A fast edge location measurement with subpixel accuracy using a CCD image

Abstract

This paper describes an analogue-based approach to fast subpixel-level edge detection. Although the most common subpixel edge detection methods employ digital-based approaches, they at present show certain drawbacks such as difficulty of fast, in-line subpixel edge detection, the necessity of implementing expensive high-resolution A/D converters, and difficulty in performing continuous, dynamic subpixel edge detection. We propose a new analogue-based first derivative subpixel edge detection approach to surmount these drawbacks. Our method involves the approximation of the distribution curve of the first derivative of the output of a CCD by using the second-order polynomial, and thus can accurately detect a peak position of the differential curve by means of the interpolating calculation, the operation of which is realized primarily through the use of analogue circuitry. The measurements of a concrete form using the prototype system demonstrate that its resolution under ideal conditions is about 1/10 pixels. We thus conclude that the approach described is effective for fast subpixel edge detection.