The surface infrared radiation is an important part that contributes to the infrared image of the airplane. The Monte
Carlo method for the infrared image calculation is suitable for the complex geometry of targets like airplanes. The
backward Monte Carlo method is prior to the forward Monte Carlo method for the usually long distance between targets
and the detector. Similar to the non-gray absorbing media, the random number relation is developed for the radiation of
the spectral surface. In the backward Monte Carlo method, one random number that reverses the wave length (or wave
number) may result deferent wave numbers for targets’ surface elements on the track of a photon bundle. Through the
manipulation of the densities of a photon bundles in arbitrary small intervals near wave numbers, all the wave lengths
corresponding to one random number on the targets’ surface elements on the track of the photon bundle are kept the same
to keep the balance of the energy of the photon bundle. The model developed together with the energy partition model is
incorporated into the backward Monte Carlo method to form the spectral backward Monte Carlo method. The developed
backward Monte Carlo method is used to calculate the infrared images of a simple configuration with two gray spectral
bands, and the efficiency of it is validated by compared the results of it to that of the non-spectral backward Monte Carlo
method . Then the validated spectral backward Monte Carlo method is used to simulate the infrared image of the SDM
airplane model with spectral surface, and the distribution of received infrared radiation flux of pixels in the detector is
analyzed.
In an infrared optical system, the thermal radiation of high temperature components is the major noise as stray radiation that degrades the system performance. Backward Monte Carlo method based on radiation distribution factor is proposed to perform the stray radiation calculation. Theoretical deduction and some techniques are presented, considering the semitransparent element like IR window as radiation emitter. The radiation distribution factors are calculated with ray tracing from the detector to radiation sources. Propagation of stray radiation and its distribution on the detector are obtained simultaneously. It is unnecessary to implement ray tracing again to study the effect of different temperatures for a given system, expect that the geometry or radiative property is changed. An infrared system is simulated using this method. Two different situations are discussed and the analysis shows that stray radiation is mainly created by IR window and lens tube.
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