Infrared Detector Arrays

23 .26 IMAGING DETECTORS
the maximum detector-limited peak D * in the absence of excess bias-induced noise (both
l / f and shot noise) is
D * pk q
2 – R 0 A
(14)
kT hc
The R 0 A product of a detector thus describes detector quality even though other
parameters may actually be more relevant for FPA operation .
Percentage of BLIP
Whereas D * compares the performance of dissimilar detectors , FPA designers often need
to quantify an FPA’s performance relative to the theoretical limit at a specific operating
background . Percentage of BLIP , B L I P , is one such parameter and is simply the ratio of
photon noise to composite FPA noise
N
B L I P 2 1/2
PHOTON
N 2 PHOTON N 2 FPA
(15)
NE T
the NE T of a detector represents the temperature change , for incident radiation , that
gives an output signal equal to the rms noise level . While normally thought of as a system
parameter , detector NE T and system NE T are the same except for system losses
(conservation of radiance) . NE T is defined :
NE T n T
V s
Q
T
n
Q V s
(16)
where n is the rms noise and V s is the signal measured for the temperature dif ference
T . It can be shown that
NE T ( τ o C T BLIP
4 N c ) 1 (16 a )
where τ o is the optics transmission , C T is the thermal contrast from Fig . 1 , and N c is the
number of photogenerated carriers integrated for one integration time , t i n t :
N c A d e t t i n t Q B (16 b )
The distinction between an integration time , and the FPA’s frame time must be noted . It is
often impossible at high backgrounds to handle the large amount of carriers generated
over frame times compatible with standard video frame rates . The impact on system D * is
often not included in the FPA specifications provided by FPA manufacturers . This practice
is appropriate for the user to assess relative detector quality , but must be coupled with