GENERATION, CONTROL, AND MEASUREMENT 161 



it is possible to calculate the change in color temperature dTc and from 

 this the change in spectral emission of tungsten for small changes in volt- 

 age dV /V . A 1 per cent increase in voltage causes a 12°K increase in 

 temperature, which results in the following increases in spectral emission: 

 1 per cent at 4000 m^u, 2 per cent at 1000 m^, 2.5 per cent at 800 m/x, 

 3.5 per cent at 600 m^, and 7 per cent at 350 m^t. It is thus evident 

 that large gains in radiant-energy output can be obtained in the near 

 ultraviolet by operating filaments at maximum voltage. In the infrared 

 the increase in intensity is approximately proportional to the change in 

 voltage. 



The voltage stability required of a power source to maintain a specified 

 stability in radiant flux at various wave lengths can be determined from 

 these data. To keep the radiant-flux variation to within 1 per cent in 

 the infrared, the voltage instability must be equal to or less than 1 per 

 cent; in the visible the voltage instability must be less than 0.3 per cent; 

 and in the ultraviolet, less than 0.1 per cent. This accounts in part for 

 the instability frequently encountered in the ultraviolet when a spectro- 

 photometer is operated with an incandescent source. Even a storage 

 battery is difficult to maintain with a voltage instability of less than 

 .0.1 per cent. An instrument that may be sufficiently stable in the infra- 

 red may be quite unstable in the ultraviolet. 



Tungsten has a high temperature coefficient of resistivity, which also 

 increases with temperature. The resistivity at room temperature, or 

 300°K, is 5.64 ^ohm-cm and at 3000°K is 96.2 ^ohm-cm, an increase of 

 17 times. As a result, there is a large inrush current at the instant of 

 applying power to an incandescent lamp. In actual practice the inrush 

 current is 8-12 times the normal operating current. With high-wattage 

 lamps it is essential that the power and switching equipment be capable 

 of handling these large currents. For example, a 1000-w 120-v lamp 

 with a normal current of 8.3 amp has an inrush current of 65 amp, which 

 can damage small switch contacts normally able to carry the rated current 

 of the lamp. 



Application. The most common applications of incandescent lamps in 

 biological research are the general irradiation of biological material and 

 the use with optical systems where it is essential to obtain the maximum 

 energy through a relatively small aperture. 



For general irradiation, bare lamps, either with clear or inside-frosted 

 envelopes, may be used with reflectors. The type of reflector to use 

 depends upon the general flux distribution required. This information is 

 generally available from manufacturers of lighting equipment. 



The internal-reflector lamp has greatly simplified the problem of general 

 irradiation of large areas, since no external reflector is required. For use 

 in greenhouses for supplementing solar irradiation, the internal-reflector 

 lamp is much superior to other types, since it introduces a negligible shade 



