An interferometer for use in aerodynamic and heat transfer studies has been designed and tested. As in a Mach-Zehnder interferometer, the light beam is large in cross section (to permit a large field of view in the test apparatus) and traverses the test section only once. Using a continuous wave laser as a source, the light beam is split and eventually recombined when small in cross-sectional area. This permits all optical components, other than two paraboloidal mirrors, to be much smaller than the field of view in the test section. The new design results not only in a less expensive unit, since most of the components are small, but also one that is much simpler to adjust than a conventional Mach-Zehnder interferometer.