LIMITS OF INFRARED IMAGING

Abstract

The background photon shot noise, induced by the signal itself and all other in-band surroundings, provides the fundamental infrared imaging sensitivity limit (background limited performance or BLIP). Optical diffraction constrains resolution angle to values greater than the ratio of the detection wavelength to the optics diameter. Imaging technology tries to attain BLIP at spatial frequencies approaching the diffraction limit while minimizing system size, weight, power, and life-cycle cost. To minimize costs, detector array technology must raise the detector operating temperature while keeping dark currents and other noise mechanisms below background shot noise. This requires reducing material and process defects. Recently HgCdTe has seen the MWIR-LWIR device dark currents to be limited only by fundamental mechanisms over a wide range of temperatures and wavelengths. Novel materials systems (e.g. Type-II superlattices) in theory have even lower fundamental limits than HgCdTe , but as yet lag in demonstrated performance. At least for now HgCdTe provides a convenient well-characterized benchmark and gives insight into further improvement possibilities Infrared imaging technology must also exploit advanced processing and packaging techniques to reduce detector pitch to a few wavelengths of detected light. Mid- and long-wave infrared imaging arrays with ~15μm detector dimensions are now approaching this goal.