398 



TYLER AND PBEISENDOBFEB 



[chap. 8 



CALIBRATED 

 RECORDER 



Fig. 1. Schematic drawing of a radiance, or Gershun, tube. The output of the photo- 

 detector area, A, is proportional to the incident flux, P, the soUd angle, Q, and the area, 

 A, of the photodetector. The solid angle and area are fixed instrumental constants. The 

 angle 6 of equation (1) is here zero. 



Fig. 2. Radiance-distribution diagrams in the vertical plane including the sun : (a) for 

 clear sky with sun at 56.6° altitude, depth 53.7 m ; and (b) for overcast sky, depth 

 55 m (sun at 40° altitude but not visible). These two diagrams have the same relative 

 magnitude. Because of the scale used, the fixed point of observation below the surface 

 appears to be on the perimeter at the bottom of each diagram. Actually this point is 

 inside the diagram (cf. Fig. 19). 



