14-24] 



SYSTEM CONSIDERATIONS 



791 



occur in the 2- to 5-micron region. It is necessary to choose the most 

 sensitive detector and the one with the best spectral response in order that 

 as much of the radiation as possible may be utilized efficiently. Assuming 

 other important considerations (discussed below) to be favorable, the 

 combined value of good absolute and wide spectral response determines 

 the degree of contrast that may be obtained. A picture is created not 

 because of the temperature of object space as a whole, but by the changes of 

 temperature (and emissivity, of course; this may be neglected if an effective 

 temperature is considered) between instantaneous field of view regions in 

 object space. Fig. 14-25 shows a spectral and absolute response comparison 

 of the common detectors. 



This paragraph has dealt with the fundamental considerations of thermal 

 mapping, namely, the angular coverage and picture quality. The criteria 

 for quality are resolution and contrast. The effect of these basic considera- 

 tions on system parameters is discussed in the following paragraph. 



14-24 SYSTEM CONSIDERATIONS 



The elements of a typical IR ground mapping system are shown in block 

 diagram form in Fig. 14-26. The main parameters associated with each 



v/h INFO ► 



■* — y/h INFO 



Target Info. 

 Modified by 

 Atmosphere 



Fig. 14-26 Typical IR Mapping System and Parameters. 



system element are also listed. The choice of these parameters determines 

 the system characteristics. The modification of any one parameter is never 

 independent of all of the others. This paragraph discusses the interrelation- 

 ships of these parameters and shows how the choice of them is influenced by 

 operating requirements and environment (speed, altitude, etc.). 



The total lateral angular coverage required determines the number of 

 scan lines attainable per revolution of the scanning optics. High-speed 

 scanning precludes anything other than rotatory motion. Fig. 14-27 

 illustrates perhaps the simplest method for obtaining a line scan in object 

 space. Energy from object space is incident upon the scanning flat and 



