Phase Shifting Technique in Laser Speckle Image Processing

A simple technique of speckle photography has been applied to measure small changes/deformation of the surface of laser scattering materials. Low-cost commercial charge coupled device photo camera provides the images of Laser speckle pattern with the beam splitter arrangement. A speckle pattern has been taken with the system at rest and then second image captured after the deformation was made in the surface of the material. By simple subtraction of the digital pictures a fringe pattern obtained called as specklegram; it gives information about modification of the position of surface of the material. Keywords— Laser speckle; subtraction of speckle images; Phase shift; fringe pattern; CCD Camera; image processing; deformation. INTRODUCTION Quick measurement of surface shape and deformation of mechanical parts of various materials is the ardent need of industries. Experimental methods in solid mechanics highly depend on surface displacement measurements. The conventional instruments cannot be used for those of soft materials and complex shape. Scanning method of mechanical probes takes much time hence it is not suited for quick and in-process measurement. Interferometry techniques are free from these issues have been mainly applied to optically smooth surfaces of materials. Speckle metrology, a simple and widely used non-destructive evolution tool in metrology [1] is used in surface deformation analysis under mechanical or thermal loading conditions and determination of in plane translation [2]. Speckle interferometry is used to measure the deformation of micro electromechanical systems [3, 4].Other applications range from medical studies on bone dynamics to quality inspection of various products [5] and the measurement of the refractive index of a liquid in a cell was reported [6]. The most important ones are based on liquid penetrant, ultrasound, magnetic particle, eddy current, acoustic emission, radiology, active thermography and optical methods [7].When an optically rough surface is illuminated with a coherent beam, a high contrast granular structure, known as speckle pattern is formed in the space is known as ̳objective speckle pattern‘ as shown in Fig.1. It can also be observed at the image plane of a lens and it is then referred as ̳subjective speckle pattern‘ as shown in Fig.2. The scattering regions are statistically independent and uniformly distributed between –π and π. The speckles in the pattern undergo both positional and intensity changes when the object is deformed. The randomly coded pattern that carries the information about the object deformation provided to develop a wide range of methods, which can be classified into three broad categories: speckle photography, speckle interferometry and speckle shear interferometry. Speckle photography includes all those techniques where positional changes of the speckles are monitored, whereas speckle interferometry includes methods that are based on the measurement of phase changes and hence intensity changes. If instead of phase change, we measure its gradient, the technique falls into the category of speckle shear interferometry. All these techniques can be performed using digital/electronic detection using a CCD and imaging processing system. Illumination of a rough surface with coherent light produces a random intensity distribution in front of the surface, called speckle pattern [8]. Because the speckle pattern follows the movement of the scattering surface the speckle can be used for displacement/deformation measurement [9]. Beam division and combination can be analysis on the basis of either by amplitude (Michelson, Fizeau, Mach-Zehnder and Jamin) or by wave front division (Young and Fresnelbiprism) methods. SPECKLE INTERFEROMETRY The optical setup for speckle interferometry is based on the Michelson interferometer [10]. The pattern that results by imaging a rough surface with a lens is itself a speckle field. The minimum size ρs of the image speckles is related to the optical system f-number F and the magnification M, and is given by: ρs =1.2(1+M) λF ... (1) Where λ is the wavelength of the laser, and ρs is the radius of the Airy disc that is formed for the given optical imaging configuration. The resultant intensity of each point of the object before deformation is given by: Ibefore = Iobj + Iref +2√Iobj√Iref cos(φ0) ... (2) where, Iobj and Iref are the local intensities of the object and reference beams respectively and φ0 is the unknown, and random, initial phase distribution of the speckle pattern at that point. International Journal of Engineering Research and General Science Volume 2, Issue 6, October-November, 2014 ISSN 2091-273