Quantitative Analysis of Static and Vibratory Displacements by Holographic Processing of Projected Fringes

Abstract

The use of projected fringes to measure object shapes and deformations is part of the general field of metrology using structured light, where any of a wide variety of patterns is projected onto an object in order to measure its shape and deformation. A very comprehensive review of this field has been provided by Geng.[1] The purpose of this paper is to present a variation of this technique that grew out of the development of a real-time, image-plane, digital holography system in the late 1980s.[2] The initial system was called PCHolo32 and it has been upgraded in the last decade to a newer system called HoloFringe300K. Digital holography allows measurement of static and vibratory displacements of objects, but it is limited to small displacements, typically only a few micrometers. The phase stepping routine used by PCHolo32 and HoloFringe300K is applicable, however, to any sinusoidally varying irradiation pattern so it is possible to illuminate an object with a sinusoidal interference pattern from a Michelson interferometer and step the phase of that pattern. If the illumination is at an angle to the object surface (typically 45 deg) and the observing camera essentially normal to the surface, out of plane movement of the surface creates a lateral shift of the observed fringe pattern that is proportional to displacement. Because the observed pattern does not involve the laser speckles, this technique can be used to compare the shape of one surface to another. Furthermore, the pseudo phase-step vibration measurement technique of the holographic processor can be used with fringe projection to measure large scale vibrations.