Developments In Real-Time Speckle Metrology Employing Spatial Light Modulation

Abstract

ABSTRACT Liquid crystal televisions are being evaluated increasingly as spatial light modulators in a wide variety of optical image processing applications. Relevant performance characteristics of these devices and their application to the implementation of optical logic gates is discussed. A description is given of a novel technique which employs liquid crystal televisions to demonstrate real-time speckle metrology. Some modifications to the TV which may increase the potential of these devices as spatial light modulators are briefly considered. 1. INTRODUCTION Laser Speckle Photography (LSP) is an established technique for measuring the in-plane surface displacement of an object.^ It uses double exposure photography to record the speckle images observed when the test target surface is illuminated with coherent light. The fundamental principle of LSP is that if the surface is displaced between the exposures, then the resulting photographic transparency consists of two superimposed images modulated by identical but laterally displaced speckle patterns. The displacement field occurring between the two exposures is usually mapped out by probing the developed photograph with a narrow laser beam. The diffraction pattern formed in the focal plane of a convex lens (the Fourier Transform lens) consists of Young's fringes which modulate the diffraction halo of the speckle. From the fringe spacing and direction both the magnitude and direction of the surface motion can be determined.The analysis of a double exposure photograph usually involves taking measurements from numerous fringe patterns. Whilst sophisticated fringe analysis algorithms are being developed and leading to important new measurement techniques,2 the Fourier processing is computationally intensive and time consuming. An alternative approach is to use optical methods for the image processing rather than conventional digital computational techniques, the advantage being the gain in processing speed which arises mainly due to the ability of optical processors to perform parallel operations. However, exploitation of the optical methods requires the availability of suitable Spatial Light Modulators (SLMs) to provide the interface between the required input signal and the parallel optical processing system.Various types of SLM have been developed which operate under quite different principles.^ Current systems include the use of electrooptic crystals, thermoplastics, liquid crystal displays and magnetooptics. Well-known commercial examples which are based on the latter methods are the Hughes Liquid Crystal Light Valve4 and the Semetex Sight-Mod.5 In the area of LSP and Particle Image Velocimetry (PIV) analysis, optical autocorrelators have been developed which are based on Bismuth Silicon Oxide (BSO) crystals.6Since 1985 there has been some considerable interest in the use of modified, pocket-size liquid crystal televisions (LCTVs) as SLMs. A key feature of these devices is that they are low-cost. Whilst their performance in terms of optical quality, spatial resolution and pixel contrast is quite limited in comparison with commercial SLMs, they have proven to be invaluable for demonstrating the feasibility of several new and important image processing techniques.7'%This report covers some of the important characteristics of the LCTV and discusses how several such devices may be employed to implement optical logic gates which can be used in a number of image processing applications including real-time LSP.