Estonian company Difrotec developed phase-shifting common path point diffraction interferometers for measuring surface form and wavefront quality of precision optical components and systems. Due to its perfectly spherical reference wavefront diffracted from a pinhole interferometer D7 provides the higest accuracy measurement accuracy: 0.6nm @ lambda 632.8nm. D7 tests concave, convex, asphere, freeform, seagull, cylindrical or any other optics with diameters ranging from a fraction of a millimeter to ca 1000 millimeters (also suitable for ELT mirrors).
D7 measures surface forms and wavefronts of optical components and systems with sub-nanometer accuracy. D7 provides 100 times higher accuracy than common interferometers. Due to its perfectly spherical reference wavefront diffracted from a pinhole D7 is the best for testing high precision optics, especially DUV/EUV optics, space mirrors (e.g ELT mirrors), x-ray optics and other complex optical surfaces including aspheres with departure over 2 mm, and freeforms. D7 has ability to regulate contrast of fringe patterns depending on the surface reflectivity. Point-diffraction principle of D7 does not need to use expensive transmission spheres and saves time on their exchanging.