138 RADIATION BIOLOGY 



approximate limits of the curve are 380 and 770 m^u, with a maximum at 

 555 m/i. The curve for rod vision (Illuminating Engineering Society, 

 1942; Weaver, 1937), as obtained with the dark-adapted eye, is shifted 

 about 45 m^ to the shorter wave lengths, with a maximum at about 

 510 m/x (Fig. 3-2). 



ENERGY EQUIVALENT OF LIGHT 



The absolute luminous efficiency or absolute luminosity K for any 

 source is given in lumens per watt of radiant energy, which is obtained 

 by integrating the radiant flux for small wave-length intervals over the 

 relative-luminosity curve. The maximum luminosity i^max is the lumi- 

 nosity of monochromatic flux at 555 mju, which has a value of about 

 650 lumens w~^ (Illuminating Engineering Society, 1942; Optical Society 

 of America, 1944b, 1953). Values obtained by different observers vary 

 from 625 to 680 lumens w"^ The least mechanical equivalent of light is 

 /-^sOj or 0.0015, w of monochromatic energy at 555 m/i and will produce 

 1 lumen of luminous flux. At 410 and 720 mpt the relative efficiency is 

 0.001, and therefore 1.5 w is required to produce 1 lumen at these wave 

 lengths. 



The absolute luminous efficiency of radiant energy should not be con- 

 fused with electric-lamp efficiency, which is usually given by lamp manu- 

 facturers in lumens per watt of electrical-energy input and involves the 

 efficiency of conversion of electrical energy to radiant energy as well as 

 the absolute efficiency of the flux. The lumens per radiant watt K is 

 always larger than the lumens per electrical watt; both are often referred 

 to as "luminous efficiency." 



The use of psychophysically derived quantities as the basis of radio- 

 metric evaluation is quite logical, providing the spectral limitations of 

 the units are properly appreciated. For the animal or plant physiologist 

 dealing w^ith nonvisual photochemical problems, the spectral limitations 

 imposed by use of the psychophysical units are often compensated for 

 by the convenient availability of "light" meters calibrated in lux or foot- 

 candles. The use of such units frequently yields more meaningful data 

 than a total-energy measurement obtained with a nonselective detector 

 §uch as a thermocouple, where the total unfiltered, or "white," energy 

 of a source is measured. Where narrow spectral regions are employed, 

 there is usually little justification for expressing intensities in any photo- 

 metric type of unit. 



PROPAGATION OF RADIANT ENERGY 

 INVERSE-SQUARE LAW 



Since the propagation of radiant energy through space is rectilinear, 

 the irradiance H produced by a point source of intensity ./ varies inversely 

 as the square of the distance d from the source such that H = J /d^- 



