Selectivity of spatial filtering velocimetry of objective speckles for measuring out-of-plane motion

We probe the dynamics of objective laser speckles as the axial distance between the object and the observation plane changes. With the purpose of measuring out-of-plane motion in real time, we apply optical spatial filtering velocimetry to the speckle dynamics. To achieve this, a rotationally symmetric spatial filter is designed. The spatial filter converts the speckle dynamics into a photocurrent with a quasi-sinusoidal response to the out-of-plane motion. Our contribution presents the technology and discusses the selectivity of the spatial filter. Specifically, we discuss how the selectivity of the spatial filter with regard to radial speckle motion is influenced by a concurrent in-plane speckle motion. The spatial filter is emulated with a CCD camera, and is tested on speckle acquisitions obtained from a controlled set-up. Experiments with the emulated filters illustrate performance and potential applications of the technology.