For a severely defocused incoherent system, its optical transfer function (OTF) has isolated zeros; therefore, an exact inverse filtering cannot be performed. Isolated zeros in the OTF can be avoided by choosing an annular aperture with a proper radius ratio, as the aperture can extend the depth of focus of the system. However, in the process of increasing the depth of focus of the system, this method results in a loss of image contrast. A simple hybrid optical/digital image processing system in which a TV camera device is coupled with an annular aperture is considered. Annular-pass filtering to compensate for the loss of contrast is performed by a digital computer. Experimental results are presented.
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