LIGHT MEASUREMENTS 839 



tween these units is shown in Table 25. III. However, without information 

 as to spectral composition, the indication of the energy flux in ergs, or 

 calories, is even less revealing than that of the intensity of illumination in 

 lux, because 60% of direct sunlight and about 95% of the energy flux from 

 incandescent lamps belong to the far red and infrared, and are not used 

 by plants for photosynthesis. Unless the proportion of these radiations is 

 known, quoting the energy flux may easily give an entirely erroneous con- 

 cept of the quantity of light available for photosynthesis. 



Of the common photometric devices, only thermoelements and bolom- 

 eters react uniformly to radiations of all wave lengths. All other instru- 

 ments—vacuum photocells, barrier layer cells, actinometers— possess a 

 selective spectral sensitivity. Some investigators suggested that instru- 

 ments insensitive to infrared light, e. g., selenium barrier layer cells ("pho- 

 tronic cells"), should be used in preference to thermopiles or bolometers 

 in the measurement of light intensities in the work on photosynthesis, in 

 order to avoid measuring infrared together with visible light. However, 

 this is almost equivalent to a return to visual photometry, since the eye, 

 too, can be described as an infrared-insensitive photometer. In fact, the 

 sensitivity curve of the selenium barrier layer cell is quite similar to that of 

 the human eye. Both drop rapidly above 600 m^— right in the middle of 

 the main red absorption band of chlorophyll (c/. fig. 25.2A) . Vacuum type 

 photocells also show strong variations in sensitivity with wave length (fig. 

 25.2B). No photocells, actinometers or photographic plates possess uni- 

 form sensitivity throughout the visible spectrum (although some of them 

 are reasonably constant in the region of the shorter waves). Therefore, no 

 instruments of these types can give reliable photometric data in nonmono- 

 chromatic light, (c/. Mestre 1935). 



Warburg and Schocken (1949) have developed an actinometer based 

 on Gaffron's earlier observations in Warburg's laboratory of sensitized 

 autoxidation of allyl thiourea (chapter 18, page 509). Thiourea was sub- 

 stituted for allyl thiourea as oxidation substrate, and pyridine for acetone 

 as solvent; it was found that, with ethyl chlorophyllide (or protoporphyrin) 

 as sensitizer, a quantum yield equal to 1.0 ± 0.1 (molecules oxygen con- 



* We will use the abbreviation kerg foi- 1000 erg. 



