Comparison of digital focus criteria for a TV microscope system.

Existing focusing methods, such as standard deviation of gray values, gradients, information content of the image and the lateral inhibition function, were compared, and a new combination criterion was developed. This combined method consists of using the standard deviation of the image for coarse focusing followed by a modified form of the lateral inhibition for the fine focus. The lateral inhibitor model of the eye is a recursive filter that can be simplified to a nonrecursive filtering algorithm, as postulated by Rechenberg. The reported focus method operates over a focusing range (z-axis) of +/- 300 micron and is accurate within +/- 0.1 micron in the transmission, dark-field, and phase contrast microscope.