166 



BIOPHYSICALLY ACTIVE LIGHT 



water it would be necessary to use a source of ultraviolet radiation from 

 a quartz mercury-vapor lamp of high intensity. This type of lamp is 



o 



especially rich in ultraviolet radiation at 2536, and 2654 A, and even 



1800 2200 2600 



Wavelengths in A 



3000 



Fig. IV-20. This curve is constructed from data by Kreusler [1901]. It shows 

 the opacity of a layer of water 1.7 cm thick to a source of artificial ultraviolet radi- 

 ation. Air does not transmit sunlight below 2950 A. Note that bacteria cannot be 

 killed by wavelength region around 2600 A, if immersed in water to any great 

 depth. Coefficient of absorption is 0.0025 for water at 2600 A. 



this form of radiation cannot be efficient if water is treated in layers 

 greater than 2 cm thick, and if the flow is so fast that a single bacterium 



erg. 



cannot absorb its lethal dose of 25 X 10 6 



Ultraviolet Absorption in Air 



In the ultraviolet the absorption at atmospheric pressure of a layer of 



o 



air 1 meter thick is negligible for wavelengths greater than 2300 A. 

 It is about 1 per cent at 2200 A and 2 per cent at 2050 A, and increases 

 rapidly towards shorter wavelengths. This absorption is due to the 

 presence of oxygen. It sets a lower limit of about 1850 A to the wave- 

 length of the ultraviolet radiation which can be recorded with an optical 

 instrument in which the beam traverses 50 cm of air at atmospheric 

 pressure. 



