That portion of the electromagnetic spectrum visible to the human eye, which we call light, is contained principally between the wave lengths 380 and 760 millimicrons (m¡u). By using very intense artificial sources, one can stretch the limits of human vision somewhat, obtaining the limits of 310 to 1050 millimicrons. Lying generally within this slightly wider range of wave lengths but mainly enclosed between 380 and 760 millimicrons, we find also the vision of other animals, the bending of plants toward light, the oriented movements of animals toward or away from light, and, most important, all types of photosynthesis. This is the domain of photobiology. According to Wald (1959), these same limits must be applicable everywhere in the universe.
Daily illumination intensities for active growth in green plants must fall between certain definite, but not too clearly established, limits. If the intensity of illumination is too low, for example, active photosynthesis can not proceed at a rate high enough to be useful; and if the intensity is too high, growth is inhibited by what has been termed "solarization." These lower and upper limits of illuminance may be set at approximately 0.02 and 30 lumens per square centimeter. (The maximum illuminance due to direct and scattered sunlight at the surface of the Earth is about 15 lumens per square centimeter.) The highest growth rates for terrestrial plants are encountered at intermediate levels of illumination. For some common species of algae, for example, the highest growth rates were found in the approximate range of 0.3 to 3.0 lumens per square centimeter (Krauss and Osretkar, 1961). On the other hand, human beings can see well enough to walk around if the illuminance is as low as 10~9 lumens per square centimeter, but they find that the level of over-all illuminance becomes painfully bright when it rises above about 50 lumens per square centimeter (Wulfeck et ah, 1958). This is an extreme oversimplification of a very complicated phenomenon, that of "glare discomfort," because many factors must be considered in addition to over-all illuminance: the reflectivity and distribution of surfaces and objects in the vicinity, the presence of shade and shadows, et cetera. Even ordinary levels of illuminance due to sunlight at the Earth's surface become intolerably high when one is surrounded by an unbroken field of diffusely reflecting material with a high albedo, such as fresh snow, giving rise to the well-known phenomenon of snow blindness.
[The above discussion of human tolerances for light refers to over-all illuminance levels, not to illumination directly entering the eye. Much lower values of illuminance must be specified in order for them to be considered tolerable to a man looking directly at a point source of light. For this, the upper limit is apparently of the order of 0.05 lumens per square centimeter (Hopkinson, 1956; Metcalf, 1958), which corresponds to a star of apparent visual magnitude about —21. The absolute lower limit of naked-eye detection of faint point sources in a very dark sky is about 1013 lumens per square centimeter, which corresponds to a star of magnitude 8, although under the best typical viewing conditions, it is difficult to see stars fainter than magnitude 6.5.]
Thus, illumination requirements are set primarily by the needs of plants and are such that during the growing seasons, mean daytime illumination levels must lie between 0.02 and 30 lumens per square centimeter.
Another factor of great importance in affecting the growth of plants is the periodicity of illuminance. Especially in the temperate regions of the Earth, plant growth cycles are determined by the relative or absolute lengths of days and nights, as well as by temperature patterns. There is, of course, a close relationship between temperature and light on planets such as the Earth, which have normally transparent atmospheres and are illuminated by incandescent bodies such as the Sun. As will be brought out later, most habitable planets must receive their primary supplies of heat and light from the same type of source; hence, not too much can be made of the requirement for a certain level of light intensity as an independent variable.
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