GENERATION, CONTROL, AND MEASUREMENT 



163 



interfaces, and the total energy is reduced 77 per cent. The transmitted 

 energy is increased in luminous efficiency by a factor of nearly 4. A 

 10-cm water filter increases the luminous efficiency nearly six times. 



MISCELLANEOUS INFRARED SOURCES 



Any thermal source of radiant energy is an efficient source of infrared 

 by virtue of the very high proportion of energy emitted in the infrared. 

 The wave length of maximum energy X„ for the short-lived photofiood 

 lamp at 3360°K is about 0.8 n; for the general-service lamp at 2900°- 

 3100°K, X„ is about 0.9 m, and the hfe is 1000 hr. At 2200°-2500°K the 



400 



800 



1200 



1600 2000 2400 2800 3200 36O0 



WAVE LENGTH , ITIM 



Fig. 3-9. Relative spectral energy distribution of infrared sources. {From lES 

 Handbook, 1952.) 



\m is extended to 1.0 or 1.2 n, and the life to several thousand hours. 

 This is the operating condition for the industrial infrared lamp used for 

 drying materials and baking industrial finishes. Low-temperature emit- 

 ters, such as the carbide or Globar emitter, operating at 1000°-1400°K, 

 are used extensively in infrared spectrophotometry (Taylor et al., 1951) 

 and as industrial low-temperature sources. The spectral energy distribu- 

 tions of various infrared sources are given in Fig. 3-9. 



Plant and animal tissues are chiefly water and therefore transmit Httle 

 infrared energy beyond 1.5 fi. Practically all the energy of the longer 

 infrared wave lengths is dissipated in the tissue surfaces. Mammahan 

 animal tissues and chlorophyllous plant tissues also absorb strongly in 

 the visible, owing principally to the presence of porphyrins and other 

 pigments. The only region of high transmission is in the near infrared 

 from about 0.7 to 1.5 n. Therefore the general-service incandescent lamp 

 with its maximum emission at about 0.9 m is the most efficient source 

 for obtaining maximum penetration of energy and the heating of tissues 

 to an appreciable depth. 



