An arrangement is made to filter out most of the daylight which 

 would interfere with the transparency measurements by a honeycomb 

 of blackened drinking straws being placed between the photocells and 

 the filters. 



f. Tri-filter hydrophotometer 



This hydrophotometer is basically identical to the dual-filter 

 hydrophotometer. It is, however, lighter and easier to maintain. The 

 electrical circuit has been redesigned to simplify operation and pro- 

 vide better linearity. In addition to the blue and red filters on the 

 dual-filter hydrophotometer, a green filter is used on the tri-filter 

 hydrophotometer. This instrument is designed for use in turbid 

 estuarine and coastal regions and is not considered suitable for deep 

 water transparency measurements. 



g. Clarke bathyphotometer 



Two basic elements comprise the Clarke bathyphotometer: 

 the submerged unit carrying the photomultiplier tube and the depth 

 meter sensing element, and a deck unit consisting of a high voltage 

 supply, the depth meter indicator, and a vacuum tube microammeter. 

 Supplementary equipment includes an oscillograph amplifier, a recorder 

 on which flashes of light from luminescent fishes can be recorded, 

 and a field calibration unit. 



The spectral sensitivity of the instrument extends approximately 

 from 3,200 to 6,500 A with a maximum at 4,800 A. Illumination as 

 low as 10 -11 gm cal/cm 2 /min. and depths as great as 610 meters 

 can be read directly on deck. 



h. Water clarity meter (photocell type) 



This meter, developed by the Visibility Laboratory of the 

 Scripps Institution of Oceanography, was designed to permit deter- 

 mination of underwater visibility by swimmers. It will measure and 

 record surface illumination, the decrease of ambient light with depth, 

 the attenuation of a beam of light beneath the surface, and the depth 

 of the instrument. 



The underwater unit consists of an Alpha-meter which is a light 

 source at a fixed distance from a photocell to provide a measure of 

 the attenuation of a beam of light, an h- meter which is a photocell 

 to measure the ambient light, and a pressure transducer. The spectral 



VIII- 6 



