It is readily seen from the figure that the scattering peaks strongly in the 

 forward direction and in fact may be orders of magnitude higher than the scat- 

 tering at, say, 90°. This forward scattering has an important relationship 

 when considered in terms of the performance of an optical system underwater. 

 The resolution of any underwater viewing system is very strongly dependent on 

 the small angle scattering function. 



One of the most interesting features of the small angle scattering is its 

 variability. This variability of the small angle scattering could be due to 

 turbulence within the water. If this is true, then one would expect the small 

 angle scattering to be strongly time dependent with time constants character- 

 istic of turbulence within the water. This means that the beam cannot be 

 enclosed, as is characteristic of optical instruments used in the sea. There- 

 fore, the experiment described here is aimed at the measurement of small angle 

 scattering with attendant measurements of scattering at other angles and with 

 the light beam external to the instrument. 



OBJECTIVE 



The objectives of the experiment are to measure the distribution of light in a 

 plane normal to a parallel beam of light in the ocean. These measurements are 

 then converted to a measurement of small angle scattering through an integral 

 relationship. This scattering will be referenced to scattering measurements 

 at other values of the scattering angle 6 . 



EQUIPMENT 



The requirements for the instrument can be summarized as: 



1. Deployable in both shallow and deep water. 



2. Produce a parallel, narrow beam of light. 



3. Beam external to the instrument. 



4. Measurement of light distribution in a plane normal to beam 



(including unscattered flux) . 



5. Measurements of scattering at 6 = 45°, 90°, 135°, and near 180°. 



6. Beam near 5000 A (to minimize absorption). 



7. All high voltages internal to instrument. 



8. Real time measurement and recording. 



9. Variable length. 



The instrument was designed to accommodate all of these requirements. The basic 

 instrument is depicted in Figure 2. An 8" diameter tube served both as an opti- 

 cal bench and as a housing for the electronics. This was intersected by housings 



ELECTRICAL 

 INPUT 





fHUlUMLLl IfLlEKS 



liMAGE DISSECTOR 



ARGON- ION LASER 



Fig. 2. Basic instrument layout 



VII-34 



