ACCURACY OF AIRBORNE INFRARED THERMOMETRY 



By John L, Frank 



Barnes Engineering Company, Stamford, Connecticut 



INTRODUCTION 



A diagram of the measurement situation appears in Figure 1. The Infrared Thermo- 

 meter is airborne and looks downward more or less vertically to the surface of the ocean. 

 It is normal to speak of the Infrared Thermometer receiving radiation from the ocean sur- 

 face; that is, the radiation is thought to flow upward from the ocean to the radiometer. This 

 is not accurate, since the Infrared Thermometer is actually a differential radiance com- 

 parator: it both receives radiation from the ocean, and radiates to the ocean (the ocean re- 

 ceives radiation from the Infrared Thermometer). ^ The actual signal developed by the 

 Infrared Thermometer is the difference between the radiation received from and emitted to 

 tlie ocean; and since the Infrared Thermometer's optics are at a higher temperature than 

 the ocean, the ocean receives more energy from the Infrared Thermometer than vice versa; 

 the net flow of radiant energy is downward from the Infrared Thermometer to the ocean sur- 

 face. However, the convention of considering that the radiation flows upward from the 

 ocean to the radiometer will be adopted. 



Radiation from the ocean passes through the atmosphere, whose characteristics will 

 be considered to be approximately uniform over the entire path length, usually less than a 

 thousand feet. Some of the radiation from the ocean is absorbed by this atmosphere; 

 furthermore, the atmosphere also emits radiation of its own. One other radiation signal 

 must be considered, and that is the reflected signal from the sky; the ocean is not a perfect 

 emitter and does reflect some foreign radiation as indicated in Figure 1. Thus, the Infrared 

 Thermometer receives radiation from three different sources, only one of which is of in- 

 terest: reflections from the sky, emissions from the atmosphere, and the radiation of in- 

 terest from the ocean. Furthermore, the radiation from the ocean is somewhat modified by 

 the atmosphere before it reaches the Infrared Thermometer. 



CORRECTION OF ERRORS 



F undamentals of Infrared 



All objects in the universe, whose temperature is above absolute zero, emit radia- 

 tion. The wavelength of this radiation depends upon the absolute temperature of the object 

 according to Planck's equation and Wien's displacement law. Planck's equation is as 



follows: \\\= C^\~^ ----- x-U-1. . -1 



exp (CgX ■''T Vl 



If this equation is plotted for any parti- 

 cular value of T (absolute temperature), a curve will result indicating that the radiation 



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