Holographic Interferometry and Speckle Metrology: a Review of the Present State

The review paper will describe the present state of holographic and speckle techniques as applied to industrial measurement. The basic principles of both techniques will be outlined with special attention given to their advantages and limitations. Current developments in the field will then be described including the t~stlng of rotating automobile tyres by holographic interferometry and a heterodyne technique to galn an insight into noise generation mechanism. Methods for real-time holographic and speckle recording to facilitate the use of the techniques in an industrial situation will be discussed. Introduction and historical background Holographic interferometry and speckle techniques are widely used; the major applications :~~i~~~ i~eO~~!~~v:~ ~~ ~~~~~~!~gi~e~~:Z~~:;0~!:~a~:m~~~4 ~~b~~~~~~~e~~r::~ :~~.~~!~~~nl was a mile stone in the development of the application of the laser in metrology. Five years later J.Leendertz 2 , at the ICO meeting in Reading, presented a paper on interferometry with diffusely reflecting objects by means of laser speckles. In 1968 Burch and Tokarski' published a paper in Optica Acta on multiple exposures of speckle patterns with equal displacements between, which led to the development of speckle photography techniques. In the field of holographic interferometry. double exposure , multiple and time average exposure techniques were introduced, as well as beam modulation and stroboscopic exposures ~,s. In addition, fringe localizations together with fringe pattern analyses in three dimensional space have been investigated. By 1970, published material was available predicting the object motion accurately from the fringe pattern. Methods were established to extract vectorial object displacement from the fringes, their parallax and their localization. The difficulty with these fringe analyses, however, was the amount of calculation require~ in their applications. The search for Simpler techniques of describing fringes in holographic interferometry initiated various studies on fringe analysis. The theories developed so far simplify the analysis and make it easier for the engineer to understand and apply it. very often, however, they are too difficult to be of practical value for many engineering problems, although they can be very useful for special applications. In recent years,matrix methods and tensor calculus have been introduced for fringe analysis, leading to a number of strain analysis techniques. Phase detection has been significantly improved to one part in 1000 by heterodyne interferometry'. Now at last, holographic interferometry is beginning to show its true potential in structural metrology . The field of speckle interferometry and photography has passed through an interesting development phase. The first of the techniques were described as speckle interferometry and are analogous to classical interferometry in that they use interference between two or more randomly speckled fields z • Detection of this random interference is based upon the cycliC repetition of the combined speckle pattern with every lw phase change between the fields, leading to interferometric sensitivity. By contrast, speckle photography has extended the range of displacement measurements. It is based on the displacement of speckles