GENERATION, CONTROL, AND MEASUREMENT 129 



"irradiance." The psychophysical terms " ilkimmation " and "bright- 

 ness" are often used erroneously in connection with non visual appli- 

 cations. It is seldom that the biologist has occasion to use such terms 

 except when dealing specifically with the eye, with vision, or with visual 

 instruments such as the polarimeter or the visual colorimeter. 



Several inconsistent photochemical expressions also are widely used. 

 "Light reaction" should be hmited to the psychophysics of vision; 

 "photoreaction," "photochemical reaction," and " photoprocess " are 

 preferable physical terms. The term "dark reaction" as used in photo- 

 chemistry usually refers to a nonphotochemical reaction that may or 

 may not proceed in a radiation field. It may be thermochemical, thus 

 requiring high-energy collisions that are a function of temperature, or it 

 may be a physical process, as in diffusion. It seldom is, as the expression 

 implies, a reaction that takes place only in the absence of radiant energy. 

 "Nonphotochemical" reaction is a more meaningful general term, and 

 such terms as "thermochemical" reaction and "diffusion" reaction are 

 more specific. 



electromagnp:tic spectrum 



The electromagnetic spectrum is an orderly arrangement of radiant 

 energy according to wave length and frequency and extends from the 

 very-long-wave low-energy photons of the radio region, as produced by 

 oscillatory electrical circuits, to the extremely high-energy particles of 

 the short-wave cosmic rays; this is presented graphically on a logarithmic 

 scale in Fig. 3-1. There is a continuous transition in physical properties 

 in passing through the spectrum, even though each spectral region is fre- 

 quently treated as if it w^ere a sharply defined entity. 



The various spectral regions have been somewhat arbitrarily delimited 

 by certain of their most evident properties. The visible spectrum extends 

 from about 380 m^ in the violet to 770 m/i in the red, as determined by 

 the Hmits of the spectral sensitivity of the average light-adapted human 

 eye (see Fig. 3-2). The near ultraviolet is usually taken as the region 

 from the short-wave-length limit of the visible to the furthest limit of 

 transmission of optical glass at about 320 m^u; the spectrum of the sun 

 extends slightly further to about 290 m^. The far ultraviolet continues 

 to 180 m^u, where quartz and air begin to absorb strongly. Beyond this 

 is the vacuum or extreme ultraviolet, which can be studied only in 

 evacuated systems. 



On the long-wave-length side of the visible is the infrared, which goes 

 from about 770 m/x to thousands of microns. The region of principal 

 biological interest, however, is limited to the very near infrared between 

 770 and 1500 m^t. Since water begins to absorb very strongly beyond 

 1500 mM, most biological materials are quite opaque to energy of longer 

 wave lengths. Here the energy of the quantum is too low to affect the 

 electronic energy levels of the atoms, and only the rotational, vibrational, 



