Abstract
ABSTRACT A radiance inversion technique, in which in-flight aircraft plume radiance record ings 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 aI ) 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 abso rption coefficient can be calculated for which the associated plume spectral transmittance and spectral emissivity calculations exhibit the correct qualitativ e 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 va lue in the image) and its th eoretical projected radiance. After the temperature values within the plume were determin ed, 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 emissi vity are then used in calcul ating the expected spectral radi ance 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. Keywords: Radiance inversion, 3D radiance model, aircraft plume, mid-wave infrared, hyperspectral
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