152 RADIATION BIOLOGY 



mals are exposed to a maximum irradiance of the order of 1000 w m~-, 

 or 1.5 cal min^' cm^^. This approaches the maximum flux tolerated by 

 most organisms in an air environment. Solar radiant flux has a lumi- 

 nous efficiency of about 100 lumens w~\ or 6500 ft-c per cal min~^ cm"^. 

 The maximum solar visible irradiance is then about 10,000 ft-c. Incan- 

 descent-lamp energy of 2000 ft-c at 20 lumens w~^ will produce the same 

 total irradiance and nearly the same heating effect as noon sunlight. If 

 water is used to absorb the infrared from an incandescent source, the 

 luminous efficiency of the radiated flux can be increased close to that 

 of solar flux, and the total amount of visible flux that can be tolerated is 

 increased by a factor of 5 (Gordon, 1930). 



Radiance and Brightness. The intensity of a source may be specified 

 in terms of radiance, which is power per unit solid angle and area; radiant 

 emittance, power per unit area; or radiant intensity, power per unit solid 

 angle. The photometric analogue of radiance is brightness, candles per 

 unit area, which is equivalent to lumens per unit solid angle and area. 

 These quantities are available for many commercial sources and serve as 

 useful guides in comparing the total and visible intensity of sources. 



Sources of high intensity must be selected when it is necessary to pass 

 the maximum flux through an optical system such as a monochromator. 

 For the irradiation of large areas where optical systems are not involved, 

 low-intensity or low-brightness sources may be used. Sources employed 

 for the general irradiation of plant material or for general lighting may 

 be of this type. The fluorescent lamp is a typical source of low bright- 

 ness, and the carbon arc, of high brightness. 



THE SUN 



The sun is an incandescent stellar body having a gaseous outer envelope 

 that is maintained at about 6000°K by nuclear reactions occurring in the 

 interior. The spectrum at the earth's surface appears as that of a thermal 

 radiator modified by the emission and absorption spectra of the lighter 

 elements in the sun's envelope and absorption by the various components 

 of the earth's atmosphere. The sun is unsurpassed as an economical 

 source of visible energy, and the high irradiances prevailing during clear 

 weather are difficult to reproduce with artificial sources. The nature of 

 the sun and its radiation (Menzel, 1949; Nicolet, 1943; Roberts, 1952) 

 and the manner in which solar irradiance changes as a function of time 

 and place over the earth's surface have been discussed extensively by 

 many authors (Benford, 1947a,b, 1948a,b; Hand, 1937, 1941, 1950; 

 Kimball, 1924; Moon, 1940). 



Solar Constant. Measurements made on the intensity of solar energy 

 after it has passed through various thicknesses of the earth's atmosphere 

 have made it possible to calculate the solar irradiance at normal incidence 

 just outside the earth's atmosphere at the mean solar distance of the 



