Simulating systematic scene-change artifacts in Fourier-transformspectroscopy

摘要

Improved understanding of midwave infrared (1-5μm) spectral emissions from detonation fireballs is needed todevelop a battle space optical forensics capability. While Fourier-transform spectrometers (FTS) are an attractive tool,interferometer-based spectroscopic measurements can be corrupted when the observed scene intensity systematicallyvaries during the measurement time. Approximating a detonation fireball as a blackbody radiator with a time-varyingtemperature T and modified by atmospheric attenuation τ(v), double-sided interferograms from an ideal FTS werecalculated and converted to measured spectra L_m(v) to characterize the nature and magnitude of scene-changeartifacts. T(x) decreased exponentially with optical path difference x, –x_m < x < x_m,at various rates relative to theMichelson mirror speed so that changing scene spectra could be simulated on 1700 < v < 7900 cm-1 at δv = 3.64 cm-1resolution (x_m=0.25 cm, Hamming apodization). The real part ofLm(V),ReILm(v)}, is well approximated by theinstantaneous spectrum at zero path difference, L(v,x = 0). In regions where i(v) is highly structured, both theimaginary component Im{L_m(v)} and the differences between Re IL_andL(v,0) exhibit spectral features, ingeneral |Im{L_v)}|》RеlL_m(v)} -L(v,0)i.In a region of highly structured absorption, 2800 < v 3500cm~(-1),600Kdecrease in temperature produced RMS values of 62 and 5μW/ (cm~2· sr ·cm-1)in Im{Lm(v) 1 and ReILm(– L(v, 0)},respectively, compared with an RMS value of 1924 μW/ (cm~2.sr · cm~(-1)) in Re {L_m(v)}. A method based on theoreticalexpressions developed by Kick etal. is devised to interpret L_m(v)and provide estimates of the temporal evolutionT(x)when its functional form is not known a priori.