Holographic particle-streak velocimetry.

We present a way to measure the positions and instantaneous velocities of micrometer-scale colloidal spheres using a single holographic snapshot obtained through in-line holographic video microscopy. This method builds on previous quantitative analyses of colloidal holograms by accounting for blurring that occurs as a sphere moves during the camera's exposure time. The angular variance of a blurred hologram's radial intensity profile yields both the magnitude and direction of a sphere's in-plane velocity. At sufficiently low speeds, the same hologram also can be used to characterize other properties, such as the sphere's radius and refractive index.