SECT. 4] 



441 



some kind was used, deep data and data taken on overcast days should be 

 quite comparable to the diffuse attenuation function defined in equation (38). 

 This reasoning does not hold, however, for measurements of the reflection 

 function or the absorption coefficient. The directional distribution in the upper 

 hemisphere underwater is quite different from that in the lower hemisphere 

 (see Fig. 22) and the proportionality factor, which connects irradiance with the 

 instrument reading, will be quite different for the two cases, thus leaving a 

 significant residual error in the value of the reflectance function if the collector 

 is imperfect. 



Fig. 22. Distribution of flux from upper and lower hemispheres. Data shown is for overcast 

 hghting at a depth of 42.8 m (from Tyler, 1960) ; lower hemisphere data has been 

 enlarged to exhibit the difference in shape. For sunny conditions the difference in 

 shape would be still more pronounced. 



An instrument suitable for the determination of the diffuse attenuation 

 function. A', and the reflectance function, R, is shown in Fig. 23. 



Recently irradiance-measuring instruments of advanced design have been 

 developed by Boden, Kampa and Snodgrass (1960) at the Scripps Institution of 

 Oceanography and by Hubbard and Richardson (1959) at the Woods Hole 

 Oceanographic Institute. 



The former instrument employs interference-type filters to isolate narrow 

 spectral bands. Desaturation of the band because of large angles of incidence 

 is avoided by a coUimating tube located between the filter and the irradiance 



