Design of Lens Central Thickness Measuring Optical System

Abstract

The designed lens central thickness measuring optical system is a kind of new system based on chromatic confocal principle, which can fast and automatically measure center thickness of lens without contact of lens surfaces. In order to expand measuring range of device, multiple configuration modes are established to eliminate monochromatic aberrations within each configuration, and enlarge the D-value of image distance to increase the longitudinal chromatic aberration. In this way, the longitudinal chromatic aberration of the system can reach about 30 mm in the wavelength range from 400 nm to 1000 nm. The maximum measurable lens thickness can be more than 30 mm by using this optical system. Measurement of three different lenses whose central thicknesses are 6, 15 and 30 mm is done. The results show that the measuring accuracies for the three lenses are ±2.5, ±3.5 and ±5.0 μm.

Applications of Chromatic Confocal Technology in Precision Geometric Measurement of Workpieces

“…The monochromatic light is converged at specific positions on the optical axis depending on its wavelength. Assuming that the wavelength of the light converging on the upper surface of the lens is λ1, with the incident and reflection angles being both θ0; the light of wavelength λ2 is converged on the lower surface of the lens through lens refraction, with the angle of reflection being θ1 and the angle of reflection being θ2; after reflection by the lower surface of the lens, the incident and reflection angles become both β, the difference between the original convergence points of the two beams of light is h. The refractive index of air is n1, the index of refraction in the lens is n2, and the radius of curvature of the upper surface of the lens is R. Hence, the thickness of the central thickness D of the lens [12] can be expressed as: Yan [13] designed a chromatic confocal thickness measurement system for transparent materials, which was designed to precisely measure the thickness of a variety of transparent materials, with a measuring range of 1 mm and a measuring accuracy of ±5 μm. Liu [14] built an accurate contour and thickness measurement device based on chromatic confocal technology, by which they measured the thickness of the front glass panel of a mobile phone.…”

“…Feng [15] developed a single-probe chromatic confocal thickness measurement system, by which they measured the thickness of flat transparent standard with a thickness of 0.53 mm, achieving an average measurement error of 2.66 μm. Zhou et al [12] designed a lens central thickness measurement system based on the principle of chromatic aberration and achieved a minimum error of ±2.5 μm in the measurement of a 6 mm-thick lens.…”

“…They calculated the accuracy error of chromatic confocal sensors to be 216.5 nm and modeled uncertainty evaluation. Zhou et al [12] developed an online integrated surface roughness measurement system based on chromatic confocal technology and conducted tests using surface roughness standard samples with triangular wave-form patterns whose arithmetic mean deviation of surface roughness Ra had a calibration value of 0.37. The test results showed that the measurement repeatability of parameter Ra was 0.0026 μm and the measurement efficiency was improved nearly 30% as a whole.…”

“…Feng[15] developed a single-probe chromatic confocal thickness measurement system, by which they measured the thickness of flat transparent standard with a thickness of 0.53 mm, achieving an average measurement error of 2.66 μm. Zhou et al[12] designed a lens central thickness measurement system based on the principle of chromatic aberration and achieved a minimum error of ±2.5 μm in the measurement of a 6 mm-thick lens.Chromatic confocal technology is particularly applicable to thickness measurement of transparent materials owing to its unique principle of measurement. It has no special requirements for workpiece material, surface smoothness, and thickness uniformity, and two obvious spectral crests can be obtained by the spectrograph as long as the dispersed light can penetrate the transparent material.…”