d. Photomultiplier tube 



Barrier layer cells have insufficient sensitivity for measuring 

 ambient light at depths greater than 200 meters even in the clearest 

 •water. However, photomultiplier-type photometers are sufficiently 

 sensitive to measure illumination as little as 10 gm cal/cm /min. 



and thus can be used to a depth of 500 to 600 meters. It is necessary 

 to use some type of shielding device when using a photomultiplier tube 

 in illumination greater than 5 x 10""^ gm cal/cm^/min., and a neutral 

 filter of density 4 is necessary if the instrument is to be used at the 

 surface. 



A photomultiplier tube is more sensitive to short wave lengths than 

 is a barrier layer cell. Its peak sensitivity is around 4,500 A. In deep 

 water it should be necessary to use only a blue-green filter; however, 

 an automatic filter changing device has been developed. 



Although it is remarkably sensitive, a photomultiplier tube, has 

 a number of disadvantages. The tube requires a high input- voltage 

 in the order of 1,000 volts and produces a signal current which ranges 

 from .01 microampere to one milliampere. The output at constant light 

 level is strongly dependent upon the supply- voltage, so that a well 

 regulated power source is necessary. The signal at constant light 

 level and constant voltage is also a function of the orientation of the 

 tube in the magnetic field of the earth; hence the tube requires magnetic 

 shielding. 



e. Dual-filter hydrophotometer 



This instrument was developed by the Chesapeake Bay Insti- 

 tute. It is capable of giving in situ readings of two wave lengths within 

 the visible part of the spectrum. Measurements can be made at day 

 or night. 



Essentially the instrument consists of a constant light source 

 placed at a set distance of 15 centimeters from two photocells, each 

 one fronted by a suitable filter. The electrical output is proportional 

 to the amount of light striking each photocell and, therefore, is a 

 function of the transparency of the medium. 



The underwater unit contains a six- to eight- volt sealed beam 

 spotlight unit and the two photocells. The spotlight is an automobile 

 type, having an angular spread of approximately four degrees. General 

 Electric type PV-10 photocells are used. The filters are type B.G.-12 

 for blue and type R.G.-l for red. 



VIII- 5 



