KEYWORDS: Hyperspectral imaging, Sensors, Principal component analysis, Reflectivity, Absorption, Signal to noise ratio, Air contamination, Detection and tracking algorithms, Target detection, Mahalanobis distance
Anomaly detection in hyperspectral imagery seeks to identify a small subset of pixels whose spectra differ most significantly from the background. The challenge is to characterize the background and noise well enough to recognize which observations are truly distinct and not simply noise outliers. The covariance-based RXD operator was developed to select low-probability pixel spectra and is therefore sensitive to noise. We compare the RXD operator to a Euclidean metric weighted by the inverse of the estimated spectral noise variance. We then combine the weighted Euclidean metric with RXD using a Lagrange multiplier and demonstrate that this formulation retains RXD's emphasis on small clusters while controlling the impact of noise. An optimum value of the Lagrange multiplier is determined based on the number of bands. We explore the utility of normalizing the pixel spectra as a step in anomaly detection. Results for the RXD, weighted-Euclidean, and Lagrange approach are presented using AVIRIS and HYDICE imagery. Based on these results, we conclude that the Euclidean, although robust to noise, does little more than emphasize the brightest pixels. The Lagrange detector selects the same regions as RXD while significantly reducing the impact of noise.
Development of target detection algorithms and simulation models for present and future multispectral and hyperspectral sensor systems requires accurate characterization of the reflectance and thermal emission of natural and man-made materials. Fourier transform spectrometry is one method for obtaining relatively high spectral resolution, in-situ measurements of surface reflectance. This paper discusses the performance characteristics of the SOC-400T FTIR and its application to field measurements. The SOC-400T is a relatively small and portable FTIR reflectometer that was designed to measure the directional reflectance and calculate the directional thermal emittance of surfaces in the spectral range from 2 to 25 ημm. The SOC-400T uses a silicone carbide glowbar to illuminate samples. This permits accurate results to be obtained in the MWIR. We recently deployed this instrument to the field to perform measurements on various materials of interest to the military. Prior to the deployment, the instrument was evaluated to assess its performance under true field operating conditions. This paper specifically examines noise characteristics, warmup time, transients induced by reorientation of the sensor, spurious detector artifacts, and sensitivity to vibration. We also address the practical issue associated with positioning, stabilizing, and calibrating the instrument for field measurements of irregular or arbitrarily oriented surfaces.