130 



RADIATION BIOLOGY 



and translational energy levels are changed; therefore few if any photo- 

 chemical reactions occur here, and the radiant energy is degraded to heat. 

 The infrared has been commonly referred to as the "heat-radiation" 

 region of the spectrum because most of the energy radiated by all high- 

 intensity sources is in the infrared. However, the energy of all spectral 

 regions is ultimately degenerated to heat when absorbed by matter and is 



FREQUENCY, Cycles /sec 



10 



24 



■,22 



10 



20 



10' 



10 



10 



l8 



10^ 



10" 



WAVE NUMBER, Cm"! 



10 



12 



lO'O 10^ 10^ 



n — I — I — i — r 



10" 



I02 



10^ 



1 — ^ r 



10-2 



1 — ^ — r 



10" 



10" 



COSMIC RAYS 

 1 



GAMMA RAYS 



I 1 ■ 



X RAYS 



ULTRAVIOLET 

 I 1 



VISIBLE 



INFRARED 



LIGHT 



HERTZIAN WAVES 



BLUE GREEN YELLOW RED 



RADAR, TELEVISION, RADIO 



400 



500 600 700 m/l 



X UNIT 



A m^ 

 I I I 



/' 



cm 



m 



.L^ 



.i_l 



km 



_J 



I0-' 



10"' 



10" 



10" 



10" 



I0<- 



10' 



10^ 



WAVE LENGTH, .cm 



Fig. 3-1. The electromagnetic spectrum. 



the basis for the quantitative measurement of radiant energy by thermal 

 detectors. The infrared is intrinsically no more a region of "heat radi- 

 ation" than any other spectral region. 



SPECTRAL UNITS 



Radiant energy may be characterized quantitatively by either wave 

 length or frequency. For any periodic wave motion the wave length is 

 defined as the linear distance between two similar points on adjacent 

 waves, such as the distance from crest to crest or trough to trough. The 

 frequency is the number of waves passing a given point in an interval of 

 time, which is usually the second. 



In a vacuum, electromagnetic waves have a velocity (the velocity of 

 light) which is never exceeded in value and is a universal constant. In 

 accordance with the laws of wave motion, the velocity is equal to the 



