In-plane coplanarity measurement by Talbot interferometer

Abstract

The monitoring and measurement of in-plane coplanarity is important in the production and assembly of smaller and more fine-pitched optical and mechanical systems. Owing to its features such as greater accuracy, resolution, instantaneous response and non-intrusive nature, interferometry is advantageous and extensively utilized in a broad spectrum of applications. Interferometers possess high resolution, long measurement range and fast response times. Infra-Red diffraction interferometer is broadly used for coplanarity measurement [1-2]. Recently grating interferometer has been also used for the coplanarity measurement [3]. Circular grating Talbot interferometry has been widely used in many metrological as well as microscopic applications [4-10]. The benefits of using circular gratings are that they are easy to align, and no prior knowledge is needed for the measuring direction [10]. In this paper, a fast and cost-effective 3D (x, y, z- axis) measurement system for in-plane coplanarity inspection of an optical table is proposed. Different from the line-scanning measurement of traditional laser-based systems, the proposed, circular grating Talbot interferometer system with fringe shift measurement can provide the 3D (x, y, z -axis) measurement of coplanarity. With the conventional practice of using the zero Talbot area, it is not possible to obtain fringes of the non-coplanar surfaces unless the Talbot distance is much greater than the distance between the surfaces. The Talbot area is employed to measure the deformation of the two non-coplanar surfaces simultaneously. But this approach has a critical limitation, and the optical setup will be extremely case-sensitive. In the proposed system, Talbot interferometric fringes are extracted from the background image using image processing operations of background normalization to reduce the unevenness in the background intensity and the morphological grayscale dilation operation to eliminate the grating lines [9-10]. In the extracted Talbot interferometric fringes of these interferograms, the intensity distribution profile is plotted along the center of the grating. The center of the shifted fringe in intensity distribution profile is obtained after averaging the position of fringe. The shift between the center of two gratings determines the coplanarity. The resolution of this method is limited by the grating pitch and the capabilities of the imaging equipment.