Reporting NETD: why measurement techniques matter

Abstract

For over 30 years, the U.S. Army Aviation and Missile Research, Development, and Engineering Center (AMRDEC) has specialized in characterizing the performance of infrared (IR) imaging systems in the laboratory and field. In the late 90's, AMRDEC developed the Automated IR Sensor Test Facility (AISTF) which allowed efficient deployment testing of aviation and missile IR sensor systems. More recently, AMRDEC has tested many uncooled infrared (UCIR) sensor systems that have size, weight, power, and cost (SWAPC) benefits for certain fielded U.S. Army imaging systems. To compensate for relatively poor detector sensitivities, most UCIR systems operate with very fast focal ratio or F-number (f/#) optics. AMRDEC has recently found that measuring the Noise Equivalent Temperature Difference (NETD) with traditional techniques used with cooled infrared systems produce biased results when applied to systems with faster f/# values or obscurations. Additionally, in order to compare these camera cores or sensor systems to one another, it is imperative to scale the NETD values for f/#, focus distance, and waveband differences accurately. This paper will outline proper measurement techniques to report UCIR camera core and system-level NETD, as well as demonstrate methods to scale the metric for these differences.

Measuring Collimator Infrared (IR) Spectral Transmission

“…It is used to compensate for any temperature drift inside the instrument housing. The following form of the Planck equation was used to calculate the radiance for both this internal blackbody and the external source: 1 where, C1 = 1.1909*10 4 (W µm 4 sr -1 cm -2 ); C2 = 1.4388*10 4 (µm 4 K); and ε(λ) = emissivity of the blackbody. The internal blackbody is assumed to be 1, while the external and collimator blackbodies are given as 0.97± 0.02.…”

“…Several IR systems [1] have been measured looking down a collimator using a large surface blackbody with a single fold/relay mirror and an off axis parabolic mirror to simulate an object at infinity. These systems were also measured apart from the collimator by directly viewing a large surface blackbody up close.…”

Signal intensity transfer function determination on thermal systems with stray light or scattering present

“…To keep the same ratio, the noise must also be scaled by 1 (1−s) (or the SITF would have to be multiplied by a factor of S). As by definition, s is less than 1, 1 (1−s) leads to an increase in noise.…”

“…To keep the same ratio, the noise must also be scaled by 1 (1−s) (or the SITF would have to be multiplied by a factor of S). As by definition, s is less than 1, 1 (1−s) leads to an increase in noise. This is essentially scaling the SITF to the situation where the scattering or stray light has little to no effect (only the scattering and stray light from the collimator contribute, which are assumed small).…”

“…1 One of the more interesting claims of this work is that the SITF of a thermal imager would change as a function of distance from a blackbody source. It is important to note that the observations that are made in the previous paper are consistent with those that are made in this research (there is a difference in the measured SITFs that is configuration dependant), but that the causal relationship is different.…”