Laser differential confocal ultra-large radius measurement for convex spherical surface

A new laser differential confocal ultra-large radius measurement (LDCRM) method is proposed for high-precision measurement of ultra-large radii. Based on the property that the zero point of a differential confocal axial intensity curve precisely corresponds to the focus points of focusing beam, LDCRM measures the vertex positions of the test lens and the last optical surface of objective lens to obtain position difference L₁, and then measures the vertex positions of the reflector and the last optical surface of objective lens to obtain the position difference L₂, finally uses the measured L₁ and L₂ to calculate the radius of test lens. This method does not require the identification of confocal position. Preliminary experimental results and theoretical analyses indicate that the relative uncertainty is 0.03% for a convex spherical lens with a radius of approximately 20 m. LDCRM provides a novel approach for high-precision ultra-large radius measurement.