A method is described for measuring flow velocity in a microscopic region by means of a differential-type laser velocimeter with a transmission grating. The principle of this velocimeter is described on the basis of spatial filtering of the transmission grating. The filtering characteristics are theoretically analyzed using a power spectral density function. The velocimeter is used to measure flow velocities and their spatial distributions in a small (700-µm diam) glass tube. The theoretical and experimental results show that this differential-type velocimeter is more suitable and practical than the conventional one for measuring the flow velocity in a small probe.

© 1985 Optical Society of America

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