Traceability of line scales using image processing

Abstract

Metrological traceability, is defined as the "property of a measurement result that allows the result to be related to a reference through an unbroken chain of calibrations, each contributing to the measurement uncertainty." Calibrating a line scale consists of determining the distances between the center of the marks along its length. This type of calibration is usually performed by a comparative method, where a standard scale of higher accuracy is positioned next to the scale to be calibrated, and its marks are visually compared using a graduated magnifying glass. This methodology has some points that can be improved in order to make the calibration faster, less dependent on an operator. The calibration method proposed and presented in this paper uses computer vision techniques for calibrating line scales, turning a digital camera into a measurement standard with its traceability referenced to a length standard. The method consists of applying an image registration technique, where images are captured in sequence along the scale, and redundant points in the images are automatically. The proposed procedure is capable of generating results with uncertainties ranging from 0.05 mm to 0.30 mm for scales up to 1 meter.