Abstract: An interferometer is provided, comprising a source, a unit under test (UUT) with at least a first surface and second surface, a reflective optic, a detector and light from the source. The light is transmitted through the unit under test and reflects off of the reflective optic, which directs the light back to the unit under test. A first portion of light is reflected off the first surface of the UUT. A second portion of light is reflected off the second surface of the UUT. The first and second portions of light are then reflected by the reflective optic and are then transmitted through the UUT. The two portions of light are incident on the detector, where the first and second portions of light coherently add and the interference pattern is detected by the detector. A method for measuring the transmitted wavefront of the UUT is also provided.