GENERATION, CONTROL, AND MEASUREMENT 157 



est on Dec. 21. The relation is reversed for the southern hemisphere. 

 In Table 3-9 are given the day lengths for the twenty-first day of each 

 month for each 10° of latitude from the equator north to the seventieth 

 parallel. The day length was taken as the time from the first appearance 

 of the upper edge of the solar disk in the morning to its last appearance 

 in the evening. It takes into account the refraction of the sun's rays 

 by the atmosphere (List, 1951; U.S. Naval Observatory, 19-4:6). 



INCANDESCENT TUNGSTEN LAMP 



The tungsten-filament incandescent lamp is undoubtedly the most 

 versatile and generally useful of all artificial sources for the visible and 

 near infrared. It has highly stable electrical and radiation characteristics 

 and ordinarily requires no special auxiliary electrical equipment. It is 

 manufactured commercially in a wide variety of bulb and filament sizes, 

 shapes, and power ratings ranging from the small 0.17-w surgical lamp to 

 the 10-kw airport floodlight lamp (Weitz, 1950). 



Radiation Properties of Tungsten. The total and spectral character- 

 istics of the radiation emitted by a tungsten lamp are wholly dependent 

 upon the thermal-radiation properties of incandescent tungsten metal 

 within the limits imposed by the envelope. The electrical power required 

 serves only to heat the filament; it contributes nothing intrinsically to 

 the radiation. An incandescent tungsten filament is a selective thermal 

 radiator whose emission, total and spectral, is primarily a function of 

 temperature and secondarily one of filament configuration. 



Tungsten has a melting point of 3653°K (3380°C), the highest of any 

 known metal. In commercial lamps it is operated as a filament to as 

 high as 3350°K, which is within 300° of its melting point. The radiation 

 properties of tungsten from 1500°K to its melting point are given in Table 

 3-10, and the spectral energy distribution for a series of filament temper- 

 atures in Fig. 3-7 (Barnes and Forsythe, 1936a; Jones and Langmuir, 

 1927a,b,c; Forsythe and Worthing, 1925). The emissivity increases with 

 temperature, and total emissivity varies between 0.19 at 1500°K and 

 0.35 at 3500°K (Coblentz ei al, 1926; Coblentz and Stair, 1936, 1944; 

 Conn, 1951). The color temperature of tungsten is slightly higher than 

 the true temperature because of its selective emission in the shorter wave 

 lengths, a fact that contributes materially to the high luminous efficiency 

 of tungsten filaments. 



The spectral energy distribution of a uniformly heated straight round 

 filament may be obtained from the product of the spectral emissivity ex 

 and the spectral radiant intensity for a complete radiator at that temper- 

 ature. The spectral emissivity is a function of both temperature and 

 wave length (Table 3-10 and Fig. 3-7). The emissivity rises slowly with 

 decreasing wave length to a maximum in the vicinity of 300 myu, after 

 which it falls sharply (Forsythe and Adams, 1945; Ornstein, 1936). 



