The Shack cube interferometer is a modification of the laser-based Fizeau that can be used to test the surface figure of concave surfaces. The advantage of this interferometer is that only the curved surface on the Shack cube beamsplitter needs to be of high quality since every other optic is common path. In order to test lenses or convex surfaces, additional high-quality optics are required. This interferometer is lower cost than a Twyman-Green or regular Fizeau, but not as flexible. A pellicle can be used to help match the beam intensities.
