horizontal white or black disc that in operation is lowered into the 

 water to the greatest depth at which the disc is visually detectable. 

 Drawbacks of a Secchi disc observation are that it depends upon a 

 number of variable factors: the eyesight of the observer, sea state, 

 accuracy of the line markings, whether the disc is in sun or shadows, 

 etc. Therefore, a Secchi disc reading unsupported by other evidence 

 cannot be interpreted in terms of the more fundamental optical con- 

 stants. It gives only an approximate index of the transparency of the 

 surface layers. 



b. Water clarity meter (movable disc type) 



This water clarity meter is a visual photometer that over- 

 comes some of the disadvantages of the Secchi disc. The instrument 

 consists of two discs of different colors and sizes connected by a 

 vertical shaft. The upper (gray, smaller) disc slides on the shaft 

 so that it can be lifted above the fixed, lower (white, larger) disc by 

 an auxiliary line. The upper disc is separated from the lower disc 

 until the two appear to be equally luminous. Once the two are properly 

 separated they will continue to match in homogeneous water regardless 

 of the depth to which the apparatus is submerged. 



The separation of the discs is a direct measure of the clarity of 

 the water and can be converted to hydrological range by a multiplying 

 factor. The hydrological range is that range at which the apparent 

 contrast is two percent; the apparent contrast is a function of the 

 reflectivities of the two discs, their average depth, and the angle of 

 sight to the discs. This device was developed a number of years ago, 

 and it is believed to have received very little use. 



c. Barrier layer cell 



For ambient light measurements in the upper 50 to 100 

 meters a barrier-layer-type cell generally is used. One of the better 

 known types is the "Weston photronic cell. It responds to light wave 

 lengths between 3,000 and 7,500 A with a maximum response around 

 5,500 A. With the proper detection device this cell has sufficient 

 sensitivity to measure radiation of as little as 10~ 4 gm cal/cmr/min. 



Filters generally are used in conjunction with barrier layer cells. 

 However, the total radiant energy between 3,800 and 7,120 A can be 

 determined with filterless barrier layer cells by using the proper 

 factor. This factor is a function of the depth of measurement and ratio 

 of the amount of subsurface radiation to the surface radiation. 



VIII- 4 



