2002 Volume 2 Issue 3 Pages 153-160
Laser-speckle is a random granular pattern appearing in the light scattered from a laser-illuminated diffuse surface or in its image. When the roughness structure is finer than the size of the incident spot or that of resolution of the imaging system, the size distribution of speckles is independent of the roughness and depends only on the optical system. The pattern exhibits displacement accompanied by decorrelation as a result of surface displacement or deformation. It can be thus used as a distinct marking for deformation measurement, whose size can be adjusted by optical systems to match the resolution of image detectors such as CCD. After the appearance and the minimum size of speckle are explained for various surface roughness and optical systems, speckle displacement and decorrelation due to surface deformation are discussed with physical backgrounds. Applications to measurement of surface deformation are then described. The first category is speckle interferometry that delivers contours of surface displacement by speckle multiplication. The second one consisting of speckle photography and speckle correlation detects speckle displacement by optical Fourrier transformation of specklegrams and correlation analysis of vidco signals. The speckle correlation methnod has been installed to encoders and strain gauges.
