DAYLIGHT, ARTIFICIAL. The wide use of artificial light in civilization has created a demand for lamps which show coloured objects truthfully; i.e., which do not change colours from their hues as seen by daylight. The problem is thus one of producing an artificial light whose spectrum (see LIGHT) closely resembles that of sunlight. Artificial light always contains too high a proportion of red, orange and yellow rays.
A gas-filled electric lamp is employed, as its filament tempera ture is high and its form the most economical. The spectrum obtained from a black body, heated to 5,000° C., is found to be approximately that of average diffused daylight. According to the temperature at which the filament of an incandescent electric lamp burns the amount of red and orange contained in the spectrum of its light varies, becoming greater as the temperature is lowered; a metal filament bulb burns at 2,200° and has more red and orange in it than the gas-filled bulb, which burns at approximately 3,000°. Therefore, the gas-filled bulb is used in artificial daylight devices, but its light has to be subjected to special treatment to correct its undue yellowness and redness.
Various means are employed for this purpose, such as filtering the light through a coloured transparent medium ; i.e., glass, talc or varnish, or by the use of a coloured reflector, or by a series of coloured mirrors fixed in a reflector. Coloured reflectors with certain pigmentations have proved successful, and by this means it is possible to match practically any type of daylight, but this method absorbs a very large quantity of the initial illumination. By the use of tinted or coloured glasses, one or more different colours being placed one behind the other, the same effect can be obtained; the loss of light by this method, however, is not so high as in the former.
Artificial daylight was at first mainly used for the purpose of matching colours. It has come into use for general illumi nation, however, as the value of artificial daylight in resting the eyes has become appreciated. Hospital staffs, dentists, physicians and surgeons find artificial daylight of great value. The method is also employed in dye and colour works. See LIGHTING.