796 



AIRBORNE NAVIGATION AND GROUND SURVEILLANCE 



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WAVELENGTH (microns) 



Fig. 14-28 Infrared Vertical Transmission from to 30,000 Feet. 



response requirement is determined by the spectral distribution of the 

 received radiation. In general the broadest spectral response available with 

 present cells, and within the following requirements, is best. This statement 

 is not categorically true, but holds for present detectors. Absolute response 

 to received radiation is equally important. The greater the detector 

 sensitivity, the greater its capability of detecting small signal variations — 

 and contrast is all-important in mapping. The required scan rate and 

 instantaneous field of view determine what the detector time constant 

 should be. This may well be an impossible figure. Time constants range 

 from about 1 msec for thermistors and 25 to 100 Msec for cooled lead selenide 

 and lead telluride to 0.2 Msec for P-type gold-doped germanium. The 

 detector time constant may provide the limit on scan rate and resolution 

 and thus the limit on maximum value of velocity-to-height ratio with given 

 instantaneous field of view. In other words, the data-rate capability of the 

 detector is limited in part by its time constant and in part by its sensitivity. 

 Another possible limitation on scanning parameters is that of system 

 sensitivity. The sensitivity of the detector as degraded by the remainder of 

 the systeyn determines system sensitivity. The sensitivity of detectors is 

 usually given in terms of noise equivalent power (NEP) or detectivity 



( D* = ^ ^^^^^ j- Required supplementary information includes center 



frequency at which measurements were taken, bandwidth, detector size, 

 and source temperature. Noise equivalent power is defined as the incident 

 power required to produce a signal-to-noise ratio of 1. It might be noted 

 that the type of limiting detector noise must be investigated to assure 

 optimum operating conditions. 



