838 METHODS OF KINETIC MEASUREMENTS CHAP. 25 



Table 25.11 

 Characteristics of Incandescent Lamps 



White fluorescent lamps emit, as visible light, ~18.5% of energy input. 

 Their brightness corresponds to 6-10 lux/sq. cm. Their spectral dis- 

 tribution peaks are at 480 m^ ("blue-white"), 490 and 585 m/x ("standard 

 cool white"), 585 mju ("standard warm Avhite"), 520 and 640 mju ("deluxe 

 cool white") and 530 and 620 m^ ("de luxe warm white") ; cf. Barr (1950). 

 The two "standard whites" emit little energy > 650 m^; the two "warm 

 whites," little energy <500 mju. 



The quanta of visible light are from 5 X 10"^^ to 2.5 X lO"^'' erg each. 

 In sunlight, the intensity maximum lies at about 575 niM, corresponding to 

 /^ = 3.5 X 10~i2 erg. Thus an illumination of 1 lux from the sun corre- 

 sponds to 1.2 X 10^2 quanta or 2.0 X lO'^^ einstein/(cm.2 sec.) of photo- 

 synthetically active light. In artificial light, X is much nearer the red 

 end of the spectmm. If one assumes, for this light, hv = 3.2 X 10"^^ erg, 

 1 lux becomes equivalent to 1.4 X 10^^ quanta or 2.3 X lO^^^ einstein/ 

 (cm.^ sec.) of photosynthetically active light. 



In direct sunlight at noon, about 10" visible quanta impinge per second 

 on one square centimeter of horizontal surface. A pigment molecule situ- 

 ated on this surface, whose average molar absorption coefficient for visible 

 light is of the order of 3 X 10^ will absorb about twelve quanta every 

 second. 



The frequency of absorption acts is determined, for a molecule with the molar ab- 

 sorption coefficient a, by the equation: 



(25.1) ?i = 4 X lO-^'aA^;,;, 



where Nh^ is the light flux in quanta/ (sec. cm.''). 



Instead of calculating, as above, the energy flux and the number of 

 quanta impinging on the illuminated surface from the intensity of illumina- 

 tion in lux, it is of course much better to measure this flux directly, by 

 means of a thermoelement, bolometer, photocell or actinometer. This is 

 also the only way to define the intensity of colored light, which cannot be 

 measured in lux. The energy flux can be expressed in ergs or calories (per 

 unit surface and unit time) or watts (per unit surface). The relation be- 



