This paper describes a diameter measurement method for micro-spheres via coherent scanning interferometry (CSI) with a gauge block as the reference. The CSI system measures the height difference between the sphere and gauge block surface from both the front and back sides; then, the diameter is calculated from the measured heights via CSI and the gauge block length. For the glass sphere measured in this study, the diameter was found to be 270.556 µm with an uncertainty of 0.16 µm (k=2). Interestingly, by selecting a gauge block that matches the sphere diameter, the measurement uncertainty remained virtually unchanged, even for different sphere diameters; the proposed method achieved a relative uncertainty of 10^-3–10^-4. By utilizing the calibrated reference and the highly sensitive CSI system, and based on the comparator principle, the proposed method enables accurate diameter measurement without requiring specific measurement instruments.

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