OPTICAL FILTERING TECHNIQUES TO ENHANCE SPECKLE CONTRAST VARIATIONS IN SINGLE-EXPOSURE LASER SPECKLE PHOTOGRAPHY.

Single-exposure speckle photography can be used to map velocity variations in a flow field. The velocity variations are coded as variations in speckle contrast, since the recorded speckle pattern will be blurred to an extent that will depend on the velocity of the scatterers producing the speckle. Contrast variations are difficult to detect visually, but can be converted to intensity variations by means of high-pass optical filtering. The design of optimal filters for this task is discussed. A two-step filtering procedure to code the contrast variations as colour variations is also described.