SECT. 4] LIGHT 446 



8. Scalar Irradiance (Spherical Irradiance) 



The use of a spherical diffuse collector for measuring spherical irradiance 

 was proposed by Gershun (1939). Very little use has been made of this device 

 in underwater light measurements. In current theory, scalar irradiance is 

 required for the determination of important optical properties such as the 

 distribution functions and the absorption coefficient. Spherical irradiance can 

 also be used to evaluate the iC-functions and in this role has the advantage of 

 being insensitive to tilting — a problem encountered sometimes with a horizontal 

 irradiance collector. 



Fig. 26. Instrument for determining Rj-^K and D. 



An instrument was described by Tyler (1955) which was used for the deter- 

 mination of K, R and D functions in lake waters. This instrument is shown in 

 Fig. 26. 



9. Absorption Coefficient 



An instrument for the measurement of the absorption coefficient, a, of 

 horizontally stratified water has been described by Tyler (1960). This instru- 

 ment is based on the theoretical work of Preisendorfer (1957), and provides a 

 direct method for the determination of a independent of the scattering of the 

 water. 



The instrument shown in Fig. 23 has an accessary spherical irradiance 

 collector and can be used to determine the absorption coefficient of large 

 bodies of water. 



