'tt,tB«TH.WK*ONS 



Figure 3. Spectral Emissivity of Ocean Surface at Normal 

 Incidence and 60° from Normal 



sufficiently low that this stratification can be ignored. (More than 90'7 of the water vapor in 

 the atmosphere occurs at altitudes below 15,000 feet; it is necessary to fly much lower than 

 this to make accurate measurements.) Of course, since a good absorber is a good emitter, 

 the very water vapor which absorbs the infrared radiation also emits infrared radiation of 

 its owTi. Before considering the absorption by water vapor in detail, it should be mentioned 

 that another atmospheric phenomena which attenuates infrared radiation is scattering, 

 caused by liquid and solid particulate matter, and dependent onthe number of particles 

 present, their size, shape, density, and electrical characteristics. If treated from a funda- 

 mental standpoint, the problem of calculating the scattering effect is complex. It is neces- 

 sary to make measurements only on days in which the particulate matter in the atmosphere 

 is at a minimum; it is not practical to try to make accurate ocean measurements from an 

 aircraft during a snow storm, fog, rain, or extraordinarily heavy smoke. By confining 

 measurement to good weather days, scattering phenomena may be ignored and, in fact, it 

 is possible to ignore absorption by all gases except water vapor. Figure 4 shows the trans- 

 mission of the atmosphere in the wavelength region of interest. A striking feature of this 

 phenomenon is that the infrared absorption by the various gases in the atmosphere is local- 

 ized into regions or bands of high absorption. These bands are vibration-rotation bands of 

 gas molecules. Homo-nuclear molecules, such as oxj^gen (O^ and nitrogen (N ), do not have 



absorption bands of this type. Between these absorption bands lie regions of moderate or 

 no selective absorption. These regions of relatively good transmission are called "atmo- 

 spheric windows" and are defined and numbered for convenience. The region from 6m to 

 15/Li is usually called window No. 8, and it is in this region that most of the radiation from 

 the ocean target occurs. Figure 4 indicates the spectral transmission in this region over 

 a sea level path 1.000 feet long. More detailed data in Reference 4 presents measurements 

 of the atmospheric transmission over pathlengths of 1,000 feet, 3.4 miles, and 10,1 miles 

 over ChesaiJeake Bay. There is relatively little absorption within the region from 8m to 

 13m • Tliis suggests tkit by utilizing optical filters in a radiometer system to exclude all 

 wavelengths shoi'ter than 8m and longer than 13m , absorption will be minimized. As a 



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