Why doesn't it damage your eyes to look at light after they've been dilated?

November 9, 2009 4:52 PM

I'm posting for my wife who just got her eyes dilated at 4 p.m. (so she can't look at the computer to type this herself). I tried to Google my question and couldn't come up with any satisfactory answers. The question is why doesn't dilating your eyes and then viewing any form of light harm your eyes?

The pupils adjust to regulate how much light is taken in. Is this a safeguard? I don't see how it's not damaging your eyes to shine a bright light in them when your pupils are fully dilated, because, and I may be wrong, but I've never seen pupils naturally get this big. But then again, you need light to view your eyes so maybe they do in a pitch dark room.

I read Catch Me If You Can, and when Frank was imprisoned in a French dungeon, deprived of any light, they had to slowly reintroduce him to even the dimmest light so not to damage his eyes. Why was this and how would this relate to dilation?

I hope I'm being clear enough. I just want to know why it isn't damaging because the eye doctor told her to come in every year, and it seems like over the course of a lifetime that would be a lot of damage, if damage is occurring. Thanks so much!

posted by LillyBird to health & fitness (10 comments total)

Staring into the sun damages your vision due to UV rays. Staring into a laser can damage your eyes because of burns from the power level involved, or from photochemical damage (Wikipedia).

There are no UV rays or lasers in the visible lights used to examine your eyes-- they're just very bright, like spotlights-- so you're cool. It may be unpleasant, but no lasting damage will occur. That being said, make sure your wife has a good pair of sunglasses for the walk out to your car afterwards-- sunlight post-exam is no fun.
posted by fairytale of los angeles at 5:09 PM on November 9, 2009 [3 favorites]

One amazing thing about your eyes (and visual system) is the incredible range of brightness that it can function under. If you view the pages of a book under normal indoor illumination, the text looks black, and the pages look white. If you take that book outside, the text looks black, and the pages look white.

In actuality, the black text outside is reflecting about 100x the luminous intensity of the white background of the page inside.

Over exponential scales of light intensity, I would imagine that dilation of your pupil is pretty negligible in terms of appreciably modulating the intensity of light hitting the retina.
posted by Maxwell_Smart at 5:16 PM on November 9, 2009

Viewing non-dim light while your pupils are dilated definitely doesn't feel good and can hurt/be nauseating. Im sure prolonged exposure will cause permanent damage, but so will looking at the sun while your eyes aren't dilated.
posted by wongcorgi at 5:36 PM on November 9, 2009

Are you referring to the fact that the doctor shines a light into your eyes after dilating them, or the fact that after you leave the office they remain dilated for some time? For the latter, at least every doctor I've been to hands out disposable sunglasses for you to wear home. Additionally I find that the body compensates to reduce the light input in other ways, mainly squinting.
posted by Rhomboid at 5:48 PM on November 9, 2009

You can damage your eyes when they're dilated, if the light is too bright. So tell your wife not to experiment.

But the reason your irises can dilate as far as hers currently are is that in some lighting conditions you need that much light to be able to see well.
posted by Chocolate Pickle at 6:46 PM on November 9, 2009

I had my eyes dialated once. I went inside and pulled all the drapes and turned off all the lights but during the day it was still too bright. I put on some sunglasses and everything was fine. And I put on a second pair to go outside. I looked ridiculous, but it worked.
posted by zardoz at 6:48 PM on November 9, 2009

My dad has to get this done semi-regularly. He uses welding goggles afterwards (not sure which kind).
posted by Decimask at 8:00 AM on November 10, 2009

Hi everyone--thanks so much for responding. I'm the wife--the one who got her eyes dilated--and I guess I forgot just how uncomfortable this was to have done since I last had it as a kid. I suppose I was equating *pain* with damage, and the mere fact that a tiny red bulb on my coffee maker was causing discomfort, even while wearing sunglasses, made me concerned. It's been 21 hours and I still need sunglasses to view brighter light. (I have light blue eyes, and I hear that makes a difference.)

There was one interesting thing that happened. I was wearing my old (too weak) glasses because the poorer vision helped cut the brightness. At 10pm, I noticed that I could read a digital clock from across the room--something unheard of in these glasses. My only thought was that I was seeing out of a larger surface area, which maybe corrected for some of the curvature in the eye.

Maxwell_Smart: That's very interesting. I had never considered that change in intensity under different lighting circumstances.

Decimask: My husband and I were teasing that I need to borrow my aunt's welder's mask. I can certainly understand why your father does this!

And thanks so much everyone else! You helped calm my nerves over the last 21 hours of almost total darkness. :)
posted by LillyBird at 11:12 AM on November 10, 2009

Think of it this way-- it's not much different, in the context of the brief exposure to intense light and dilating drops, than changing the f-stop on your camera. :)
posted by fairytale of los angeles at 11:57 AM on November 10, 2009

I wanted to update this, as I had problems after the dilation and asked the doctor this question.

First, he said that exposure to light while the eyes are dilated CAN harm the eyes. To quote him, "If we camped out with the light 'here' for a minute, then yes, it can damage the eyes." He said that the brief exposure, however, shouldn't damage the eye. But then I called my brother-in-law's brother, who is an eye surgeon, and he said that he shines a light in a patient's eye for a 1/2 hour with cataract surgery and they are fine. So I really can't say there's a clear answer.

However, I am experiencing a very odd problem that the eye doctor said he truthfully doesn't know what it is and has referred me to a retinal specialist. Ever since the blurriness cleared yesterday evening, I am seeing a light and dark splotch in my right eye, very similar to the shadowy afterimage you get after viewing light. (So it's been over 24 hours.) He rechecked my eyes today and sees no damage. He listed a variety of things from sinuses to migraine to coincidence, but admitted he has no idea what's causing it. My brother-in-law's brother said to go to an ophthalmologist to have more tests done. He can't see me because he lives across the country. I'm not sure if this is "allowed" to sort of ask another side question within a question, but if anyone has experienced this shadowy/bright afterimage after a dilation and knows what it is and, more importantly, if I can expect it to go away, I'd greatly appreciate it. At this point, I'm very leery of having more light shined in my eyes because that makes it worse. Thanks everyone!
posted by LillyBird at 5:03 PM on November 11, 2009