Alien Eyes
Published on Aug 22, 2007 at 7:23 pm.
1 Comment.
Filed under amateur astronomy.
Yesterday I had an eye exam. My eyes require a bit different exam than most people every two to three years. This involves, among other things, extra strong eye drops that dilate my pupils. I can’t drive afterwards, so my girlfriend drove me to the appointment. But, she couldn’t hardly look at me afterwards, saying that my eyes were freaky looking. The pupils were so dilated that she didn’t even see any iris, just the black pupils. She said that I looked like an alien!
But, this incident gave me an idea for my next blog entry: dilated eyes and astronomy. People often ask me if getting their eyes dilated would help them see the stars at night. The short answer is “No!”
Now, for the longer answer, let me explain. Everyone who has ever tried to observe the stars is well aware of the fact that you have to be dark adapted in order to see the sky well. If you walk outside from a bright building, then you can’t see much of anything. If you are outside and dark adapted and look at a star chart using a bright white light, then you lose your dark adaption and can no longer observe well. If you are at the telescope at a star party and you are observing something when someone’s headlights play across the telescopes, you can lose your dark adaption, and it takes a while until you can again see the sky well. And, anyone who has had their eyes dilated knows that this makes you extra sensitive to bright lights, so it would seem that it would also make you extra sensitive to dim light, and thus automatically dark adapted. Well, that is a nice hypothesis, but it is largely wrong. Your eyes are a bit more complicated than that.
Most people are aware that the iris of your eye opens and closes the pupil in response to variations in light intensity. When you are exposed to bright light, the iris closes up, making the pupil very small, in order to limit the amount of light entering the eye. This keeps the retina from being damaged from very intense light. In low light conditions, the iris opens the pupil to allow more light in. This is very analogous to the way the f-stop works on a camera lens. The lower the light conditions, often the more open that you want the lens iris, and the brighter, often the narrower the iris is set (Note: this is a rule of thumb, but you can adjust the iris to adjust depth of field, too.). The change in pupil size is the most obvious thing that you notice as you go between bright and dark conditions. So, it is no surprise that people would wonder if you might be better dark adapted with chemically dilated pupils. But, as I said, your eyes are more complicated that most people suspect. Dark adaption is not just the size of the pupils.
In fact, the pupils dilate and contract fairly quickly. But, you dark adaption doesn’t change nearly as quickly as your pupils change, so that isn’t the sole determiner of your dark adaption. Also, astronomy requires more than just just large pupils, particularly at the telescope. And, that is the problem when your eyes are dilated. You see, the iris opens and closes in response to muscles in the eye. To dilate the iris, the eye drops temporarily paralyze the muscles that cause the iris to contract. Unfortunately, this also tends to have an effect on the muscles that are used to focus the lens of the eye. Anyone who’s eyes have been dilated may also have noticed that they had trouble focusing on a page to read it. Obviously that would have an effect if you were trying to read a star chart or look through a telescope!
So, dilating your eyes doesn’t do the same thing as getting dark adapted.
But, there’s more! In order to see, light entering your eye triggers chemical reactions in the rods and cones of your eyes. The cones are used for color vision, and the rods are used for black and white vision. The different cones and the rods use chemicals in the eye that are photoreactive (they undergo reactions when exposed to light, much like the chemicals in film emulsions). The more light that enters the eye, the more of the chemicals that are used. The cones require a higher light intensity level than the rods to see. So, under very low light conditions, you see with your rods. The rods, remember, are black and white receptors. So, under low light conditions you see in black and white. That is why when you look through a small telescope, you normally don’t see the pretty colors of nebulae that you see in photographs. The most important chemical for the rods is rhodopsin (sometimes called visual purple). Since the rods use rhodopsin, the more rhodopsin that is in the eye, the more sensitive the rods will be to low light levels. Rhodopsin is very photoreactive, which is why even low light levels cause the chemical reactions that the rods need to detect light. But, this reactivity means that rhodopsin is used up quickly when exposed to bright lights. It takes a while for the body to make more rhodopsin. In fact, it can take many minutes to make enough rhodopsin to see in low light levels again. Your dilated eyes allow more light in, and that can even cause bright light to use up even more rhodopsin. That may make your eyes even less dark adapted soon after you go from bright lights to the dark than if you did not have dilated eyes.
So, in summary, don’t expect to be able to go out observing after an eye exam where you have gotten your eyes dilated.
-Astroprof
Seeking Solace on August 22, 2007 at 7:54 pm: 1
I try to schedule my appointments when I have to get the drops on days where I can go home afterward. My eyes are so sensitive when I get the drops.
Btw, I tagged you for a meme.