Comparison of IRST systems by SNR

Abstract

Infrared (IR) cameras are widely used in systems to search and track. IR search and track (IRST) systems are most often available in one of two distinct spectral bands: mid-wave IR (MWIR) or long-wave IR (LWIR). Many have compared both systems in a number of ways. The comparison included field data and analysis under different scenarios. Yet, it is a challenge to make a right decision in choosing one band over the other band for a new scenario. In some respects, the attempt is like choosing between an apple and an orange. The signal-to-noise ratio (SNR) of a system for a point-like target is one criterion that helps one to make an informed decision. The formula for SNR commonly uses noise equivalent irradiance (NEI) that requires front optics. Such formalism cannot compare two bands before a camera is built complete with front optics. We derive a formula for SNR that utilizes noise equivalent differential temperature (NEDT) that does not require front optics. The formula is further simplified under some assumptions, which identifies critical parameters and provides an insight in comparing two bands. We have shown an example for a simple case. Keywords: Infrared search and track (IRST), MWIR, LWIR, SNR, NEI INTRODUCTIONThe first goal of an infrared search and track (IRST) system is to detect signatures as far as possible. Signatures of interest may include aircraft, helicopters and missiles. At long range a target is viewed as a point source when its angular subtense is smaller than the instantaneous field-of-view (IFOV) of the system. IRST systems are available in two distinctive spectral bands: mid-wave (MW) from 3 to 5 μm [1, 2] and long-wave (LW) from 8 to 12 μm [3]. Many have compared the performance of MWIR IRST with that of LWIR IRST for various cases [4]. Each has pros and cons. Ideally, both would be used simultaneously to enjoy all the pros. Performance metrics and the weighting of those metrics peculiar to the application often drive the choice.The key metrics for evaluating IRST system performance are signal-to-noise ratio (SNR), resolution and clutter rejection. Clutter rejection is related to the environment such as the sun and clouds. In this paper, we focus on SNR. Further, we exclude any noises due to the scan for search and non-uniformity among the pixels for focal plane array (FPA). It is common to use noise equivalent irradiance (NEI) in defining SNR [5]. NEI, however, depends on the camera system as a whole and is inadequate for comparing MWIR IRST with LWIR IRST when one is interested in making a choice before building the corresponding system with front optics. On the other hand most manufacturers provide noise equivalent differential temperature (NEDT) [6] for both MWIR and LWIR detectors that do not require optical systems during measurement.We introduce the measurement of NEDT and derive its relation to the corresponding temporal noise. The SNR for a point target in the field is obtained by approximating target signature. The signal in SNR means the signal difference between the signal an...

Digital readout integrated circuit (DROIC) implementing time delay and integration (TDI) for scanning type infrared focal plane arrays (IRFPAs)

“…Infrared imaging technology has been developed first in the 1950s to be used in military surveillance systems and it has been a very useful tool for many military and civilian applications such as IR search and track [5] [6], medical examination [7], astronomy [8], forward-looking infrared (FLIR) systems [9], missile guidance [10], security [11], process control [12], environmental monitoring [13] and spectroscopy [14]. The main motivation for the IR technology was military demands at the beginning, and it is still the main driving force to develop high-performance IR detectors and imaging systems.…”