A method for inspecting double-sided high-sloped structured surfaces based on dual-probe wavelength scanning interferometer

Double-sided high-sloped structured surfaces such as V-groove surfaces and Fresnel lenses are widely used in optical fibre positioning, retro-reflection, grating, light guiding and light concentration for solar power installation. Both the surface finish as well as the dimensions of the structured surfaces play important roles in the quality of the final products. Numerous efforts have been put into the study of characterisation of these types of surfaces. However, only part of the parameters can be acquired and analyzed. It is still impossible to measure and generate the whole topography of these types of structured surfaces. This results in the manufacturing process suffering from high scrap rates. In this paper, an orthogonally placed dual probing system based on Wavelength Scanning Interferometry (WSI) aiming to measure the whole topography of the double-sided high-sloped structured surfaces simultaneously is presented. Each of the probes form an interferometer, and measures the facets of the double-sided high-sloped surfaces in one direction and acquires part of the topography. The whole topography is then stitched together using the two datasets based on the relationship between the coordinate systems of the two probes. The relationship between the two probes is acquired through the calibration of a specially designed 3D artefact. The artefact contains geometric features on each of the facets and is calibrated by a combination of several measurement methods to establish the space coordinates of the features. By matching the corresponding features on the measurement results acquired with each of the probes of the new setup to the reference topography using a 3D registration algorithm such as ICP (Iterative Closest Points) and its variants, the relationship between the coordinate system of each probe and the coordinate system of the reference topography can be calculated. Then the relationship between the coordinate systems of 2 probes can be determined, which can then be used to stitch the whole topography. The setup and the math model has been built and some initial results have been acquired.