Effects of light

Visual effects

Light allows us to perceive our surroundings using our eyes. The visual effects of light on humans result from the stimulation of special receptor cells ("rods" and "cones") in the retina of the eye. The stimuli acting on the receptors are relayed via the optical nerve to the areas of the brain responsible for processing visual signals. The cones are used for colour vision, and the more light-sensitive rods allow us to see at night or in twilight – albeit only in black and white.

Non-visual (melanopic) effects

Light is not only used for sight but also has an influence on endogenous messenger substances and sets the "body clock". The ability of light to act as a timekeeper is primarily thanks to a third type of receptors, which are also localised in the retina: the photosensitive (light-sensitive) retinal ganglion cells. This receptor type responds primarily (but not exclusively) to blue light and mediates, among other things, the suppression of the secretion of the "sleep hormone" melatonin from the pineal gland. This is the mechanism by which light influences the sleep/wake cycle, promoting alertness during the day and having a beneficial effect on performance and mood. In the evening and at night, as the rising melatonin level promotes the onset of sleep and the body prepares itself for a period of rest, light can inhibit sleep, especially if it has a high proportion of blue light.

Too much light can be harmful

If the irradiance exceeds certain levels, light can also be harmful – especially to the eyes. This is particularly true of the high-energy blue region of the visible spectrum ("blue light hazard"). Energy absorbed from the light is converted into chemical reaction energy. For example, this can result in the formation of reactive forms of oxygen that then go on to attack cellular structures and even DNA. This can cause photochemical damage to the retina of the eye (photoretinitis).

In general, it is not advisable to look into a bright source of radiation from a short distance for a prolonged period of time. This particularly applies to children, as the lenses of their eyes are more transparent to visible light (and also to UV-A radiation) than those of adults. Normally, people find it unpleasant to look at an (excessively) bright light source anyway, which is enough to ensure that they refrain from doing so.

Thermal effects predominate in the long-wavelength, red region of the visible spectrum – as well as in the infrared region. The damage essentially results from heating, although this is only possible at higher irradiances.

Lasting damage to the retina can be caused by looking at the sun or powerful sources of artificial light, e.g. lasers, without eye protection.

Glare

If the eye is exposed to excess luminous intensity, its ability to adapt is overwhelmed. The person is dazzled. The extent and duration of dazzling depend on several factors, including:

• the luminance and size of the light source
• the point of projection on the retina
• the ambient brightness
• the eye’s state of adaptation
• individual factors such as the age of the affected individual.

In so-called "disability glare", the perception of visual information is actually reduced in a measurable way, whereas so-called "discomfort glare" causes light to be perceived as unpleasantly bright or distracting. The latter form in particular is heavily influenced by subjective perception.

Glare limits a person’s ability to see for a certain period of time. Even though this effect is temporary and does not constitute damage to the eyes, it can significantly increase the risk of accidents.

The dazzling of vehicle drivers or pilots by light sources such as powerful laser pointers is very dangerous and can result in the loss of human life. Dazzling a pilot is a criminal offence in Germany (section 315 of the German Criminal Code).