Design considerations for advanced MWIR target acquisition systems

In this paper, mid-wave infrared (MWIR) sensor optimization is provided as a function of the parameter F λ/d, where F is the f -number, λ is the effective wavelength, and d is the detector pitch. For diffraction limited systems, acquisition range is related to the instantaneous field of view (detector limited operation) when F λ/d < 1, and to the optical properties (optics limited operation) when F λ/d > 2.0. Range performance is a combination of detector and optics resolution limits when F λ/d is in between. When the system is not strictly diffraction or sampling limited, the optimal F λ/d depends on other system component characteristics and conditions. Optical system aberrations affect system resolution and decrease range performance. As background shot noise, dark current shot noise, and read noise increase, range decreases. In the infrared spectral region, atmospheric absorption leads to reemission of thermal energy. The detected reemission creates additional shot noise. Atmospheric attenuation greatly affects MWIR sensor range performance. Next-generation MWIR sensors will have smaller detectors, larger arrays, and better sensitivity to enable F λ/d-based optimization. Previous studies (1T = 4 K for tracked vehicles) suggest that an initial design point is F λ/d ≈ 2.0. When detecting low contrast targets (1T ∼ 0.1 K), sensor gain is used to increase the signal for a desired displayed contrast. This gain increases displayed noise and reduces acquisition range. This is typically not an issue for long-wave infrared sensors due to the excess number of photons in the 8–12 µm band but poses a problem for MWIR sensors, which are photon starved. Under such scenarios, the optimum F λ/d appears to be about 1.5 for MWIR sensors. The results here provide reasonable strategies for MWIR system optimization and a direction associated with future MWIR focal plane development.