The components of a scatterplate interferometer are mounted compactly within a rugged housing which is adjustable along Cartesian coordinates so that it can be aligned relative to the optical component under test. The illumination section of the instrument receives light by way of a fiber bundle attached to an external light source. This light is imaged by a lens system onto a spot mirror on a diagonal window and the reflected light is transmitted through a scatterplate and imaged onto the optical component under test. The scattered return light from the scatterplate is collimated and directed back through the diagonal window and imaged onto an exit pupil plane behind the window. The unscattered return light is observed by the mirror so that an eye in the plane can view the interference fringes formed at the scatterplate without seeing the bright spot at the center of the plate caused by unscattered light. To facilitate aligning the instrument, the diagonal window can be replaced by a beam splitter and the optical train modified so that the operator can view the scatterplate directly.
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