Remote surface motion measurements using defocused speckle imaging

When an object with a rough surface is illuminated using a coherent laser, the scattered light rays interfere to create a three-dimensional speckle field that fills the adjacent space. If the object is displaced or tilted, the entire speckle field moves in response. Therefore, the motion of an object can be measured remotely by tracking the movements of the objective speckle pattern recorded on a lensless imaging sensor placed within the speckle field. It is demonstrated that the same image acquisition can be made using a defocused camera, where the recorded speckle pattern corresponds to the image that would be recorded by a lensless sensor placed at the camera's focal plane. In this paper, this feature is experimentally demonstrated by showing the correspondence between the captured objective and defocused speckle patterns. The effective speckle field sampling position can then be freely chosen by controlling the camera focus distance, rather than needing to move the sensor physically. Furthermore, the defocused speckle patterns are scaled by the optical system in-focus magnification ratio, enabling further tuning of measurement sensitivity. This paper presents a series of displacement and tilt measurements designed to investigate the sensitivity characteristics of defocused speckle imaging. The measurements made at various object distances up to 16 m demonstrate the method's suitability for high-sensitivity remote measurement applications. A strategy to separate linear and rotational components under multiaxial object motion is also proposed.