Analysis of method for measuring thickness of plane-parallel plates and lenses using chromatic confocal sensor

Abstract

Noncontact optical metrology based on the chromatic confocal principle is becoming increasingly important for fast and accurate measurements of surface topography, distance, and layer thickness in engineering and industry. These sensors are based on the wavelength dependence of longitudinal chromatic aberration of optical systems, and the distance or thickness of the measured sample is coded into spectral information. We provide a theoretical analysis of a problem of the thickness measurement of transparent samples (glass plane-parallel plates or lenses) with respect to material dispersion. Our work deals with a description and analysis of induced measurement errors in the cases of measurement of the thickness of a plane-parallel plate and the central thickness of a lens. Relations are derived for a quantitative evaluation of these errors and a method is presented for minimizing the influence of these errors on the accuracy of measurement.

Development and Beam-Shape Analysis of an Integrated Fiber-Optic Confocal Probe for High-Precision Central Thickness Measurement of Small-Radius Lenses

“…Non-contact techniques that have been widely employed for thickness measurement of transparent plates and optical lenses are mostly based on low-coherence interferometry [ 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 ] and confocal microscopy [ 9 , 10 , 11 , 12 , 13 ]. Interferometry-based techniques can be used to measure thicknesses of transparent objects in the range of ten microns to a few millimeters, with sub-nanometer resolution [ 1 , 2 , 4 , 5 ].…”

“…There are reports of using a reflection confocal sensor to measure the central thickness of a lens [ 9 , 10 , 11 , 24 ]. During depth scanning, the optical distance is calculated from the interval between confocal peaks corresponding to the top and bottom surfaces of the lens.…”

Frequency-Shifted Optical Feedback Measurement Technologies Using a Solid-State Microchip Laser

“…A machine vision method [88] has advantages of continuous measurement, it is flexible in the production line, and the accuracy is less than 0.005 mm. Chromatic confocal sensors [89] are based on spectrally broadband light to realize a special optical probe. The measuring range and the accuracy are influenced by the designed probe and the dispersion of the measured object material.…”

New method for lens thickness measurement by the frequency-shifted confocal feedback

“…Contact measurement can only be utilized when machining and assembling the lenses, which has lower accuracy and even causes damage to the coated lens. The non-contact measurement methods mainly include machine vision method [9], chromatic confocal microscopy [10,11], low coherence interferometry [12], and laser differential confocal technique [13,14]. Machine vision method has advantages of continuous measurement and flexible in production line with an accuracy of less than 0.005 mm.…”