Analysis of uncertainties in infrared camera measurements of a turbofan engine in an altitude test cell

Abstract

The infrared (IR) signature of a jet aircraft engine in altitude operation is a key component for the design of effective IR countermeasures and low-emission engines. Predicting the signature with radiometric models is widely accomplished, but measurements in situ are crucial for model verification. The altitude test cell provides a venue for measuring the IR signature in a simulated altitude environment, but the facility is designed for testing engines, not IR imaging. As a result, the imaging in the test cell is laden with measurement uncertainty due to stray radiation from the facility structure, hot exhaust gases, and the measurement equipment itself. Post-processing using correction factors is necessary to extract the engine signal from the stray radiation. This research investigated the uncertainties in measuring the IR signature of a turbofan engine inside an altitude test cell. The engine is measured by an IR camera immersed in the hot exhaust gases 35 feet downstream from the on-engine axis view. A protective enclosure and zinc selenide (ZnSe) window shield the camera from the heat and vibrations of the plume. The requirements for the IR measurement system include the apparent intensity and radiance of the visible engine surfaces in three bands of operation, two Medium Wave IR (MWIR) bands and one Long Wave IR (LWIR), with a spatial resolution of 1 in. To explore the extent of the measurement uncertainties, a radiometric model of the altitude test cell is formulated to quantify the engine and stray flux. To increase the fidelity of the model, the ZnSe window, a source of stray radiation, is characterized through measurements and experimentation. The resulting data is employed in the radiometric model. Specific measurement conditions at which the stray radiation is 5% or less of the total radiation are then derived, thereby decreasing the necessity for post-processing correction factors. These conditions are derived for the 3–4-, 4.5–5-, 8–9- and 8–12-μm bands using a parametric analysis. Two LWIR bands are considered to provide insight into specific previous measurements made with a quantum-well IR photo-detector (QWIP, roughly 8–9 μm), as well as potential future measurements made using broader band imagers (e.g., HgCdTe at 8–12 μm). A sensitivity analysis in the style of a Monte Carlo simulation is also performed to gauge the uncertainty in the radiometric model calculations.