Electronic Military & Defense magazine was developed for engineers, program managers, project managers, and those involved in the design and development of electronic and electro-optic systems for military, defense, and aerospace applications.
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Techniques Electronic Military & Defense Annual Resource, 4th Edition 42 • Full well capacity is 12,000 e - • Readout noise is 50 e - /pixel RMS • Dark current is 2.5 fA/pixel (at 15˚C sensor temperature). Remembering that: Using an exposure time of 33 ms, the NEI at 1550 nm will be: NEI Conversion In W/cm 2 In order to obtain the NEI in W/cm2, we need to use the energy per photon, which is determined by the wavelength according to the following relation: • Planck's constant h = 6.626 069 573x10 - 34 J·s • Speed of light c = 299 792 458m/s Camera Sensitivity Comparison The NEI is a powerful way to compare wildly differing cam- eras. Since the quantum efficiency is a function of wave- length, and the dark current is a function of the exposure time, it is important to use the wavelength and exposure time of interest for the given application. Silicon Cameras Table 2 and Figure 1 compare the NEI of typical EMCCD, CMOS, and CCD at 550 nm for exposure times ranging from 10 ms up to about 15 min. EMCCDs are best for short exposure times (milliseconds), while CCDs are best for long exposure times (minutes). For exposure times in seconds, all technologies are in a similar order of magnitude. Still, the NEI allows a precise compari- son and ranking. InGaAs Cameras Table 3 and Figure 2 compare the NEI of InGaAs FPA detec- tors at 1550 nm. The readout noise and dark current are very large compared to silicon-based detectors and are not compensated for by the high QE and large pixel size. Long exposure times are also prohibited by the large dark cur- rent, which fills up the full well capacity at about 1 second. • 1C oulomb = 1(A . s) = 6.241 509 629 152 65 x 10 18 (e - ) • Dark current (e - ) = Dark current(A) x C • Noise dark current = Dark current √ NEI= 50 2 + ( 2.5 X 6.241 X 10 15-18 X 0.033) 2 (15 X 10 6 ) 2 X 0.033 X 80% ~ 9.2 X 10 8 (photons/(cm 2 · s)) X 10 -4 √ √ E Photon(J) = h X c λ E Photon(J) = 6.626069573 X 2.99792458 X 10 -34+8+9 1.986 X 10 -16 λ(nm) λ(nm) ~ NEI(W · cm -2 ) = NEI(Photons/(cm - 2 · s)) X 1.986 X 10 - 16 λ(nm) 9.2 X 10 8 X 1.986 X 10 -16 ~ 1550 ~ 1.18 X 10 - 10 (W · cm ) -2 Figure 1: NEI of silicon-based cameras at 550 nm EMCCD CMOS CCD QE 47.3% 51.5% 78% Pixel size (µm) 10 5.5 4.54 Full well capacity (e - ) 24,000 14,000 12,000 Readout noise (e - /p) <1 7 7 Dark current (e - /p/s) 1 9 0.08 NEI (picoW/cm 2 ) @ 33 ms 38 493 477 NEI (picoW/cm 2 ) @ 10 s 1.6 2.7 1.6 NEI (picoW/cm 2 ) @ 10 min 0.103 0.233 0.027 Table 2: NEI of silicon-based detectors at 550 nm Table 3: NEI of InGaAs FPA detectors at 1550 nm 320 SWIR 320 Vis- SWIR 640 Vis- SWIR QE 72.9% 82.1% 80% Pixel size (µm) 30 30 15 Full well capacity (e - ) 170,000 170,000 12,000 Readout noise (e - /p) 150 150 50 Dark current (e - /p/s) 190,000 190,000 15,000 NEI (picoW/cm 2 ) @ 33 ms 100 45 118 NEI (picoW/cm 2 ) @ 0.5 s 13 11 14