A Tentative Analysis of the Status of Speckle Interferometry in Experimental Mechanics

Abstract

The paths followed by a new technique before being widely accepted are generally not straightforward. Speckle interferometry (SI) certainly does not escape the rule. As far as I remember, the articles by Leendertz, [1], and Butters, [2], aroused a considerable interest in the early 70s among the holographic interferometry (HI) community. Scientists working in the field of optical metrology were astonished to discover that the direct recording of a 2-beam interference pattern between a reference and a speckle wave carries in itself the phase information of the speckle wave, without the need to reconstruct its 3D complex amplitude as in holography - a step absolutely obvious nowadays. This was the birth of SI. Though most of the first recordings were made with very high spatial resolution emulsions - the same as in holography - the new paradigm, in practice, was clearly leading the way for low spatial resolution media. Indeed, video cameras soon became a standard, and the name ESPI, for Electronic Speckle Pattern Interferometry, was coined. The choice of low spatial resolution - by no means a necessity but widely adopted for evident reasons of convenience - produced at first two opposite outcomes: a rapid acceptance of the new method, due to its simplicity and the availability of photoelectric imagers and analogue processing electronics, by someone, and, on the contrary, a rejection by others, belonging mostly to the HI circles, who didn’t accept the heavily speckled aspect of SI fringes, see Figs. 1 and 2.