Tilt measurement using digital speckle shear interferometry R.S. SIROHI, A.R. GANESAN, B.C. TAN Speckle shear interferometry is an established technique to measure derivatives of displacement components. It can also be used to measure the tilt of diffuse objects. Electronic detection with speckle shear interferometry makes it a real- time technique for tilt measurement. The paper describes the various shear types that could be used for this purpose. KEYWORDS: interferometry, speckle, measurement Introduction Laser speckle interferometry is an established technique for the measurement of displacement, deformation, vibration amplitude and tilt of diffuse objects . ’ 2 The technique involves observing the correlation between the speckle patterns corresponding to the two states of an object under study. The correlation fringes obtained depict the contours of constant displacement. On the other hand, linear speckle shear interferometry responds to the derivatives of the displacement. Different kinds of shear, namely lateral, radial, rotational, inversion and folding, have been incorporated in speckle shear interferometry3. It has been shown that speckle shear interferometry with a photoemulsion as a recording medium can also be used for the measurement of tilt4-“. Use of electronic detection equipment makes this technique almost a real-time technique. It may be noted that lateral shear, although responsive to slope, cannot be used to measure tilts. The other shear types however, can be used for the measurement of tilt in addition to slope measurement. It is shown that a sensitivity better than that obtainable from holographic interferometry can be realized in digital speckle phase interferometry (DSPI). Experimental set-up and procedure The schematic of the experimental arrangement is shown in Fig. 1. The object is illuminated normally by a collimated laser beam and is imaged on to the face plate of a video camera via two right-angled RSS is in the Department of Physics, Indian Institute of Technology, Madras 600 036, India. ARG and BCT are at the Institute for Advanced Studies, University of Malaya, 59 100 Kuala Lumpur, Malaysia. Received 25 February 1992. Accepted 20 July 1992. prisms A and B. The prisms are mounted on suitable stages which can provide required translation and rotation to the prisms’ as indicated in Fig. 1. Two images of the object with appropriate shear are superimposed on the camera detector surface. Any required type of shear can be introduced between the images by proper translation or rotation of the prisms. The video camera is connected to a digital image processing system which is interfaced to a host computer. The speckle pattern corresponding to the initial state of the object is stored in the frame grabber. The Cal lunoted loser beam I I TV comero Fig. 1 Schematic diagram of the experimental arrangement for the measurement of tilt using digital speckle shear interferometry 0030-3992/92/050257-05 0 1992 Buttetworth-Heinemann Ltd Optics 8 Laser Technology Vol24 No 5 1992 257