Stilwell 



t 



simplest outline of axi optical computer. The use of a laser is not 

 absolutely necessary but the light intensities available from other 

 sources are too low to be convenient. The laser beam diameter is 

 enlarged after traversing the telescopic lens system to a size 

 suitable for the data format, i.e., the negative size. The colli- 

 mated light impinges on the transparency placed in the front focal 

 plane (input plane) and is transformed by the lens into the output 

 plane. The light amplitude in the output plane is proportional to 

 the Fourier transform of the light amplitude in the input plane. 



The basic problem then is to determine the relationship 

 of the light amplitude after passing through the optical system in 

 terms of the information on the sea as recorded on the scene photo- 

 graph. To accomplish this, it is necessary to establish the 

 correspondence between a sea parameter and the optical density on 

 the film, determine the exact form of the transform, and to relate 

 the recorded optical density of the transformed information to the 

 value of the sea spectrum. In what follows the first order theory 

 for the Fourier components analyzed optically will be presented and 

 the conditions under which the technique is valid will be indicated. 



SCENE PHOTOGRAPH REQUIREMENTS 



The initial problem in the photographic analysis is to 

 determine the relationship between the optical density at a point 

 on the negative with some parameter of the surface point it 

 represents. Figure 2 is a plot of the characteristic curve of 

 photographic emulsions for which the linear range has an eq_uation 

 of the form 



D = 7 log K u T (l) 



where D is the optical density of the developed negative, 7 is 

 the slope of the straight line part of the curve, K is a constaxit 

 relating to the sensitivity of the film, t is the exposure time, 

 and u is the power density incident on the film. Optical density 

 is defined by the equation 



u = u 10 "-"^ (2) 



o ^ 



where u is the light power density transmitted through the film 

 with optical density D and u is the incident intensity. From 



these equations it is obvious that a knowledge of the light in- 

 tensity leaving a point on the surface in a direction toward the 

 camera is sufficient to determine the resulting optical density. 



The camera illumination (observer direction in Figure 3) 

 is due largely to the light reflected from the surface since light 

 leaving the water is normally of much lower Intensity. Figure k 

 is a plot of the reflectivity of water with angle. Visualizing 



172 



