Focus Variation – a Robust Technology for High Resolution Optical 3D Surface Metrology

Abstract

This article describes and evaluates the focus variation method, an optical 3D measurement technique. The goal is to analyse the performance of the method on a series of typical measurement tasks including roughness measurements, form and wear measurements. First, a comparison of roughness measurements between the proposed method and a tactile device on a newly developed roughness standard is made. Results show that both systems deliver R a values that are comparable to each other with differences of a few nanometers. Afterwards form measurements are performed on a calibration standard with hemi-spherical calottes, showing a repeatability of sphere measurements < 100 nm. Finally, two typical engineering applications are provided. The first is wear measurement of cutting tools, the second the inspection and classification of welding spots. Both applications demonstrate the ability of the method to measure steep surface flanks up to 80° and surfaces with difficult reflectance behaviour.

Ion distribution and ablation depth measurements of a fs-ps laser-irradiated solid tin target

“…The microscope has a 50Â imaging objective with a numerical aperture (NA) of 0.42, yielding a depth of focus of 3 lm and enabling the determination of crater depth by straightforward optical inspection of a selected number of holes. The same microscope, equipped with a 5Â imaging objective and a motorized stage for automated focus scanning to provide a complete picture of the hole, is used for an automated ablation volume determination by means of the focus variation technique 33 which combines the images acquired by the microscope with computational techniques to provide 3D reconstructions of the ablated sample surfaces. A 2D Gaussian fit to the reconstructed surface profile is performed, and the integral of the fitted curve then provides an estimate of the ablated volume.…”

A newly discovered antler flint-knapping hammer and the question of their rarity in the Palaeolithic archaeological record: Reality or bias?

“…The Alicona Infinite Focus microscope (AIFM) used in this analysis provided a truecolour rendering of the pedicle surface in three dimensions (Bello and Soligo, 2008;Bello et al, 2011;Danzl et al, 2009). Images were captured using a 2.5x objective lens (magnification x 45.72) and a vertical and lateral resolution of 10 µm and 3.47 µm respectively; a 5x objective (magnification 91.44x, vertical resolution = 1.74 µm, lateral resolution = 1.49 µm) was used to record features of the embedded lithic fragments and morphology of the surface marks.…”

An optical method for measuring surface roughness of machined carbon fibre-reinforced plastic composites

“…The optical system uses focusvariation of an optic, taking multiple images at varying vertical distances and using optical sharpness to create a full high resolution 3D surface image. R. Danzl et al [16] and [17], details the system operation and demonstrates its capability to achieve similar results to a calibrated stylus device using a standard roughness specimen. This system has not been applied to the characterization of machined fibrous composite surfaces in any known literature.…”