Ultra-small spectral confocal displacement sensors based on GRIN lenses

Spectral confocal displacement sensors have been widely used in various precision measurement fields such as aerospace, biology, and medicine due to their advantages of high accuracy, non-contact, fast response, and high resolution. In this paper, a new method of spectral confocal measurement of a ultra-compact probe based on a GRIN lens, to our knowledge, is proposed. Through ray tracing analysis, it is found that the axial dispersion is close to linear in a specific wavelength range, and a theoretical model of spectral confocal measurement with the GRIN lens is deduced. A GRIN lens with a diameter of 1 mm and a length of 6.59 mm was prepared by the ion-exchange method and used as a dispersive objective lens in a homemade spectral confocal measurement system. The system was calibrated to have an axial measurement range of 440 µm, an axial resolution better than 0.5 µm, a maximum standard deviation of 0.749 µm, and a relative error of less than 2%. In addition, the system was successfully applied to 3D surface contour scanning, demonstrating its potential for 3D scanning. The results show that the system has the advantages of simple structure, low cost, easy integration, and excellent performance.