ABSTRACT
A method of optical metrology perhaps not as widely used as laser triangulation, structured light, and machine vision is to use information about the focus of an optical system to provide a means of measurement. One of the simplest forms of this method is used in most autofocus cameras today. A simple analysis analyses an image based upon overall contrast from a histogram of the image, the width of edges, or more commonly the frequency spectrum derived from a fast Fourier transform of the image. That information might be used to drive a servo mechanism in the lens, moving the lens until the quantity measured on one of the earlier parameters is optimized. Moving from a fuzzy image to a sharp image is something just about anyone can do instinctively with a manual camera. Given an image like Figure 11.1, the left-hand image shows an in-focus image with distinctive surface texture evident, while the right-hand image is clearly out of focus and the surface detail is lost. The resulting FFT images shown in Figure 11.2 show more high-frequency content (Figure 11.2a) for the in-focus image versus the blurred image (Figure 11.2b).
