416 TYLER AND PREISENDORFER [CHAP. 8 



optical documentation of the sea and of the underwater hght field are reviewed 

 below : 



Radiance distribution N{Z, 9, (/>) 



Attenuation coefficient a 



Volume scattering function a{d) 



Irradiance H 



Scalar irradiance h 



Diffuse attenuation function K 



Distribution function D 



Reflectance function R 



Absorption coefficient a 



Total scattering coefficient s 



Path function N^^: 



Beam transmittance T 



The first three can be regarded as having primary importance since the remain- 

 ing nine can be derived from them (Tyler, Richardson and Holmes, 1959). 

 However, in the determination of these latter nine properties it is sometimes 

 more convenient to make direct measurements, and considerable attention has 

 been given to specialized instrumentation for this purpose. 



3. Radiance Distribution 



The early exploratory measurements of radiance distribution underwater 

 were made with instruments that were simple but ingenious and well designed 

 for detecting the major features of radiance distribution underwater. The work 

 of Pettersson (1938) and Johnson and Liljequist (1938) is of particular interest 

 since it represents one of the major advances in the optical exploration of the 

 ocean and focused attention on radiance distribution as an important parameter 

 in optical oceanography. 



The existence of "characteristic diffuse" light (or asymptotic radiance 

 distribution) was deduced from these early measurements. A limited amount of 

 data was also obtained on the influence of the light field above the water 

 surface and the rate of change of radiance with depth. 



Complete radiance distribution data for lake water under clear sunny and 

 overcast lighting conditions was obtained at several depths by Tyler in 1957 

 (Tyler, 1960). These data are given in Tables I through XII. 



These data are, of course, characteristic of optically identical water under 

 the same lighting conditions anywhere in the world. 



The instrumentation used by Tyler is shown in Figs. 8 and 9. This instrument 

 employs a gyrosyn compass for azimuth position information and a servo 

 mechanism with positive propeller drive to maintain the azimuth position of 

 the instrument underwater to better than 1°. The zenith angle is controlled by 

 means of a synchronous motor and event marks which appear on a 

 synchronously driven strip chart along with the data. The angular field is 6.6°, 

 obtained by means of a Gershun tube. The instrument has a dual measuring 



