Correlation spectroscopy with diffractive grating synthetic spectra and orthogonal subspace projection filters

We evaluate the feasibility of implementing correlation spec- troscopy for remote sensing with a passive IR sensor that incorporates a diffractive grating on which a synthetic spectrum is imbedded. The spec- tral diffraction reflectances of diffractive gratings are measured and com- pared to the theoretical design spectra and to the library absorbance spectra. Five gratings are evaluated: sulfur hexafluoride (SF6), ((dimethoxyphosphinothioyl)thio)butanedioic acid, diethyl ester (malathion), diisopropyl-methylphosphonate (DIMP), methylphosphono- fluoridic acid,(1-methylethyl) ester (Sarin or GB), and O-ethyl-S-(2- diisopropylamino)ethyl methylphosphonothioate (VX). While the gratings were designed from liquid phase infrared spectra, the measurements and models provide a satisfactory proof of principle for the exploitation of synthetic spectra in correlation spectroscopy. The predicted noise equivalent concentration pathlength (NECL) for detecting GB and VX in a desert environment is computed for distances up to 5 km. The device was found to have high sensitivity in terms of NECL. The selectivity of the grating is found to be poor, but can be improved dramatically with the use of orthogonal subspace projection (OSP) filters that reduce the false alarm (improve selectivity) from misidentifying a GB as VX (and vice versa) at the ''cost'' of a slight increase of the NECL (reduce sensitivity).