Figure 4. I-Y Recorder 

 for water transparency 

 (light transmission) vs. 

 depth for vertical casts 

 with the hydrophotometer. 



Laboratory. The photometer consisted of a light source and 

 a photocell mounted one -half meter apart. The light source 

 was mounted in a watertight housing with a glass port, and 

 the light was directed through the water column to a photo- 

 cell in a second housing. The light source was a 6 -volt 

 light bulb (G. E. 964), with power provided by a variable 

 power supply for calibration of the instrument. The light 

 passed through a lens system to form a cylindrical beam of 

 parallel rays in the water. 



Several vertical baffles were mounted along the light 

 path for the purpose of filtering out ambient light. The 

 receiver, or photocell end of the photometer, was calibrated 

 to receive the cylindrically limited bundle of rays. The 

 photocell was a Weston 856 -RR. The electrical output of the 

 photocell was fed to the Z-input (pen drive) of a Leeds and 

 Northrup Speedomax Z-7 Recorder (fig. 4). The 7-scale was 

 depth of observation. The Z-scale was calibrated in per cent 

 light transmission per half-meter with reference to distilled 

 water. The calibration was made in air in which the full 

 scale reading was set at 93.5 per cent. The 6. 5 per cent 

 correction is due to increased reflection at the glass ports 

 of the lens systems in air. The photometer should read 

 100 per cent transmission in pure distilled water. 



Material in the water, such as plankton, sediment, or 

 dissolved substances, reduces the percentage of light trans- 

 mitted. The resulting values of transparency have been 

 related to visibility by divers. Although considerable spread 

 exists in the data, there is always a direct relation between 

 the hydrophotometer readings and the horizontal distance 

 at which a Secchi disc may be seen. Averages of such data 



