IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 57, NO. 6, JUNE 2008 1261 Noncontact 2-D In-Plane Speckle Velocimeter Lorenzo Scalise and Andrea Di Donato Abstract—In this paper, the authors present a noncontact 2-D speckle velocimeter, which is realized by means of a compact setup and based on an improved binary correlation algorithm for image processing. Starting from the results obtained in our previous work, where we demonstrated the feasibility of the method for the assessment of the surface velocity for one direction, we have extended the study to a 2-D velocity and displacement sensor by overcoming the previously encountered limits. In addition, the configuration is further simplified by introducing a low-coherence source and a simpler electronic circuit. Sensor performance is compared with that measured using a triangulation laser sensor. The main advantages of the proposed sensor, as compared with those of the traditional velocimeters, are the simplicity of the opti- cal setup, the easily controlled device, and the small dimension of the sensor. The velocity of frame acquisition (15.6 frames/s) is the principal limiting factor of the sensor bandwidth (BW =3.5 Hz). Such a noncontact optical sensor is suitable for small 2-D in-plane vibration measurements and allows one to overcome the limits of the classical Doppler velocimeters affected by speckle noise in this range of displacement. Index Terms—Binary correlation, laser Doppler, noncontact velocity sensor, rough surface, speckle. I. I NTRODUCTION T HE ANALYSIS of digital speckle images for the measure- ment of surface displacements and deformations repre- sents an interesting research area in the field of optical sensors. Particularly, the application of laser speckle techniques is in- creasing in specific branches, such as noncontact measurement techniques, 2-D maps of the strain, and high-temperature appli- cations. In these fields, the speckle sensors show some advan- tages with respect to electrical and mechanical devices [2], [3]. Recently, the analysis of speckle images has been applied in the field of nanotechnologies and in the thermal strain, surface roughness, and flow field measurements [3]–[6]. In addition, there are some operative conditions in which the traditional vi- bration measurement systems can be affected by noise and, consequently, by a reduction of the signal-to-noise ratio (SNR); this is typical when a rough surface is illuminated by coherent laser radiation [9]. Speckle is known as a cause of noise for the laser Doppler vibrometers (LDVs) since it induces lowering of the SNR and a signal dropout condition on the LDV output [10], [11]. The study of the speckle pattern, which is generated by a coherent source, for sensing aim and velocimetry applications has been proposed in the past by various authors. Different tech- niques, such as interference and speckle correlation methods, have been applied to process speckle images and to measure Manuscript received July 10, 2006; revised December 11, 2007. L. Scalise is with the Dipartimento di Meccanica, Università Politecnica delle Marche, 60131 Ancona, Italy. A. Di Donato is with the Dipartimento di Elettromagnetismo e Bioingeg- neria, Università Politecnica delle Marche, 60131 Ancona, Italy. Digital Object Identifier 10.1109/TIM.2007.915456 Fig. 1. Generation of the speckle phenomena on a reference plane. rigid body properties [6], [7], [12]–[14]. The obtained results have demonstrated that the correlation techniques are less sensi- tive with respect to the interference methods, but they are often easier to implement. The main purpose of this paper, which fol- lows from the results obtained in [1], is the realization of a com- pact and simple bicomponent optical sensor that can be applied to the measurement of displacements and velocities of rough surfaces. After a short presentation of the principle in Section II, the sensor configuration is presented in Section III, and the motion of a rigid body is evaluated by using a semiconductor laser and an image acquisition system (a charge-coupled device (CCD) sensor). To process the speckle images, an improved binary correlation technique was applied. This algorithm allows one to reduce the consumption time with respect to a full- correlation method. The bandwidth and the sensor sensitivity were derived by the analysis of the static and dynamic be- haviors. The speckle velocimeter is also applied to evaluate the displacement and velocity of a target oscillating along two normal directions, and the results are compared with a reference optical sensor. II. THEORY A speckle pattern is generated in the space when a rough surface is illuminated by laser radiation. The image that can be observed on a reference plane is characterized by a speckled intensity distribution (Fig. 1). Speckle size is a relevant parameter in image processing, and it is strongly influenced by the geometry of the system. Speckle analysis for the vibration measurement requires that the mean grain dimension is stable and clear and larger than the interpixel distance of the CCD sensor to avoid aliasing error and decorrelation phenomena in the peak of the cross-correlation function [15] (Fig. 2). The variation of the mean speckle size δ depends on the laser wavelength, the diameter of the incident spot 2w, and the 0018-9456/$25.00 © 2008 IEEE