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Aircraft plume infrared radiance inversion and subsequent simulation model

A radiance inversion technique, in which in-flight aircraft plume radiance recordings are exploited to construct a three dimensional (3D) radiance model of the plume, is presented. The recordings were done with a mid-wave infrared (3 – 6 μm) camera at different altitudes. The algebraic formulation of this inversion technique, also known as an emission-absorption technique, is stated for the ideal case of spectral radiance measurements of a high spatial sampling resolution over the plume area, as would be obtained from a hyperspectral imager. The non-ideal case of having only broad-band (mid-wave) image measurements and only one spectral measurement of the plume, is then investigated. It is shown that from this incomplete information set, an effective spectral absorption coefficient can be calculated for which the associated plume spectral transmittance and spectral emissivity calculations exhibit the correct qualitative behaviour. It is also shown that, by using this effective absorption coefficient, an optimization procedure can be used to determine the temperatures and/or spectral radiance values within the plume. This optimization procedure consists of minimizing the difference between the observed line of- sight (LOS) radiance in the image (i.e. a pixel radiance value in the image) and its theoretical projected radiance. After the temperature values within the plume were determined, the observer LOS radiance is parameterized so that it can be described for an arbitrary angle with respect to the main axis of the plume. The inferred temperature, spectral transmittance and spectral emissivity are then used in calculating the expected spectral radiance at this arbitrary angle. The spectrally integrated/mid-wave broad-band radiances and intensities for aspect angles other than those used during the inversion process, are then calculated and compared with actual measurements in order to determine the adequacy of this model for incorporation into existing infrared imaging system simulation software used in the training of infrared seekerhead missiles.