Optical Nondestructive Metrology of High-aspect-ratio Micro-holes via Focus-displacement Mapping

The accurate and efficient metrology of micro-holes with aspect ratios from 5:1 to 20:1 remains challenging owing to low throughput and a high risk of damage in conventional approaches.Here, we develop an embedded optical, nondestructive measurement system that exploits a focus-displacement mapping mechanism to quantify key geometric parameters of high-aspect-ratio micro-holes.We introduce a gradient-variance-driven adaptive sharpness operator (GDANS) and a three-point parabolic interpolation scheme to rapidly locate focal planes with subpixel precision.In addition, a lightweight random sample consensus (RANSAC)based circle fitting routine is employed to extract diameter and circularity with high efficiency.The system is built around an STM32H743 microcontroller and integrates an OV7725 CMOS image sensor, a quasi-coaxial illumination path, and a high-precision geared motor module through a modular opto-mechatronic design.A state-machine-based software architecture enables closed-loop operation from autofocus scanning to parameter reporting.Experiments on calibrated samples demonstrate favorable performance in measurement speed, cost, and practical deployability, providing a feasible solution for the rapid on-site inspection of high-aspect-ratio micro-holes in industrial settings.