Correlation-Based Measurements of Camera Magnification and Scale Factor

A correlation method is described whereby the high precision of a commercial translator, which can be 10^-5 or smaller in fractional error, is transferred to the image plane of a camera system through the determination of magnification and scale factor (effective focal length). Key to this method is the averaging of pixel boundary effects with a large-area planar random-dot target. The correlation method is relatively insensitive to camera random errors, target characteristics, illumination nonuniformities and localized imperfections in the camera or the target. Random errors are reduced by the large number of pixels contributing to each correlation sum. Variations of the focal length across the camera field-of-view were measured. Both systematic errors and the angular variation of magnification (lens aberrations) limit the accuracy of the method.