Freeform surface measurement by virtual differential confocal based on reference planes monitoring

Aiming at high-precision measurement of freeform surface, a method combing virtual differential confocal and reference planes monitoring is proposed. By setting the window of the CCD and selecting two acquisition areas of different sizes, the large area is used for coarse observation of the light spot when looking for the focus, and the small area is used as perform high-speed synchronous acquisition of the light intensity of the front focus and back focus. The intensity signals of the front focus and back focus are processed by normalized differential subtraction, to obtain the anti-scattering differential confocal curve. 3D point cloud data is obtained by measuring the freeform surface, and an error monitoring and compensation model is established based on standard planes to compensate 3D motion errors in point cloud data, to achieve the high precision freeform surface measurement. Theoretical analysis and experimental verification show that the axial resolution is 2 nm, the maximum measurable inclination angle of two dimensions are both 25°, and the standard deviation of measurement repeatability of 10 times PV values is 10 nm. This method can effectively expand the observation range of the differential confocal sensor, reduce the difficulty of the radial adjustment of the pinhole, and provide a new idea for high-precision measurement of freeform surface with large inclination angles and different surface roughness.