Evaluation methods for gradient measurement techniques

Abstract

Many optical metrology methods deliver 2D fields of gradients, such as shearography, Shack-Hartmann sensors and the fringe reflection technique that produce gradients for deformation, wave-front shape and object shape, respectively.The evaluation for gradient data usually includes data processing, feature extraction and data visualization. The matters of this talk are optimized and robust processing methods to handle and prepare the measured gradients. Special attention was directed to the fact that optical measurements typically produce data far from ideal behavior and that parts of the measured area are usually absent or invalid. A robust evaluation must be capable to deliver reliable results with non perfect data and the evaluation speed should be sufficient high for industrial applications.Possible data analysis methods for gradients are differentiation and further integration as well as vector processing when orthogonal gradients are measured. Evaluation techniques were investigated and optimized (e.g. for effective bump and dent analysis). Key point of the talk will be the optimized data integration that delivers the potential of measured gradients. I.e. for the above mentioned examples: the deformation, wave-front and object shape are delivered by successful data integration. Local and global existing integration methods have been compared and the optimum techniques were combined and improved for an accelerated and robust integration technique that is able to deal with complicated data validity masks and noisy data with remaining vector rotation which normally defeats a successful integration.The evaluation techniques are compared, optimized and results are shown for data from shearography and the fringe reflection technique (, which is demonstrated in talk "High Resolution 3D Shape Measurement on Specular Surfaces by Fringe Reflection" 1 ).

Testing an aspheric mirror based on phase measuring deflectometry

“…[3][4][5] Here the trapezium rule is used for the numerical integration to complete the surface reconstruction. For a more general measurement case, if the camera cannot meet the requirement of the pinhole model, the original ray PA can be constructed through camera calibration.…”

“…To reconstruct the shape, a complex calibration method combining deflectometry with photogrammetry is needed. [4][5][6] In this presentation, a method based on basic phase measuring deflectometry ͑PMD͒ is proposed to measure the aspherical mirror. The method employs a reference screen two different distances from the testing mirror.…”

High-resolution 3D shape measurement on specular surfaces by fringe reflection

“…17c). It is possible to calculate a local radius from the reciprocal curvature for every measurement point (see 1 ). The average of all local radii (distribution in Fig.…”

Specular surface measurement by using a moving diffusive structured light source

“…The fundamental principle of such approaches is that, the periodic fringe patterns generated in computer are displayed on the screen, reflected by the measured specular surface, and then captured with a camera. Analyzing these distorted patterns results in their phase maps, from which the slope distribution of the surface is reconstructed, and further the 3-D shape is obtained by integrating the slope distribution numerically 37,38 . However, the difficulty is that, the phase values generally depend on the surface slopes coupled with the surface heights, and have ambiguities in their distributions (i.e.…”