LIGHT SOURCES USED IN HORTICULTURE USED IN THE U.S.A.
Light sources at one time could be grouped into three distinct categories; incandescent, mercury, and fluorescent. Recent developments have reduced this clear-cut separation between categories and have minimized differences to the extent that there is now some overlap in the comparative characteristics.
Manufacturers' advertising occasionally adds confusion. Each new variation of lamp is reported to its best advantage. There has been progress, but the lamp performance of new offerings must be compared with the old to form proper evaluation; for horticultural use, the final evaluation is by the plants with which they may be used.
Both plant science and illumination engineering are sufficiently developed that some preliminary predictions can be made without first-hand experimentation. For lighting information, the "Handbook of the Illuminating Engineering Society" (8) gives extensive information on electrical and other characteristics of lamps by class. There are a number of textbooks and reports concerning plant response to light.
Additional information on lighting or on specific lamps is available from the manufacturers. This engineering information is generally accepted as technically correct. It should be clearly understood that practically all such published illumination information is based on lumens and human vision. A lamp source which is efficient for human vision frequently is not necessarily efficient for plant response. Before discussing the comparative properties of various lamps, let us begin by briefly describing some of the standard features of American lamps.