It is not good to see a screen in a room without light, while watching TV in the dark won’t leave us blind, nor produces lesions pemanentes or irreversible visual loss. The eye is a well-designed body, and there are very effective mechanisms to adapt to light and darkness. The two main mechanisms are: 1. the iris. What gives the color of the eyes really is a diaphragm that regulates the amount of light that enters our sensitive retina.

In normal circumstances we live with lots of light to our surrounding, and the iris is rather closed, i.e. the pupil (the girl), which is the hole where light, is tiny. Enters sufficient light to see well, but not excessive to make damage to the retina. In circumstances of great darkness, opens the iris, the pupil becomes very big and takes advantage of the little light that can reach the eye to see. If we recall animal documentaries, the eyes of owls at night are black, i.e. very large pupil.

2. The retina. It is the most sensitive and delicate eye part, there are millions of light receptors (such as case sensitive photoelectric cells). In a previous article I explained the types of cells that there are, and put photographs. But the important thing now is highlight that these receptors are dynamic and are adapted to our conditions of life. If we live in environments with lots of light, the receivers come down your sensitivity because luminosity excess can be harmful. In situations of penumbra receptors of the retina change their metabolism to increase this sensitivity. After 10 minutes of total darkness already begins to notice a remarkable increase of sensitivity, i.e. a small stimulus visual before it was invisible, after 10 minutes in the dark, we already see it. But this adaptation to darkness is exponential, and 16 days of total darkness (Yes, this experiment has been done in humans) we are able to see a lot more than in normal conditions. We almost become owls then have these 2 mechanisms of adaptation. If we are in an environment with luminosity average is enough to maintain the conditions which are called photopic, in other words, with an abundance of light. Put another way, rather small pupil and retina with low sensitivity. As soon as we turn off the light, adaptation to the medium escotopico, namely the darkness begins. If at that time we put ourselves in front of a TV or a monitor, we received a few rays of light in the center of vision that weakened our adaptation to darkness.