Single image geometry inspection using inverse endoscopic fringe projection

Abstract

Fringe projection is an important technology for the measurement of free form elements in several application fields. It can be applied to measure geometry elements smaller than one millimeter. In combination with deviation analysis algorithms, errors in fabrication lines can be found promptly to minimize rejections. However, some fields cannot be covered by the classical fringe projection approach. Due to shadowing, filigree form elements on narrow or internal carrier geometries cannot be captured. To overcome this limitation, a fiberscopic micro fringe projection sensor was developed [1]. The new device is capable of resolutions of less than 15 m with uncertainties of about 35 m in a workspace of 3x3x3 mm. Using standard phase measurement techniques, such as Gray-code and cos-patterns, measurement times of over a second are too long for in-situ operation. The following work will introduce an approach of applying a new single image measuring method to the fiberscopic system, based on inverse fringe projection [2]. The fiberscopic fringe projection system employs a laser light source in combination with a digital micro-mirror device (DMD) to generate fringe patterns. Fiber optical image bundles (FOIB) are used as well as gradient-index lenses to project these patterns on the specimen. This advanced optical system creates high demands on the pattern generation algorithms to generate exact inverse patterns for arbitrary CAD-modelled geometries. Approaches of optical simulations of the complex beam path and the drawbacks of the limited resolutions of the FOIBs are discussed. Early results of inverse pattern simulations using a ray tracing approach of a pinhole system model are presented.