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).

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