Uncertainty Evaluation of Profile Projector Calibration

Abstract

A profile projector is an optical measuring instrument that magnifies a workpiece's surface features to allow measurement on a linear or circular scale. It is also referred to as an optical comparator, since dimensions can be measured directly on the screen or compared to a standard reference at the correct magnification. Profile projectors are usually used for measuring profiles, dimensions and angles of workpieces. They have been widely used in electronics, semi-conductors, and mechanical industries. Profile projectors are common used as the basic inspection equipment in many factories and laboratories. The accuracy of profile projector is very important, since it influences the finishing quality and reliability of products directly. Thus, profile projector shall be calibrated periodically to ensure its quality of inspections and measurements. This paper detailed describes the uncertainty evaluation procedure of profile projector calibration in accordance with the ISO/IEC Guide 98-3:2008. The effects of error sources in this calibration system are analyzed to estimate the corresponding standard uncertainties and sensitivity coefficients. The combined standard uncertainty and effective degrees of freedom are then calculated. The coverage factor can be found by taking the t value corresponding to the effective degrees of freedom. Finally, the expanded uncertainty can be obtained by multiplying the coverage factor and the combined standard uncertainty. The calibration method of profile projector is also briefly stated. For the calibration of a profile projector at a magnification of 10, the expanded uncertainty of magnification deviation is estimated to be 0.0096 % with a coverage factor of 2.04 corresponding to a level of confidence of approximately 95 %. For the positioning deviation calibration in a measurement range of (0 to 100) mm, the expanded uncertainty is estimated to be 0.005 mm with a coverage factor of 2.01 corresponding to a level of confidence of approximately 95 %.