High-precision laser transverse differential confocal radius measurement method.

To meet the current need for high-precision and environment-insensitive measurement of the radius of curvature (ROC), we proposed a transverse differential confocal radius measurement (TDCRM) method based on the optical system of the confocal ROC measurement. Using a D-shaped aperture and the virtual pinhole technology, two signals, analogous to the pre-focus and post-focus signals in the two-detector-based differential confocal radius measurement (DCRM), can be obtained from two segmentations of a single CCD image. The difference of these two signals can be used to precisely determine the cat's-eye and confocal positions, thereby achieving the high-accuracy ROC measurement as DCRM with a relative repeatability of 3.4 ppm. Furthermore, compared to DCRM, no optical alignment is needed after replacing the objective lens, which significantly reduces the time cost of measurements. We believe this novel and high-precision ROC measurement method will widen its application to optical manufacturing and provide an exciting opportunity for mass production of the ROC measurement instrument.