Adaptive-tracking laser differential confocal freeform surface measurement with high accuracy

This study introduces an adaptive-tracking method for high-precision measurement of freeform surfaces with unknown angles and normal directions. By integrating the laser differential confocal technology with the normal tracking method through a position sensitive detector (PSD), the proposed method achieves precise fixed-focus scanning over large angles. Subsequently, by establishing the zero-crossing and normal constraints, an adaptive-tracking method based on the height and normal direction of the freeform surfaces is presented to realize the adaptive-tracking measurement of freeform surface profiles. Experimental results demonstrate that the peak-to-valley (PV) (3σ) and the root-mean-square (RMS) (3σ) of surface profiles are superior to 39 nm and 13 nm, respectively. This method provides an effective approach for the high-precision and large-angle measurement of freeform surfaces without known expression equations.