Figure 1. Airborne IRT Measurement Situation 



from this object occurs in many different wavelengths and is a maximum at a particular 

 wavelength. These wavelengths, for most objects familiar to man's environment, that is to 

 say, in the general neighborhood of 300^ Kelvin, occur in the general vicinity of lO/x and, 



in fact, Wien's displacement law is ^ max = ^ where T is given in degrees Kelvin. The 



foregoing comments have applied only to perfect "ideal" radiators, or "blackbodies". By 

 definition, a blackbody has an emissivity of 1; it is a perfect radiator according to Planck's 

 equation, and since a good emitter is a good absorber, it will also absorb all of the radiation 

 which impinges upon it from an external source. It is "black". As a practical matter, the 

 emissivity of objects is less than 1 and varies with wavelength. This is referred to as the 

 spectral emissivity of an object, and as a matter of fact, an object may have a high emissiv- 

 ity in the infrared and a low emissivity in the visible portion of the spectrum. A good ex- 

 ample of this is ordinary paint. Enamel has an emissivity of greater than .9 throughout most 

 of the infrared region of the spectrum, regardless of its visible color. 



Another point to be made is that of the equivalence of the terms "emissivity" and 

 "absorptivity". A graybody with an emissivity of 0.5 will emit (radiate) exactly one-half 

 as much energy at any wavelength as an ideal blackbody at the same temperature. Also, 

 it will absorb exactly half the radiation which impinges upon it from an external source. 

 Thus, the oft -heard expression, "a good absorber is a good emitter". 



All objects possess the three properties of absorptivity (emissivity), reflectivity, 

 and transmissivity, and these three co-efficients must add up to 1 at every wavelength. 

 Thus a perfect blackbody whose emissivity is 1 has zero reflectivity and zero transmis- 

 sivity. A non-blackbody, or object whose emissivity is less than 1 may have transmissivity, , 

 such as a thin plastic film, or reflectivity, such as a piece of shiny metal, or both. 



Having reviewed the basic physics necessary to understanding radiometry, let us 

 apply these principles to our specific situation. Let us start with the ocean surface. \\'hat 

 are its infrared characteristics? Over different parts of the world, the temperature of the 



-26- 



