GENERATION, CONTROL, AND MEASUREMENT 151 



ation properties are a unique function of temperature and may be calcu- 

 lated with precision, it has become the practice to specify incomplete or 

 selective radiators by that temperature at which a complete radiator has 

 similar radiation characteristics. Since the spectral emissivity is a func- 

 tion of both temperature and wave length, the comparison can be made 

 only on the basis of one of the three criteria: (1) radiance, (2) brightness, 

 or (3) color, or spectral energy distribution in the visible region. 



The radiation temperature of a source is that temperature at which a 

 complete radiator produces the same total radiant flux per unit area 

 (radiant emittance) as the selective radiator. Since complete radiators 

 are the most efficient, the radiation temperature is always less than the 

 true temperature. The brightness temperature is that temperature at 

 which a complete radiator produces the same brightness (luminous flux 

 per unit area) as the selective radiator. Optical pyrometers for the 

 remote measurement of the temperature of incandescent objects, such as 

 the interior of furnaces, measure brightness temperature. The color 

 temperature of a source is the temperature at which a complete radiator 

 produces a chromaticity or color match with the source. The color tem- 

 perature may be higher than the actual temperature of the selective radi- 

 ator, because many metals, including tungsten, emit radiant flux at a 

 given temperature with the wave-length maximum shifted toward the 

 shorter wave lengths as compared with a complete radiator at the same 

 temperature. Color temperatures of tungsten-filament lamps are deter- 

 mined by various methods, the more important of which have been 

 reviewed by Harding (1950). One method involves the determination 

 of the ratio of blue to red flux, thus determining the average slope of the 

 emission curve in the visible region. 



Luminous Efficiency. The luminous efficiency of the radiant energy 

 of a source is probably the most convenient means of expressing the 

 proportion of the total spectral energy within the visible spectrum. It 

 must be recognized, however, that the term involves the evaluation of 

 the energy over the luminosity curve (Fig. 3-2). Luminous efficiency 

 may be expressed in various units, but those most commonly used are 

 lumens per watt (radiated watts, not electrical) and lux or foot-candles 

 per calorie per minute per square centimeter. The foot-candle per cal 

 min~^ cm~2, also referred to as the illumination equivalent of 1 cal min~^ 

 cm~2, has been used by the Weather Bureau for the evaluation of solar 

 energy (Kimball, 1924). 



The upper limit of irradiance which plant and animal tissues can toler- 

 ate is often determined by the heating effect of the absorbed energy. 

 Therefore, in studies of vision, photosynthesis, and other mechanisms 

 involving only the visible, the infrared is an undesirable component that 

 may represent the largest proportion of the total energy. In direct noon 

 summer sunlight in the tropical and temperate zones, plants and ani- 



