[AndroidNeox (OP)]

How much protection do the eyelids provide? In UV? In visible light? Tanning might help with protecting from an excess of visible light, but I gather from what I've read that it only blocks about 25% of UV (SPF 4).

[evan_au]

The fact that we have a blink reflex suggests that eyelids provide protection from stuff getting in your eyes.

The embedded tear glands wash dust out of your eyes, and blinking mechanically sweeps dust away from your eyes.

At night, they stop your cornea from drying out, and stop dust from getting in your eyes while you are asleep.

[Bored chemist]

I'm pretty sure that the eyelid will afford much more than factor 4 protection.
It might, however get sunburned in doing so.

[AndroidNeox (OP)]

Sorry I wasn't clear (no pun intended). I meant to ask how much protection from light do the eyelids provide? Will they protect the eyes from facing the Sun? The SPF 4 came from something I read that tanned skin offers 4 times the UV protection of untanned skin. Is it dangerous for the eye if one "looks" at the sun with their eyes closed?

[evan_au]

You can lie in direct sunlight with your eyes closed perfectly safely. You see a warm red color, which is due to the red blood cells in the capillaries of your eyelids.

But your iris also has a considerable SPF.
- In the dark it opens wide, and in bright sunlight it closes down small. This gives an order-of-magnitude variation in light intensity
- If you are looking in the sky somewhere near the Sun, your iris blocks a considerable amount of light from striking the retina (don't look straight at the Sun!)
- Slightly more if you are brown-eyed compared to blue-eyed

The cornea and lens of your eye block parts of the infra-red spectrum.

Be aware that UV damage to the eyes is cumulative, and increases the chance of cataracts.

The lens of the eye blocks UV light - it is said that people receiving glass lenses in early cataract surgery were able to see in UV light, as glass is less effective at blocking UV light. But I suspect that the UV light now reaching the retina would damage the retina.

In bright sunlight, wear sunglasses which have been tested to block UV light.

[alice123]

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