The four-detector photopolarimeter (FDP) is analyzed for an arbitrary spatial configuration and any reflection characteristics (ri, /i, Ai) of the first three detectors. The instrument matrix A, which relates the output signal vector I to the input Stokes vector S by I = AS, and its determinant are derived explicitly. The essential condition that A be nonsingular (det A $ 0) is satisfied in general with uncoated absorbing detector surfaces, assuming that the plane of incidence (POI) is rotated between successive reflections by other than 90°. Therefore no special coatings on the detectors are required, and a thin dielectric (e.g., thermal oxide) layer would suffice. The differential reflection phase shift A is unrestricted for the first and third detectors and has optimum values of ±90' for the second. The optimum rotation angles of the POI are +45 0 and +135'. The optimum values of the surface parameter ip are 27.37°, 22.50 or 67.50, and 0 or 90° for the first, second, and third reflections, respectively. The following topics are also considered: (1) the partition of energy among detectors, (2) the effect of tilting the last detector, (3) operation of the FDP over a broadband spectral range, (4) choice of the light-beam path, and (5) calibration.

An important step in the fingerprint identification system is the reliable extraction of distinct features from fingerprint images. Identification performance is directly related to the enhancement of fingerprint images during or after the enrollment phase. Among the various enhancement algorithms, artificial-intelligence-based feature-extraction techniques are attractive owing to their adaptive learning properties. We present a new supervised filtering technique that is based on a dynamic neural-network approach to develop a robust fingerprint enhancement algorithm. For pattern matching, a joint transform correlation (JTC) algorithm has been incorporated that offers high processing speed for real-time applications. Because the fringe-adjusted JTC algorithm has been found to yield a significantly better correlation output compared with alternate JTCs, we used this algorithm for the identification process. Test results are presented to verify the effectiveness of the proposed algorithm.

A division-of-amplitude photopolarimeter (DOAP) is described that uses a dielectric parallel-slab (PS) beam splitter that is coated with a high-reflectance metal on one side at oblique incidence. The instrument matrix of the PS DOAP is nonsingular, hence all four Stokes parameters can be measured simultaneously over a broad (UV-visible-IR) spectral range. The parallel, evenly spaced, ref lected beams simplify interfacing of the PS DOAP with linear photodetector arrays for both single-wavelength and spectroscopic polarimetry. The PS DOAP has several degrees of freedom that can be controlled for optimum performance.

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