star chart or fiddling with equipment. This preserved your dark adaptation so that viewing objects in the telescope was done with maximum sensitivity.
In sub warfare the idea is probably to preserve dark adaptation so that if there was a sudden need to see in near darkness it wasn’t compromised. The difficultly is that long term use of red light will still bleach the rhodopsin, and thus red light may not be useful. It may take longer, but the red illumination still wipes out dark adaptation. But for short bursts - it does help if the light is red.
My city requires bicyclists to have a rearward-facing red light if you’re riding at night. That’s the only time I ever use the red LED in head lights and I assumed that’s what they were for. I never even thought about night vision. That might make a lot more sense.
The other reason of course is that red LEDs cost less and use less power than other colors.
One other thing about hiking headlamps is politeness. If you turn toward one of your companions, a red light is (maybe) less likely to dazzle them and wipe out their dark adaptation.
I’m not totally convinced that the idea of red for hiking is fully justified, but not having tried it, I’m not going to dismiss it.
Military folk use red covers on flashlights, same principle.
NM
I always thought it wasn’t so much a matter of the reaction to visual pigments as it is that red light doesn’t cause your pupil to restrict as much or as fast as white or blue light.
If I may digress, the nocturnal creatures in a salt water aquarium come out only in the dark. They cannot see red light, and may be observed with a red LED bar.
If you use a bright white head lamp, as we recently did on a camping trip, you’ll get a huge swarm of insects in your face.
I haven’t used a red lamp yet, but I’m betting that doesn’t happen with red.
Not that the situation arises often but it is slapstick when a a head lamp wearing party stops to discuss something and the current speaker gets momentarily spotlighted (before everyone remembers to not look directly at the speaker - but it is a strong habit).
I have never used red lights, though I will often only turn the light on as I really need it. There was a particularly memorable ski descent down a Glacier headwall with the moon came up just at the right time.
In the UK, and probably in the EU, it is actually illegal to display a red light at the front of a moving vehicle, including bicycles. White lights are at the front, with the exception of a reversing light (which only comes on when the vehicle is actually reversing). Indicators and other lights are yellow/amber/orange.
In ophthalmology, we use them on lights for the technicians (to see equipment) when you’re testing a patient who’ve you specifically dark-adapted (sit in a totally dark room for 20 minutes) for testing with electroretinography.
I think the red light is probably enabling to get some limited use out of your cone vision (otherwise quite useless in very low light) without wiping out your rod vision by overloading the rods, as more light in the shorter wavelengths would tend to do.
Also useful for collecting night-crawlers for fishing.
Something interesting about blue lights: they stand out against a cluttered background of regular auto lights at very great distances.
As soon as I started living in Toronto (the only place I’ve ever been where I found this to be true), I noticed that all the TTC buses are equipped with blue lights on either side of the route display above the windscreen.
At night, they’re very easy to spot from amongst the yellow and white lights of other oncoming vehicles. It’s not just the color; they actually seem to shine more intensely than the other lights. You can tell a bus is coming from a long, long way away.
Are you sure they weren’t just equipped with HID lights, which typically run bluer & brighter?
Yes, pretty much. terentii had the full explanation, but you guys are saying the same thing.