Wavelength range and sources
The wavelength range where optical radiation is visible does not have sharp borders. Here, the wavelength band of 380 nm to 780 nm is used. There is an overlap with the UV wavelength range that extends to 400 nm and in the upper range with Infrared.
Common natural sources that produce visible radiation are the sun and fire. Common artificial sources include lamps for lighting, projectors, displays, indicator lights, welding arcs and lasers.
Effects of visible light on the body and health implications
The unique property of visible radiation, or “light”, is that it is imaged through the cornea and lens of the eye to the retina and induces a visual stimulus. Under normal circumstances, the pupil of the eye controls the level of light that reaches the retina. When this level is too high, a natural aversion response, such as squinting or blinking, occurs.
Looking into the sun or another bright source for a brief moment can lead to the transient occurrence of after-images. Additionally, “flash blindness” can also occur. These effects can be discomforting and in the case of flash blindness can be dangerous in certain situations, for example for drivers and pilots.
Staring at the sun, for example, when viewing a solar eclipse with unprotected eyes, or other very bright sources, such as welding arcs, for a longer time, will induce phototoxic reactions in the retina that can ultimately lead to loss of visual function in the affected areas. This is referred to as photochemical retinal hazard or blue light hazard.
Exposure to extremely bright sources such as high power flash-lamps at close distances, or laser radiation with intermediate or higher power levels, can also result in high local temperature rises inducing retinal thermal injury within milliseconds (i.e. much too fast for the natural aversion response to prevent injury). This retinal burn leads to a localised loss in the visual field, i.e. a dark spot. Other absorbing tissues, particularly the iris and the skin, are also at risk from burning.
Visible radiation is not thought to play a role in initiating skin cancer. However, some medications can result in the skin becoming photosensitized so that exposure to visible radiation can cause toxic photochemical reactions in the skin.
If the whole body is subjected to high levels of visible (and/or infrared) radiation, then the body core temperature can rise to a level that provokes physical heat-stress. Heat stress needs to be evaluated by considering all contributory factors, i.e. not only the irradiance originating from the source but also air movement, air temperature and humidity, as well as workload.
With regard to shorter-wavelength (i.e. blue light) visible radiation, there are some ongoing discussions that this might accelerate retinal aging.
Blue light also suppresses the secretion of melatonin, which contributes to the regulation of sleep and wake cycles. The health implications of de-regulation of the day and night (circadian) rhythm remain unclear and continue to be the subject of research.
Protection recommendations are aimed especially at the skin and relevant parts of the eye, which are at risk from excessive exposure to bright and intense light sources.
To avoid detrimental health effects from visible radiation on the eye and skin, such as thermally or photochemically induced injuries, ICNIRP provides guidance and recommends limits of exposure. There are different exposure limits to protect the skin and the eye that depend on the wavelength bands and action spectra and the wavelength dependence of the hazard. The limits also depend on the exposure duration and in some cases on the size of the source.
For some lasers, damage can occur in an extremely short time, within a fraction of a second. Of particular concern for public health are laser pointers of powers exceeding 5 mW , which may be purchased by private individuals and used by children who are unaware of the severe hazard that they present.