Physical Sciences Inc. (PSI) has developed an Adaptive InfraRed Imaging Spectroradiometer (AIRIS), comprised of a low-order tunable Fabry-Pérot étalon coupled to an HgCdTe detector array, for passive, stand-off detection of chemical vapor plumes. The tunable étalon allows coverage of the 9.5 to 14 µm spectral region with a resolution of ~7 cm^-1 and provides the capability to obtain monochromatic images (λ/δλ >100) of a scene at only those wavelengths needed for chemical species identification and quantification. The adaptive sampling capability of the étalon allows suppression of background clutter and minimization of data volume. The tuning time between transmission wavelengths is typically ~10 ms, however the mirror tuning system may be operated to obtain tuning times as short as 1.3 ms. We present results using a brassboard imaging system for stand-off detection and visualization of chemical vapor plumes against near ambient temperature backgrounds. This data shows detections limits of 22 ppmv · m and 0.6 ppmv · m for DMMP and SF₆ respectively against a ΔT of 6 K. The reported detection limits are consistent with the measured system noise-equivalent spectral radiance, approximately 2 µW cm^-2 sr^-1 µm^-1.