I have a kind of weird question. What causes retinal burns from looking at the sun? I remember when I was younger, staring at the sun for a couple minutes without blinking just cause I thought it was neat the way it'd leave a purple afterimage on things. As far as I know, it didn't cause any damage (although, like everyone in my immediate family, I wear glasses). How long would one have to look at the sun to cause irreversible damage (on average)?

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"Some see the glass half full, some see it half empty, and some see it crawling with toxic alien parasites who want to devour your pancreas." - Sgt Aarhus, from the book Ascending by James Alan Gardner

I can find all kinds of references to eye damage caused by UV radiation I have yet to find any that discuss how long it takes to cause the damage. I guess that i haven't looked hard enough yet. [img]/phpBB/images/smiles/icon_wink.gif[/img]

Well, that clears up what kinds of damage can be caused by looking at the sun, but not really how long it'd take to actually cause said damage. I guess the UV blocking on the car windows (said it right at the bottom right hand side of the glass, that I remember) helped a bit.

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"Some see the glass half full, some see it half empty, and some see it crawling with toxic alien parasites who want to devour your pancreas." - Sgt Aarhus, from the book Ascending by James Alan Gardner

I can't give you a definitive answer as to how long it would take to cause retinal damage. Part of the problem in determining this is that the damage doesn't become apparent until several hours have passed after exposure. Since there are no pain receptors in the retina the victim is not aware of the damage caused until it is too late - hence the danger.

A good explanation of the process causing the damage can found in a NASA article

http://www.offthemark.com/Images/mot.../nursery25.gif

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It's probably variable due to natural variation in the human eye and variation of viewing conditions. If you have glasses, that may actually slow down the effect somewhat, especially if you've got a special coating on the lenses for UV protection. It might be different depending on whether you are nearsighted or farsighted and whether or not you are looking through the center of the lens. I can definitely tell that my eyes are least light-sensitive when I have my glasses on; with my contacts they become painfully light-sensitive. (I'm not sure how light-sensitive I am with neither; I'm literally blind without my glasses so I don't often go outside without them.) I would bet that using no corrective lenses at all if you are either farsighted or nearsighted will cause the sun to take longer to damage your retina because the focal point of the light will no longer be directly on the most sensitive part of your retina. Astigmatism will probably also affect it, by spreading the worst damage out from the fovea of your eye.

Cloudiness, season, time of day, haziness -- these will also affect how long it takes to damage your eye. I remember when Yellowstone Park burned -- the smoke cloud actually extended over Minnesota, where I live, and I could see the Sun turned blood red in the sky by all the particulate matter in the air. It was still bright, but I could look straight at it without blinking or even having my eyes water. I don't know whether it would have eventually impaired my vision, though. It was a very strange sight -- a blood-red Sun at high noon.

Galileo, solar observing, and eye safety:
http://mintaka.sdsu.edu/GF/vision/Galileo.html

I've actually seen solar retinopathy up close and personal (I'm a practicing optometrist) but only on rare occation. This is not a common problem, by the way. It was rampent during the Vietnam war because guys would stare at the sun in order to get eye damage and get off the draft. I see similar damage in the eyes of people who have worked as arc welders for many years.

It was my understanding that solar retinopathy is not just caused by UV, but rather by the intensity of the light involved and to some degree the amount of heat transfer to the retina, espectially to tissues at the macula that are not nearly as vascular as the rest of the retina.

In fact, in theory, I could simulate the damage in my office with an instrument called a "BIO", or binocular indirect ophthalmoscope. It's essentially a head mounted lamp with a 20 diopter lens held in the middle as a magnifier. In theory, if the light and lens were held immobile for more than 30 seconds, you could start to damage to retina (difficult to do, the light is very bright and it's hard to get a patient to stay still long enough).

The upshot of all this is that the time involved is not something that you could measure easily with a stopwatch, but I would guess it's less than 2 minutes of constant exposure before damage sets in. Perhaps less than one if you've got a lightly pigmented retina.

I've actually seen solar retinopathy up close and personal (I'm a practicing optometrist) but only on rare occation. This is not a common problem, by the way. It was rampent during the Vietnam war because guys would stare at the sun in order to get eye damage and get off the draft. I see similar damage in the eyes of people who have worked as arc welders for many years.

It was my understanding that solar retinopathy is not just caused by UV, but rather by the intensity of the light involved and to some degree the amount of heat transfer to the retina, espectially to tissues at the macula that are not nearly as vascular as the rest of the retina.

In fact, in theory, I could simulate the damage in my office with an instrument called a "BIO", or binocular indirect ophthalmoscope. It's essentially a head mounted lamp with a 20 diopter lens held in the middle as a magnifier. In theory, if the light and lens were held immobile for more than 30 seconds, you could start to damage to retina (difficult to do, the light is very bright and it's hard to get a patient to stay still long enough).

The upshot of all this is that the time involved is not something that you could measure easily with a stopwatch, but I would guess it's less than 2 minutes of constant exposure before damage sets in. Perhaps less than one if you've got a lightly pigmented retina.