An oblique incidence interferometer has been devised for the testing of unpolished surfaces and non-optical surfaces such as lapped metal plates, fine ground surfaces, etc. The interferometer makes use of a wedged plate as a beam splitter. Large surfaces, at one stretch, can be examined at angles of incidence around 80°. It is also possible to vary the sensitivity of the interferometer to some extent by the readjustment of the wedge plates. It is also possible to use the interferometer to test the straightness of cylindrical rods, tapered rods etc.