798 AIRBORNE NAVIGATION AND GROUND SURVEILLANCE 



All detectors (except thermistor bolometers) that currently find use in 

 infrared mapping systems are cooled to liquid nitrogen temperature (90° K). 

 Cooling is no longer a handicap, and may be accomplished by the use of 

 available miniature cryostats or by manually pouring the liquid coolant into 

 the cell. This latter method may, of course, be a forced flow system in 

 which the liquid flow is pressure-regulated instead of actually requiring a 

 manned operation. Particularly for P-type germanium it is important that 

 good thermal contact be made between the detector's heat sink and the 

 coolant. Otherwise, minor temperature fluctuations will give rise to 

 additional noise. 



14-25 MAJOR SYSTEMS FEATURES 



No one set of criteria provides an adequate basis for comparison of 

 infrared mapping systems. However, some characteristics of a system do 

 describe it sufficiently well for meaningful evaluation: 



{a) Spectral response (c) Resolution 



{b) System Sensitivity {d) Total field of view 



((?) Maximum v jh capability 



An overall figure of merit for system performance is sorely needed. In 

 view of the diff"erent types of application this may not be completely, 

 feasible. An eff"ective and standard method for comparison of infrared 

 reconnaissance systems must include the important features of scanning 

 systems and must do the comparison in a completely fair manner. These 

 requirements have so far precluded a useful approach to the problem. 

 Most systems, however, are sufficiently similar in purpose to warrant an 

 attempt at a standard method of comparison. It would seem that a 

 proposed method open to critical comments would provide a good starting 

 point. With this in mind, let us consider the problem in more detail. 



The following pages propose a standard method which should be fully 

 applicable to mapping systems. Special-purpose systems may be subjected 

 to the comparison procedure, but due caution must be observed. These 

 system characteristics are inherent either explicitly or implicitly in the 

 comparison scheme: 



{a) Instantaneous field of view ture difi^erence 



{b) Resolution (system) (/) Sensitivity (system) 



{c) Time constant (system) {g) Scan rate 



{d) Spectral response (/;) Effect of multiple detectors 



{e) Minimum detectable tempera- (/) Size of optics 



A plot of the reciprocal of the product of minimum detectable black body 

 temperature diff'erence and instantaneous field of view versus eff'ective scan 

 rate is proposed. At this point it should be noted that scanning systems 



