| Abstract | The measurand for line scales is defined as the distance between the centres of two scale marks under specified conditions (20 °C, 101.3 kPa, and prescribed support conditions), but since no method for determining the mark centre is specified, different centre-finding algorithms can produce different results, introducing measurement uncertainty. This study quantitatively compares various centre-finding methods used in literature and calibration practices, including those for photoelectric sensors and camera systems, and investigates the effects of lighting conditions, artefact defects, defocus, and reversal measurements. Using a numerical model of a microscope system, the researchers found that light-source misalignment and optical aberrations such as coma and spherical aberration cause algorithm-dependent shifts in the calculated mark centre, explaining the focus sensitivity observed experimentally. The results show that both the measurement method and microscope imperfections significantly influence the determination of mark centres and therefore contribute to uncertainty in high-precision line-scale calibration. |
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