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Speckle Methods When looking at the laser light scattered from a rough surface, one sees a granular pattern as in Figure 8.2. This so-called speckle pattern can be regarded as a multiple wave interference pattern with random individual phases. In the years following the advent of the laser, this pattern was considered a mere nuisance, especially in holography (and it still is!). But from the beginning of 1970 there were several reports from experiments in which speckle was exploited as a measuring tool. In this chapter the basic principles of the different techniques of speckle metrology will be described | Optical Metrology. Kjell J. Gasvik Copyright 2002 John Wiley Sons Ltd. ISBN 0-470-84300-4 8 Speckle Methods 8.1 INTRODUCTION When looking at the laser light scattered from a rough surface one sees a granular pattern as in Figure 8.2. This so-called speckle pattern can be regarded as a multiple wave interference pattern with random individual phases. In the years following the advent of the laser this pattern was considered a mere nuisance especially in holography and it still is . But from the beginning of 1970 there were several reports from experiments in which speckle was exploited as a measuring tool. In this chapter the basic principles of the different techniques of speckle metrology will be described. As a spin-off from laser speckle methods based on similar principles using incoherent light have emerged. This white-light speckle photography is included in the final section of the chapter. 8.2 THE SPECKLE EFFECT In Figure 8.1 light is incident on and scattered from a rough surface of height variations greater than the wavelength k of the light. As is shown in the figure light is scattered in all directions. These scattered waves interfere and form an interference pattern consisting of dark and bright spots or speckles which are randomly distributed in space. In white light illumination this effect is scarcely observable owing to lack of spatial and temporal coherence see Section 3.3 . Applying laser light however gives the scattered light a characteristic granular appearance as shown in the image of a speckle pattern in Figure 8.2. It is easily realized that the light field at a specific point in a speckle pattern must be the sum of a large number N of components representing the light from all points on the scattering surface. The complex amplitude at point in a speckle pattern can therefore be written as u N EUk TN EUke 8-1 NN k 1 N k 1 By assuming that 1 the amplitude and phase of each component are statistically independent and also independent of the .