Projected fringe profilometry for metal surfaces

Abstract

ABSTRACT A non-scanning method to describe the 3D profile of a metal surface is proposed. A fringe pa ttern is posited in front of the metal surface. A virtual image of the fringe pattern is formed behind the metal surface. Fringes on the virtual image are deformed by topography of th e metal object. Thus, phase of the deformed frin ges is desirable to retrieve the profile of the metal surface. Keywords: fringe analysis, fringe projection, project ed fringe profilometry, optical inspection 1. INTRODUCTION Fringe projection is a well-known technique for the 3D profile measurement [1-10]. In addition to its full-field measurement capability, projected fringe profilometry is also notable for the non-contact nature, real-time inspection, and high sampling density. It uses a fringe pattern to illuminate the inspected surface and then records the fringes projected on the surface from a different vi ewpoint. Phase of the fringes contains the depth information and therefore are analyzed to reconstruct the 3D shape. Fringe projection still encounters some challenges. One of the difficulties is the inspection of specular materials. It identify the surface of a di ffusive object very well, but it fails to describe the profile of a specular object, such as a meta l surface. Only a part of the reflected light can be detected by the image sensor array. The others cannot be collected by the image sensor array, resulting in lost of the data points. On the other hand, the light wave incident to the image sensor system generally possesses a high intensity, leadin g to an image with a saturation grayscale value. In this paper, a fringe projection technique for metal surfaces is proposed. A fringe pattern is posited in front of the metal surface. A virtual image of the fringe pattern is formed behind the metal surface. Fringes on the virtual image are deformed by topography of the metal object. Thus, phase of the deformed fringes is desirable to retrieve the profile of the metal surface. To ensure that the virtual image can be observed through the entire metal surface, the size of the fringe pattern is much larger than the inspect ed object. The measurement setup is simple and robotic, and the computation cost is low. Its one-shot measurement property also makes it possible to inspect dynamic objects.