Flow visualization by means of single-exposure speckle photography

A photograph of a flow field is taken under laser illumination. If the exposure is short enough, the velocity distribution in the field will be mapped on the photograph as variations in speckle contrast. These contrast variations can be converted to intensity variations by means of a simple spatial filtering technique, to give a direct picture of the velocity distribution in the flow field. A potential application of this technique to the mapping of retinal blood flow is described.