A correction lens for laser Doppler velocimeter measurements in a cylindrical tube

Abstract

A lens was designed using analytical ray tracing techniques to correct aberrations caused by a cylindrical tube wall when measurements are made off the plane of symmetry with a dual-beam laser Doppler velocimeter. The lens keeps the probe volume stationary when the measurement axis is rotated and reduces aberrations in the image of the probe volume on the pinhole. A planoconcave cylindrical lens with a radius of curvature of 1195 mm (47 in.) was fabricated and tested for use with a 101.6 mm (4 in.) o.d., 3.2 mm (1/8 in.) thick acrylic plastic tube with air as the flow medium. Good results were obtained out to a normalized tube radius of r/R = 0.83.