Volume 64, number 3 OPTICS COMMUNICATIONS 1 November 1987 NARROW' SLIT APERTURE WHITE LIGHT SPECKLE PHOTOGRAPHY FOR NON DESTRUCTIVE TESTING N. KRISHNA MOHAN, C. JOENATHAN and R.S. SIROHI Engineering Design Center, Indian lnsitute of Technology, Madras 600 036. India Received 22 May 1987 We propose a simple technique of whit light speckle photography using a narrow slit aperture in front of an imaging lens. A detailed theoretical and experimental analysis for the fringe formation and its use for non destructive testing is presented. I. Introduction White light speckle methods have generated con- siderable interest in recent years for the measure- ment of in-plane displacements, deformation and contouring of three dimensional objects [ 1-4 ]. The advantage of this technique over the coherent spec- kle technique is that the decorrelation effects due to the speckle movement are low and hence large defor- mation of the specimen is possible. Moreover, as the artificial speckles are embedded on the object, they faithfully follow the displacement and deformation of the object. Forno [5,6] has employed high reso- lution moir6 photography for studying thermal strain at elevated temperatures and strain in transition joints. His system consists of a camera lens with modified aperture mask to record small surface deformations in metal pipe sections at temperatures upto 600 ° C. Chiang and Bailangadi [ 7 ] introduced a white light speckle method whereby a random speckle pattern is projected onto a specimen surface at an angle using a white light illumination to obtain out-of-plane deflection of a plate. The researches of Chiang and Bailangadi [ 7] and Forno [2] are of identical nature except that former projects the ran- dom speckles at an angle ¢, where as in the latter the observation is made at an angle to the normal of the object. It has been shown that the sensitivity of out- of-plane displacement measurement depends on the angle of observation [ 2,4,7 ]. In this paper, we propose a simple technique of white light speckle photography using a narrow slip aperture in front of the imaging lens. A general anal- ysis of the formation of deflection contour fringes and application of white light speckles for non destructive testing are given. It is shown that the speckle displacement depends on (i) out-of-plane displacement, (ii) slope and (iii) direction of obser- vation. By recording two specklegrams at normal and oblique directions with respect to the normal on the objet, both slope and out-of-plane displacement can be separated out. 2. Theory Fig. 1 (a) shows the recording setup when the object is illuminated by an ordinary halogen lamp. The object is first coated with white paint and then with retro reflective liquid. When such a painted surface is illuminated by a polychromatic beam, these beads in the retroreflective paint behave like individual point sources and reflect the light back along approx- imately the same direction. A camera is used for imaging the object onto a holographic plate placed at the image plane. A narrow slit aperture as shown in fig. 1 (b) is mounted in front of the camera lens. The object is focussed onto the image plane with the camera lens fully open as described in an earlier paper by us [4]. Fig. 2(a) shows the deformed and undeformed states of an object under study. As the out-of-plane 0 030-4018/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division) 229