Seeing Red
Published on May 26, 2006 at 7:58 pm.
1 Comment.
Filed under amateur astronomy, skywatching.
I don’t recall if I’ve talked about this or not, but here goes. With summer coming on, the weather is nice, and sometimes people have more free time. Often stargazing picks up a bit in the summer. There are often more public star parties through the summer, too. So, a lot of people new to astronomy and stargazing decide to do some of their first observing. Now, they know that it is dark out at night (that’s the whole point!), so they bring a flashlight with them. Here is where the problem comes in. You see, the way that your eye works, in bright light two things happen. One, your eyes adjust the size of the pupil to admit more or less light depending upon the ambient light conditions. When it is bright, your pupils contract to admit less light, and when it is dark, your pupils widen to admit more light. This allows the eye to work in much the same way over wider ranges of light. Second, your eyes use a chemical called rhodopsin, or sometimes visual purple, to see. Rhodopsin is hightly photosensitive (duh!). But, this means that it is quickly bleached and becomes ineffective when exposed to high light intensities. So, bright light uses up the rhodopsin in the eye. But, you need the rhodopsin to see. In dim light, you need all the rhodopsin that you can get. So, if you expose your eyes to bright light, then you are in the dark, you can’t see for a while. It takes about 30 minutes for the rhodopsin to regenerate, and they you can see in the dark well. Interestingly, this is why really bright security lights are exactly the wrong thing to use at night. If you are under the light, then they night blind you to everything once you step out of the light, or try to look into a shadow.
So, when you use your flashlight to see, the extra light uses up more rhodopsin, and they you can’t see in the dark. Often at star parties, people will come up with flashlights on, and we ask them to turn them off. Then, they can’t see. They are apt to bump into things. Now, we can see them perfectly fine, because we’ve not been in the bright light. Of course, most things in the sky are dim, so when you are night blind, then you can’t see them. It takes time to get dark adapted again (as I said, about 30 minutes). But sometimes, you just need some light. Starlight often isn’t enough to read charts by. Sometimes, you need to look for something that you dropped in the grass, or perhaps you are looking for something in the bottom of a telescope case, or something. Perhaps you want to walk down a tree lined path, and even starlight doesn’t penetrate. Then, what? Well, you have to resort of a flashlight. But, if you are careful, you can do so without losing dark adaption.
It turns out that your eye has two receptors: rods and cones. The cones are used for color perception. The rods are used for black and white. The rods also are more sensitive in low light conditions. When it is actully dark out, you see in black and white. Try it sometime! You are unable to perceive color in very low light conditions! That is also why many things in the telescope look black and white, even though they have pretty colors when viewed through much larger telescopes or when you take a photo of them. Now, interestingly, the rods use a little different type of rhodopsin that absorbs more green and blue light. That means that red light doesn’t bother it as much. So, if you use a red flashlight, then you can see without losing your dark adaption. Now, I should point out that you are still better off using as dim of a red light as possible.
How do you get a red filtered flashlight? Well, many sporting goods places sell flashlights like I used in the Boy Scouts that have red filters that you can insert. Outdoorsmen use these to avoid damaging their dark adaption. You can also find them in military surplus stores. Or, you can make your own by taking an ordinary flashlight and putting a red film over the front. I have even seen flashlights with red fingernail polish on the front of them. All of that works. Red socks often don’t, though, because the white light shines between the red fibers.
Many astronomy suppliers now sell special astronomy flashlights that use red LED lights. These are even better than the other forms of red lights, because the light really is red. The other forms often leak some of the other colors, but normally not enough to notice unless you are really serious.
Anyway, if you are seeing red, then you will likely see better when you turn off the light.
Enjoy your nightime viewing!
-Astroprof
Astroprof’s Page » The North Texas Skywatch Star Party on November 10, 2007 at 12:45 am: 1
[…] The event is held at the trail head part of the park. There is a large parking area there where we set up telescopes. If you bring a telescope, let them know when you arrive so that they can tell you where to go. If you don’t bring a telescope, you’ll have to park a little farther away and walk. There is only so much room at the trail head. Besides, headlights going through there while people are trying to observe and work doesn’t work. We try to keep it dark right where the telescopes will be set up. Of course, if you need light to see where you are going, try to bring a red filtered flash light. The red light will let you see without ruining the dark adaption of everyone around you. […]