LIGHT MEASUREMENTS <S37 



2. Light Measurements 



111 many older investigations, the influence of light intensity on photo- 

 s\Tithesis was studied by illuminating the plants with "white light" (of the 

 sun, or of an incandescent lamp) and introducing gray filters, or altering 

 the distance between light source and plant. The intensity was then given 

 in relative units (e. g., "Ho of full sunlight" ; or "lamp at 30 cm. distance") . 

 Other obsen'ers determined the intensity of illumination by visual com- 

 parison with a standard light source, and expressed it in meter candles 

 (also called lux, or lumen per square meter) or foot candles (1 foot candle = 

 10.764 meter candles). These figures cannot be used for the calculation of 

 the incident energij, unless the spectral distribution is known. A knowl- 

 edge of the so-called "color temperature" of the light source (the tempera- 

 ture that a black body must have to produce radiation of the same color) 

 provides some helpful information. However, no light source is a black 

 body; and, even if it were, the spectral distribution of the light it supplies 

 is modified by passage through air, glass or other material media. The 

 figures given below can therefore be used only for approximate calculations. 



The maximum illumination from direct sunlight at noon in summer (at 

 sea level at the latitude 42° N) is about 85,000 lux* (8000 foot candles), 

 its color temperature (cf. Tajdor and Kerr 1941), about 5400° K. Under 

 these conditions, the average light flux is 2.0 X 10"- cal. or 8.5 X 10^ erg/ 

 (cm.^ sec.) (including the infrared) or 10 erg/(cm.^ sec. lux). The diffuse 

 light from the clear blue sky increases the illumination by about 21,0CG 

 lux (2000 foot candles). The sky light has a much higher color temperature 

 than the sunlight — over 10,000° K. for clear blue sky, decreasing to 7000° 

 K. for hazy or overcast sky (see fig. 22.52). At sea level, when the sun is 

 in the zenith, about 40% of the incident radiation belongs to the spectral 

 region below 700 mju (photosynthetically active radiation) and about 60% 

 to the region above 700 m^ (photosjnithetically inactive, extreme red and 

 infrared radiation). Thus, in sunlight, one lux is equivalent to 4 erg/ (cm. ^ 

 sec.) of pliotos\Titheticalh^ active light. 



The characteristics of incandescent lamps (gas-filled tungsten filament 

 lamps) in table 25. TI are taken from Hardy and Perrin's The Principles 

 of Optics (1932). 



About 30% of the lamp light below 760 mju belongs to the region 700- 

 760 m/x, which is scarcely used at all by green plants, so that the proportion 

 of photosynthetically active energy is only 7-8% in medium power lamps 

 (100-200 watts) and 9-10% in high power lamps (500-1000 watts). In 

 other words, an illumination of one lux from a 100 watt lamp corresponds 

 to about 5.0 erg/(cm.- sec.) and the same illumination from a 500 watt lamp, 

 to about 4.5 erg./(cm.2 sec.) of photosynthetically active light. 



* We will use the abbreviation klux for 1000 lux. 



