Use of Correlation coefficient based image analysis scheme for improved accuracy in focal length measurement

In this paper, we propose a novel image analysis scheme based on correlation coefficient for the accurate determination of focal position of a lens. The collimated light is incident onto the lens and is back reflected using a plane mirror then after passed through a coarse grating. Because of the Talbot self-imaging phenomenon, self-images of the grating at certain fixed planes, called Talbot planes, are formed. For each position of test lens, self-images formed at the first and second Talbot planes are captured. The correlation coefficient between the pair of images is computed, and is used as a measure to determine accurately the focal position of test lens. The proposed technique does not require the use of phase measuring algorithms or the translation/ rotation stage.