A prism fingerprint sensor is described that uses a holographic grating glued to a right-angled prism. A light source normally illuminates the hypotenuse side of the prism with the finger pressed against the grating. The ridges and valleys of the finger are sensed on the basis of the principle of total internal reflection. The grating is used essentially to correct the distortion usually present with prism sensors. The quality of the fingerprint is very good: the pores on the ridges can be seen.
An autofocusing mechanism based on the concept of autocorrelation is described. The method may be performed optically with ring detectors in the Fourier domain or electronically with a special computing chip or software.
The structure and dynamics of speckles formed by a weak diffuser in the presence of severe spherical aberration have been studied. Two cases were considered. In the first case the diffuser is treated as an object, and the speckles are observed in the paraxial, least confusion, and marginal planes. In the second case the diffuser placed in various planes is illuminated by a spherically aberrated wave front from a lens. The results are illustrated by photographs, and heuristic arguments are given to account for them.
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