Micro-lenticular array for spatial-filtering velocimetry on solid surfaces

Abstract

We present a miniaturized, low-cost optical method for detection of in-plane speckle translation. The speckles are produced by illumination of a non-specular target surface with coherent light from a vertical cavity surface emitting laser. The scattered light propagates through free space to the sensor inlet. At the inlet speckle translation appears due to both translation and rotation of the target. The sensor is based on a micro-lenticular array and a spherical lens, which implement a narrow spatial band-pass filter. The spatial filter acts on the intensity distribution of the translating speckle pattern. The presented free-space propagation design provides a sensor that in principle operates within a large range of working distances, and with a low susceptibility to variations in working distance. The relation between the precision of the sensor and the decorrelation length for speckle translation at the sensor inlet is studied. Configured as a free-space propagation sensor, and in this case operating with a low power laser source, the pitch of the lens array is required to be in the order of a few tens of micrometres. Therefore, limitations due to diffraction and aberrations due to the high numerical aperture of the lenslets have been studied.