Sharpness index and its application to focus control

A sharpness index function, derived from the intensity distribution in an image aberrated by a quadratic-curvature wavefront distortion, is shown to be very effective as a measure of the correctness of focus. The function is introduced and analyzed in the spatial frequency domain. Broadly, the sharpness index belongs to a family of functions known as image sharpness functions that are potentially usable for image enhancement and restoration. The selectivity and the sensitivity of the sharpness index are theoretically derived as functions of focus error. Several sharpness index functions have been hardware implemented, and their performance as focus error signal generators have been tested in a TV imaging system in the laboratory. Finally, an outline is presented of an experimental closed-loop autofocus system using a sharpness index function, successfully implemented and tested in the laboratory.