Solar radiation is radiant energy emitted by the sun, particularly, electromagnetic energy. About half of the radiation is in the visible short-wave part of the electromagnetic spectrum. The other half is mostly in the near-infrared part, with some in the ultraviolet part of the spectrum. The portion of this ultraviolet radiation that is not absorbed by the atmosphere produces a suntan or a sunburn on people who have been in sunlight for extended periods of time. Solar radiation is commonly measured with a pyranometer or pyrheliometer.
Solar radiation is becoming increasingly appreciated because of its influence on living matter and the feasibility of its application for useful purposes. It is a perpetual source of natural energy that, along with other forms of renewable energy, has a great potential for a wide variety of applications due to its abundannce and accessibility. Solar radiation is rapidly gaining ground as a supplement to the nonrenewable sources of energy, which have a finite supply.
Recent developments in the areas of photochemistry and photobiology have also contributed  to bringing solar radiation to the limelight of science. The significant depletion of the stratospheric ozone layer, which shields the earth from much of the biologically injurious solar ultraviolet radiation (UVR), is apparently due to human activity and has now become a popular topic. Adverse biological effects of UVR on man include, among others, sunburn (erythema), conjunctivitis, and skin cancer. On the contrary, the vital phenomenon of photosynthesis is an example of the beneficial effects of sunlight in the natural environment. Other beneficial and harmful effects on a variety of living beings, especially microorganisms, are well documented. Solar radiation plays, therefore, a prominent ecological role.
Most photobiological investigations have been confined to laboratory work based on the use of monochromatic UVR produced artificially. As these model studies do not completely reflect the situation in nature, the recent trend is to supplement them with experimental work employing sunlight as a natural UV source. This produces results bearing more realistic, practical values. However, the polychromatic nature of solar radiation, its variable intensity, and the limited knowledge of its distribution at the local level tend to hamper such efforts and complicate the interpretation of the results.
Artificial sources of UVR are often used for a variety of purposes, ranging from experimenting to suntanning. Gas-discharge arcs, fluorescent lamps, and incandescent sources are some of the common artificial UVR sources, and they represent  potential health hazards  attributable to the significant amounts of biologically effective UVR emitted.
Photobiological research has been confined largely to laboratory work based on the use of UVR produced artificially in preference to solar radiation. The recent trend, however, is growing  toward the use of sunlight, so Sun radiation is considered as a supplementary source in radiation experiments.