# eli5: How does my UV flashlight have a higher uv index rating than the Sun

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When I measured the sun, it gave a UV index of 6. When I measured the uv flashlight, it gave a uv index of 196. Is the sun not more powerful in UV than the flashlight?

In: Physics

While the sun is immensely bright, it’s also far away. We can make artificial lights that, relative to the distance from the source to you, are brighter than the sun. Same concept with UV. Your UV flashlight may not be on the same order of magnitude of intensity as the sun, but it’s magnitudes closer, so more of the photons hit you and/or your sensor.

The sun is powerful but it’s very far away. The sun is also very hot but if you have a heater next to you you’ll feel that more than you feel the sun’s warmth. The sun is far away and its radiation spreads out the farther away you are. Ultimately the Earth, and more specifically the area you’re in, receives only a tiny fraction of the sun’s energy.

Radiation is kind of like sound. The farther away you are, the less powerful it gets. Someone whispering in your ear is louder to you than an earthquake on the other side of the world. The sun puts off more UV radiation than the flashlight, but it’s much farther away, AND some of that radiation that DOES make it here gets blocked by the atmosphere.

You have to look at two things:

What wavelength is the detector measuring and how much of that is absorbed by the atmosphere? Large portions of the UV spectrum are absorbed by the atmosphere, and I’m not sure, but it’s possible the Sun only emits certain wavelengths of UV.

The total output of the Sun would be trillions of times what your flashlight produces, but you may not see much of it (which is a good thing).

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There are 3 types of UV radiation based on their wavelength. [UVC, UVB, and UVA.](https://www.hepacart.com/hubfs/03-Images/Blog/UV-ForceAirQuality_UVDifference.jpg#keepProtocol) UVC is entirely absorbed by the atmosphere around the earth, UVB is partially absorbed, and UVA relatively passes through (all are absorbed to some extend, I’m talking relative to each other).

Typical UV flashlights are only submitting UVA, the higher wavelength UV radiation at 365-380nm, while UV Index focuses much more on the shorter wavelengths (UVB) as they are responsible for sunburn (not entirely true because the shorter wavelengths of UVA also cause sunburn, but for simplicity lets go with this).

UV Index is [weighted across its power spectrum](https://en.wikipedia.org/wiki/Ultraviolet_index#/media/File:Erythemal_action_spectrum.svg) based on the Erythemal action spectrum (how sensitive the skin is at a certain wavelength of light – ie. sunburn sensitivity).

UVA is weighted just very very lightly because its not very damaging compared to UVB.

There is no way that a UV Flashlight would be giving you a UV Index of **196** as it only gives out UVA light of a longer wavelength which does not cause any meaningful skin reaction. **Either that, or you have a UV-C Flashlight which you should destroy and throw away because that’s a dangerous device and NOT A TOY.**

When I take my calibrated UV Index meter into the sun I tend to get exactly the same measurements as weather stations. When I stick it against a powerful UV light for reptiles, it spikes to 13-15 which is absurdly high. My UV Flashlight doesn’t register, and it’s a 2W UVA flashlight.

I can guarantee you that unless you have a powerful UVB source and you meter it very close to it, the sun is more powerful, certainly around the tropics where you get a UV Index of 10-11 at ~sea level.

For reference at a UV Index of 15, you can end up with skinburn in as little as 7 minutes exposure. You can reach that when you are high up in the mountains at clear sky around the equator. And while exposure time doesn’t drastically go down further by a higher amount of UV radiation, 196 is very very extreme. So I hope your measuring device is broken and you are not messing around with a potentially dangerous UV source.

Eli5 is distance. The sun is massively brighter and hotter, but it is very far away.

It’s like taking a small lighter and putting your hands right on top of it and burning yourself versus being at the other side of the room to the lighter. Distance.

Right now if you go out in the sun you may get a sunburn from its heat. If you went right up to the sun directly, you’d be instantly burned to a crisp.