The human eye, unlike some animal eyes, is dependent on light for its function. The less light that surrounds us, the less shapes and outlines can be perceived. The more light that enters our eye, the more colorful and clear the world around us becomes. For this reason, the human eye has the mechanism of brightness adaptation (also called light adaptation), by means of which it can adapt to different degrees of brightness. If this does not function or functions poorly, it can lead to limitations in vision or health impairments.
What is bright adaptation?
By definition, brightness adaptation is the adaptation of the visual organ to different levels of brightness. By definition, bright adaptation is the adaptation of the visual organ to different levels of brightness. The word adaptare (German: adapt) comes from Latin and is still used in German as well as in Romance languages for the process of adaptation. The eye can adjust to different intensities of light by opening and narrowing the pupil. A healthy eye performs this task automatically – it is one of the reflexes that occur in the body without the involvement of consciousness. Automated protective mechanisms of the body, such as increased blinking and squinting of the eyes, are also secondary to the concept of bright adaptation.
Function and task
The pupil is not a skin or an organ, but an opening into the interior of the eye. All around it is bordered by the brown, green, or blue iris. The iris has two smooth muscles – the pupil dilator and the pupil constrictor – which trigger the pupillary reflex by tensing and relaxing. These are parasympathetic muscles that belong to the smooth and unconsciously controllable musculature. The pupil constrictor can be observed very well by suddenly looking into bright light, but the pupil dilator takes a little longer to react to a darker environment – this can also be observed when changing from a bright to a dark environment. The cause of this phenomenon is the rods and cones on the retina, which are responsible for color vision in high light and black and white vision in low light. They react immediately to light stimuli and send the corresponding message to the brain via the optic nerve. A functioning brightness adaptation ensures that we immediately perceive too much light, which cannot be handled by the pupillary reflex alone, as unpleasant and close our eyes, shade them with our hands, put on sunglasses or goggles, or leave the bright environment. The automatic protective measures we take also include blinking much more frequently and squinting our eyelids. This is because a prolonged look at the sun is enough to cause the temperature inside the eye, and here especially on the lens and retina, to rise by two to three degrees. A functioning brightness adaptation, however, only affects the light spectrum that can be perceived by the eyes. Large parts of the ultraviolet, infrared and blue light are not perceptible and can reach the retina unhindered through the lens – here the pupil reflex must be supported by suitable protective devices such as good sunglasses. Children in particular are at risk and must be protected at all costs. In a child in the first year of life, almost all UV rays reach the retina unimpeded; only in adulthood are they almost completely absorbed by the lens. In diabetics, the situation is similar to that in children.
Diseases and complaints
The pupillary reflex is very important for humans and their eyes, as the eye can be severely damaged by too much brightness in the long run. Continuous strong light radiation, which hits the lens and subsequently bundled on the retina, leads to injuries and thus to vision problems or loss of vision. Our eyes cannot simply be switched off, which means that as long as we are alive and awake, they must be able to process the incidence of light, and this includes not only the perceptible light spectrum but also ultraviolet light, infrared light and blue light. Not to be forgotten in this context are also the artificial light sources by which our civilization is constantly surrounded (lamps, headlights, lasers).The greater strain on the eye, in contrast to earlier times, results from higher life expectancy, changed leisure behavior (vacations, snow sports, water sports) and changed environmental conditions (hole in the ozone layer). People should be aware that snow reflects up to 80% of the sun’s rays, water reflects a quarter, and light-colored sand reflects about 10%.
Damage caused by too much brightness or reduced or insufficient brightness adaptation can primarily affect the lens, but subsequently also the choroid and retina. The cornea and conjunctiva, which lie in front of the pupil, can also be damaged by too much light and continuous exposure to light (snow blindness, blinding), but this cannot be influenced or avoided by brightness adaptation, only by appropriate protection. The lens, which concentrates the incident light, receives most of the incident radiation. Continuous exposure to light can trigger or accelerate cataracts (lens clouding, reduced visual acuity and reduced transparency). A damaged lens cannot be regenerated by the body and must be surgically replaced. The choroid, which supplies the eye with blood, is also affected by too much light, as is the retina, which it supplies. Continuous exposure to light leads to permanent damage of the retina and the macula (place of sharpest vision). Each small tear in the retina manifests itself in diminished vision, larger failures show up in blind, i.e. dark spots and other limitations in the visual field. Melanomas of these skins can also be attributed in part to constant and high light exposure. A damaged retina is irreparable. While light damage to the outer eye, i.e., cornea and conjunctiva, can be detected and treated immediately due to extreme pain, damage to the lens, choroid and retina sets in insidiously and is therefore difficult or impossible to treat.
