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When plants are grown under high irradiances of incandescent lamp radiation, temperature control becomes difficult unless the infrared is removed. The infrared energy radiated by standard incandescent lamps is from 75 to 85% of the input power (2). Excessive heating of exposed portions of the plant results from direct absorption of infrared energy since most of that energy is in the region of the water absorption bands. The most effective filters for removing the near infrared are water or solutions of the ferrous or cupric salts (3, 4, 5). If the filters consist of shallow tanks open to the air, the rapid loss of water due to evaporation necessitates constant maintenance. Since most of the radiated energy is absorbed in the top layer of water, the temperature gradient does not induce appreciable convection. While cooling coils placed in the body of the liquid may keep the bottom of the tank cool, they do not appreciably reduce the evaporation rate. In attempting to solve the problem of evaporation in unsealed systems, it was found that if the water-cooling coils were suspended in the air space above the water surface, the water vapor was condensed on the coils and refluxed back into the tank. In addition, the cold condensate dropping upon the water surface set up convection currents, further reducing the temperature and evaporation. If the coils are placed near the top of the unit around the lamps, the air temperature is kept within safe limits for a completelv enclosed but not sealed system. Two types of lamp unit which have been constructed and used extensively in our laboratories are shown in detail in figures 1 and 2. Both of these units are designed for use with a layer of water or solution and the standard line of internal reflector lamps. A 10-cm. layer of water, as used in the large lamp unit of figure 2, absorbs all infrared beyond ca. 1150 mu (1). The addition of 30% ferrous ammonium sulphate decreases the limit to ca. 760 mland copper sulphate can be used to remove both red and infrared beyond ca. 600 mu. The 75-watt incandescent internal reflector lamp is four inches in diameter, and the 150and 300-watt incandescent and the 100watt mercury arc lamps are five inches. The incandescent lamps are available as reflector flood or spot lamps. The spot lamp is especially useful for very high irradiances over a small area; the flood is better for irradiation of larger areas.

Water-cooled Lamp Systems with Refluxing Aqueous Filters