With enough intensity infrared light, could you tan your skin? Maybe this could be a safer way to darken skin without risking skin cancer.
I know for some people, inflammation causes skin to get darker without needing sunlight, so maybe if you heat your skin hot enough with a powerful infrared light you can get your skin inflamed enough to darken it.
Nope. Melanogenesis is only activated by UV radiation. IR will just heat you up, not recommended.
In other words, it’s not the heat that makes you tan, but the much shorter wavelengths of UV light.
But people can get dark spots on their skin after things like rashes . So maybe with enough heat from a source like a heat lamp the skin can get tanned if it gets inflamed.
Maybe you’re talking about scar tissue? That can look discolored next to healthier skin, but it’s not a tan, and you do not want to scar your entire body, do you? Inflammation will redden your skin due to increased blood flow, but that’s not the same as a tan.
You’ll get nicely brown due to the Maillard reaction. You may not enjoy the experience.
Neatly, that IR cannot make you tan whilst UV does is yet another example of the quantum nature of light. It doesn’t matter how intense the light, the wavelength of the light has to be short enough, - and thus each photon energetic enough, to make the triggering reaction work. You will tan under starlight - just not very fast. But you could stand next to an open blast furnace and near fry yourself, yet not tan in the slightest. Lots and lots of photons there, but none energetic enough.
Aside: I once taught an astronomy class, and one of my test questions was “Name two ways to detect non-visible light”. The answers I was expecting were the two methods we mentioned in class: A thermometer for infrared, and photographic film for ultraviolet. But one student correctly answered that you could detect ultraviolet by getting a tan from it.
While this has a nice Addams Family vibe to it, I suspect the rate of tanning under starlight << the rate that a tan fades (I am ignoring the fact that tanning during the day is under starlight :D).
Does the moon reflect any UV? A moontan might happen sooner than a startan.
It does, but considering a full moon is 400,000x dimmer than the sun, no need to grab the sunscreen. Maybe a relatively nearby supernova would do, assuming we don’t get zorched by the gamma rays.