Single-shot areal profilometry: Towards real-time surface quality control in additive manufacturing

© 2018 The authors and IOS Press. All rights reserved. Hyperspectral Interferometry (HSI) is a recently-proposed technique for measuring 3-D point clouds from an opaque object in a single shot. We propose a new application of HSI enabling single-shot 3D surface measurements of optically rough surfaces commonly found on additively manufactured and machined components. Using an additively manufactured sample, single-shot surface profiles were taken at a fixed distance to capture and reconstruct the surface profile. This enables the single-shot measurements of rough surfaces over many independent channels in a short time

Double-shot 3-D displacement field measurement using hyperspectral interferometry

A combination of a Michelson interferometer, a micro-optic element and a hyperspectral imager is used with broadband illumination to measure depth-resolved out-of-plane displacements without any scanning. Reference and deformed states of a transparent sample are recorded in single shots and used to evaluate the displacement field at different interfaces

2500-Channel single-shot areal profilometer using hyperspectral interferometry with a pinhole array

Surface profilometry techniques such as coherent scanning interferometry or focus variation require long scan times and are thus vulnerable to environmental disturbance. Hyperspectral interferometry (HSI) overcomes the problem by recording all the spatial and spectral information necessary to reconstruct a 2D surface height map in a single shot. In this paper, we present a new HSI system that uses a pinhole array to provide the necessary gaps for the spectral information. It is capable of measuring 2500 independent points, twice the previous maximum number, with a maximum unambiguous depth range of ∼825 µm and a larger maximum surface tilt angle of 33.3 mrad. The use of phase information allows height to be measured to a precision of ∼6 nm, an order of magnitude improvement on previous HSI systems

Pinhole-array-based hyperspectral interferometry for single-shot profilometry and depth-resolved displacement measurements

Surface profile and topography measurements of manufactured goods and components across all sectors of engineering are an integral component of a production process. Control measurements are taken on a regular basis to ensure items fulfil the required quality standards. Recent industry developments are driving towards more in-situ sensory measurement and monitoring systems. Depending on the dimensional layout and integral material properties of a sample, different metrological measurement techniques come into action for example Coherence Scanning Interferometry or focus variation microscopy. These techniques require high environmental stability and cannot be used for in-line inspection due to long mechanical scanning times. Recent developments in Hyperspectral Interferometry have laid the foundation for a potential single-shot absolute distance measurement technique suitable for in-line inspection. In this thesis a new version of Hyperspectral Interferometry is presented based on a pinhole array. [Continues.

900-channel single-shot surface roughness measurement using hyperspectral interferometry

A recently developed hyperspectral interferometer has been applied, for the first time, to the single-shot measurement of surface roughness. Traditional optical surface profiling techniques, such as coherence scanning interferometry (CSI) or focus variation microscopy, require long scan times and mechanical motion of the imaging objectives, making them vulnerable to environmental disturbances and thus inappropriate as an embedded metrology tool for production. Hyperspectral interferometry (HSI) answers these problems by capturing spatial and spectral information to reconstruct the surface profile, from 900 locations measured in parallel, in a single shot. We report here measurement results with both HSI and CSI from 12 individual roughness samples taken from Rubert roughness gauges covering the roughness range = 0.025 − 50 . Very good agreement was obtained for values in the range 3 – 30 µm, where the HSI values were mostly within a few per cent of (and at worst 24% away from) those measured by CSI . An alternative measure of roughness, based on averaging the widths of the Fourier transform peak of the individual HSI interference signals, is shown to be a statistically reliable measure of local roughness in the medium to high roughness regime, i.e. for values of 0.8 µm and above. The results thus demonstrate the technique’s potential for real-time surface quality inspection in manufacturing.</div