Why does red light not affect night vision

Night vision, green light instead of red light

Fran├žois-Xavier RicardouPublished on

The necessary adjustment phase

The eye is an organ that we usually use very poorly, especially at night. He has the ability to see well at night but takes a few minutes to adjust (between 10 and 15 minutes). During the adjustment, the pupil opens, its diameter increases from 2 to 8 mm. This means that when fully opened, around 16 times more light can pass through than with daylight.

This opening of the pupil is not the only adaptation to the darkness. The delicate surface of our eye, the retina, is lined with two types of receptor cells: cones and rods.

The color-sensitive cones adapt quickly to strong lighting conditions. The sticks adapt very slowly to low light conditions, but are not very color-sensitive.

Why should we avoid bright lights?

During the adjustment phase, the eye makes sticks. These are irreparably destroyed if one is blinded. In strong light it is necessary to resume night adjustment at the beginning ... Therefore it is not necessary to use strong lighting during night navigation. No lamp to light the helmsman, no lighter to light a cigarette ...

Only use a light source that is so weak that it does not allow color to be seen if necessary. It should preferably be green. Contrary to what might be thought, red light should be avoided as the eye's sensitivity to this tint is very low. This requires the use of relatively intense lighting to see details. The opposite of the hoped-for effect!

Night vision requires good nutrition!

Proper nutrition is critical to good eyesight. Cones and chopsticks use a derivative of vitamin A to convert light energy into nerve impulses. In its direct form - retinol - vitamin A is contained in quantities in foods of animal origin: fish oils, liver, egg yolks, but also ... caviar!

But the body is also able to convert beta-carotene into vitamin A. It can be found in carrots, spinach, sweet potatoes or red pepper and especially in lamb's lettuce. Melon, mango, and apricots also contain fruits. Not to forget the bilberries and black berries - black currants, bilberries - which the Allied pilots ate a lot during World War II.

Why look at the dimly lit objects on the side?

The cones are used at night for maximum sensitivity. The cones and rods are not distributed in the same places on the retina. The cones are collected near the axis of the eye (in the middle), the rods surround this central zone. The area most sensitive to low light (the rods) is offset from the center of the retina. In order to distinguish between the weakest objects, one therefore has to look slightly to the side. It is said that peripheral vision is then used.

For the sailor this means looking at the horizon without looking in the middle. Light details (boat lighting, headlights ...) are visible even at very low density.