Want to stay on top of all the space news? Follow @universetoday on Twitter
Dark adaptation is the adjustment of the human eye to a dark environment. That adjustment takes longer depending on the amount of light in the environment that a person has just left. Leaving a bright room to a dark one takes longer than leaving a dim room and going to a dark one.
The time course for dark adaptation is well known. It is a two branched function; one for the cone receptors and the other for the rods. The receptors in your eyes contain photopigments in their outer segments. When light is absorbed by these photopigments they undergo certain changes which stop them from sending visual signals to the brain. These changes are reversed in darkness.
Our visual system is most sensitive when the photopigments have not absorbed any light for about 30 minutes. After that the photopigments are fully regenerated. When the rod photopigments are exposed to light they undergo a process called bleaching. It is called bleaching because the photopigment color actually become almost transparent. In the dark they regenerate and regain their pigmentation. In the rod receptors the unbleached photopigments appears purple(sometimes called visual purple). The technical name for the rod photopigment is rhodopsin. The cone receptors also have outer segments which contain photopigments. The photopigments in the cones also bleach when exposed to light. There are three classes of cone photopigments. Each class is photochemically different so the wavelengths of light that they absorb are different.
A person whose body has trouble with dark adaptation is generally night blind(nyctalopia). The most common cause of nyctalopia is retinitis pigmentosa, which is a disorder where the rod cells gradually lose their ability to respond to the light. Patients suffering from this condition have progressive nyctalopia and eventually their daytime vision may also be affected. Another cause of night blindness is a deficiency of retinol, or vitamin A, found in fish oils, liver and dairy products.
Astronomers need to have good dark adaptation in order to successfully perform visual observations through telescopes.
We have written many articles about dark adaptation for Universe Today. Here’s an article about the first telescope, and here’s an article about the May 14-16, 2010 Weekend Skywatcher Forecast.
We’ve also recorded an entire episode of Astronomy Cast all about Telescopes. Listen here, Episode 33: Choosing and Using a Telescope.