Except at SCOPE Expedition station 9C, all 

 measurements of irradiance have been nnade 

 with a blue-green (peak transmission at 480 

 m^) gelatin filter (Wratten No. 45). On clear 

 or slightly overcast days, this cell-filter- 

 meter combination permits measurements to 

 a depth where about 5 percent of the incident 

 downwelling blue -green flux is present. 



This particular gelatin filter has the dis- 

 advantage that it becomes progressively more 

 opaque with exposure to intense light (table 3), 

 although the transmittance does appear to in- 

 crease somewhat when the filter is stored in 

 darkness between exposures to light. For- 

 tunately, the transmission peak does not shift 

 appreciably, and the increase in opacity is 

 slow enough under in situ measurement con- 

 ditions so that its effect on any one set of 

 observations from which attenuation coeffi- 

 cients are obtained is negligible. 



Relative Spectral Sensitivity of the Detector 

 in Air and in Sea Water 



The output of the detector when lowered in 

 water will be proportional to the summation 

 of the wavelength-by-wavelength product of 



the wavelength-dependent parameters (notation 

 from Tyler, 1959), namely: 



(a) the energy distribution of the light im- 

 mediately below the water surface (E;^), 



(b) the spectral sensitivity of the photo- 

 voltaic cells (S>), 



(c) the transmittance of the filter (Tp ), 



(d) the transmittance of the window (Tq ), 



(e) the transmittance of the cosine collector 



(Tc). 



(f) the transmittance of the water path (T^ ). 



Thus any output value (V) of the detector 

 can be expressed as follows (Tyler, 1959): 



V = 1^ ■^Ai E, S, Tp To T^ Tw AX 



where k is a constant of proportionality, and 

 the limits Xj^ and Xj are deternnined by one or 

 more of the wavelength-dependent parameters. 

 We may compute the spectral response of 

 the detector without initial reference to E ;i 

 and T . In table 4 the relative spectral sensi- 

 tivity of the Photronic cell (S^), the trans- 

 mittance values obtained for a Wratten No. 45 

 filter (Tf ), a plastic window (Tq), and a 



Table 3. --The progressive decrease in transmittance (percentage T) of a Wratten No. 4-5 filter 



after successive exposures to direct sunlight, 1957 



[Measured with a Beckman Model DU spectrophotometer] 



