Light is also important in biological studies, in that it provides the 

 primary energy source for food production by photosynthesis. However, 

 radiation studies as applied to biology have not been considered to 

 relate very much to fulfilling the purpose of the Hydrographic Office. 



B. INSTRUMENTATION 



Although methods of measuring radiation have not been standardized, 

 several basic types of intruments are in common use. These are used 

 either above or below water. Instruments representative of the basic 

 types are described below. 



1. Instruments used above water 



a. Eppley pyrheliometer 



This instrument is designed primarily for measurement of 

 the intensity of solar radiation upon a horizontal plane and is calibrated 

 at the Eppley Laboratory in sunshine against pyrheliometers which 

 are standardized every year at the Weather Bureau, Washington, D. C. 

 The probable error for the instrument, as compared with direct 

 radiation intensities ranging from 0.25 to 1.50 gm cal/cm^/min., is 

 ±1.5 percent. The instrument is sensitive to the wavelength range 

 between 3,000 and 50,000 A. 



Two types of pyrheliometers are available. One has ten junctions 

 for use with recorders having a range of zero to four millivolts, and 

 the other has 50 junctions for use with recorders having a range of 

 zero to 16 millivolts. The price of a 50- junction type Eppley pyrhe- 

 liometer mounted on a chromium plated base is about $300. 



2. Geir and Dunkle radiometer 



The response of the Geir and Dunkle radiometer is independent 

 of the wave length of the incident energy. For this reason this instru- 

 ment has an advantage over a pyrheliometer in that it can be used to 

 measure long wave radiation as well as solar radiation and can be 

 used both for daytime and nighttime measurements. It also can be used 

 as a net exchange radiometer to measure the net heat transfer through 

 a surface. 



The radiometer is essentially a heat flow meter and consists 

 of three bakelite plates 4 1/2 inches square and 1 / 64 inch thick. The 

 thermopile is constructed by winding 40- gauge constantan wire onto 



VIII- 2 



