This paper proposes a method for high-precision radius measurements. The proposed system uses a frequency-modulation technique to improve the accuracy of detecting cat’s eye and the confocal position of a lens. The distance between two points defined as the radius of a lens was measured by using a frequency-modulated Michelson interferometer. In the confocal part, when the tested lens was moved along the optical axis, the intensity at the detector reaches a maximum at the confocal and cat’s eye positions of the sphere. Synchronous detection technique is used to turn the intensity signal from the detector into an odd-symmetry signal through a zero point. These symmetry points coincide with cat’s eye and confocal positions and thus enhance the accuracy with which these two points are detected. A frequency-modulated Michelson interferometer is used to accurately measure the displacement when moving the test sphere between the confocal and cat’s eye points.
