Dual differential confocal method for surface profile measurement with a large sensing measurement range

To meet the requirements of the large sensing measurement range and high axial depth resolution for profile measurement, a dual differential confocal method (DDCM) is proposed in this paper. The DDCM uses the confocal signal to process separately the signal of two pinholes with axial offset, and it adds the two processed signals to obtain an axial response curve with a large slope and linear response range, thereby achieving a high-precision surface profile measurement with no axial scanning. Preliminary experiments show that the DDCM has a sensing measurement range of 0.54 µm and an axial resolution of 1 nm at the numerical aperture of 0.9. Furthermore, the sensing measurement range of the DDCM is approximately 2.9 times that of the differential confocal microscopy.