Design of a Deflectometry for Measurement of Specular Object with Curved Surface

Abstract

Deflectometry is widely used to measure the 3D profile of a specular free-form surface because of its high accuracy and short inspection time. A general deflectometry is composed only of a camera and a screen. The camera observes the reflected image of the pattern on the specular object. Distortion of the image due to the curved surface lets us know the shape of the surface. Once the correspondence between the image and the pattern on the screen is resolved, we can reconstruct the surface shape. For highly curved surfaces, however, the camera cannot observe the reflected image of the pattern on the specular object because the light from the limited size of the screen can cover only the limited slope of the object. To overcome the limitations of the existing deflectometry and enhance the performance of deflectometry, first of all, existing deflectometry is modified to a multi screen deflectometry to measure highly curved specular surfaces, and the proper calibration method and reconstruction algorithm are developed. This idea is verified by experimental results, in which phase information is successfully extracted from the area which are not normally measurable due to its high curvature surface. Secondly, a 2D phase-shifting method for computing two phases in different directions with a single set of 2D phase pattern with only five images is proposed to reduce the image acquistion time. A surface of solder joint is a typical example of specular surface. Due to the spcularity and the complex 3D geometry, the visual inspection of solder joints has been regarded as one of the most difficult tasks and thus has not guarrenteed accurate inspection result. In the final chapter, the solder joint inpection device using the multi screen deflectometry is proposed to inspect the solder joint accurately.