Energy Spectra of the Sea from Photographs 



Denzil Stilwell 



Naval Research Laboratory 



Washington, D.C. 2039O 



BACKGROIMD 



The present work was undertaken to provide an independent 

 measure of the surface conditions of the ocean in order to 

 quantitatively correlate radar cross-sections with sea state. 

 Previous attempts to define the sea state have been largely 

 qualitative definitions, dependent not on direct surface measure- 

 ments but on wind velocity and an observer's eye. Quantitative 

 measurements have been made in such efforts as the SWOP program 

 and glitter analysis, but these are not readily available techniques 

 for the routine measure of sea clutter. Stereophotographic 

 measurements have been attempted to supplement the SWOP data but 

 the difficulty in its application at the very high range of the 

 ocean spectrum (one meter wavelengths to millimeter waves) pre- 

 cludes it as a convenient tool. Normal oceanographic devices offer 

 little promise of obtaining the huge amounts of information re- 

 quired to specify accurately the energy spectrum of a real sea, 

 especially in the high frequency region, because of the tremendous 

 difficulties involved in designing accurate measurements with the 

 resolution required. Most techniques designed to obtain spectral 

 information have one common feature, recognition of photographic 

 recording as the only pi-esently practical technique capable of 

 providing a sufficiently large sample of information adequate to 

 determine a spectrum with precision, 



The technique of obtaining Fourier transforms by optical 

 techniques^ is particularly applicable to the present problem since 

 the basic data are the optical intensities which expose the photo- 

 graph and the analysis is performed on the light amplitudes (which 

 are simply related to the intensities). Optical Fourier trans- 

 forms arise because the basic transfer function for lens has the 

 form of a Fourier transform and when used with monochromatic, 

 collimated light generates a light distribution which is simply 

 related to the desired spectrun?. Figure 1 illustrates the 



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