Abstract: A 3-D LDV system for measuring three velocity components by using a He-Ne laser has been presented. Four incident beams were arranged to be an isosceles triangle with a right angle and focused by a single front lens. Such a beam arrangement has been examined as a most efficient form of the light energy utilization. It is also well suited to measure the velocity profiles close to a wall even in the situations in existing two different layers which the incident beams have to penetrate through, In order to sep...