Infrared Detector Arrays

23 .28 IMAGING DETECTORS
Spatial Noise . Estimation of IR sensor performance must include a treatment of
spatial noise that occurs when FPA nonuniformities cannot be compensated correctly . This
requires consideration of cell-to-cell response variations . Mooney et al . 42
have given a
comprehensive discussion of the origin of spatial noise . The total noise determining the
sensitivity of a staring array is the composite of the temporal noise and the spatial noise .
The spatial noise is the residual nonuniformity U after application of nonuniformity
compensation , multiplied by the signal electrons N . Photon noise , equal to 4 N , is the
dominant temporal noise source for the high infrared background signals for which spatial
noise is significant (except for TE-cooled or uncooled sensors) . The total noise equivalent
temperature dif ference is
Total NE T 4 N U 2 N 2
41 / N U 2
(17)
N 1 N
T N T
where N / T is the signal change for a 1 K source temperature change . The denominator ,
( N / T ) / N , is the fractional signal change for a 1 K source temperature change . This is the
relative scene contrast due to C T and the FPA’s transimpedance .
The dependence of the total NE T on residual nonuniformity is plotted in Fig . 12 for
300 K scene temperature , two sets of operating conditions , and three representative
detectors : LWIR HgCdTe , MWIR HgCdTe , and PtSi . Operating case A maximizes the
detected signal with f / 1 . 4 optics , 30-Hz frame rate , and 3 . 4 – 5 . 0- m passband . Operating
case B minimizes the solar influence by shifting the passband to 4 . 2 – 5 . 0 m and trades of f
signal for the advantages of lighter , less expensive optics (f / 2 . 0) at 60-Hz frame rate .
Implicit in the calculations are charge-handling capacities of 30 million e- for MWIR
HgCdTe , 100 million e- for LWIR HgCdTe , and 1 million for PtSi . The sensitivity at the
lowest nonuniformities is independent of nonuniformity and limited by the shot noise of
the detected signal . The LWIR sensitivity advantage is achieved only at nonuniformities
less than 0 . 01 percent , which is comparable to that achieved with buf fered input circuits .
At the reported direct-injection MWIR HgCdTe residual nonuniformity of 0 . 01 to 0 . 02
FIGURE 12 Total NE T versus nonuniformity at 300 K
scene temperature .