A heterodyne phase-determining interferometer comprising a Smartt point diffraction interferometer (PDI) 10 in which the pinhole plate 22 is replaced by a half-wave, partially transmitting plate 22' with a pinhole 20 therein. The output beams 26 and 24 from the pinhole 20 are propagated through a frequency shifter 12 which includes a quarter-wave plate 28 whose axis is at 45.degree. to the polarization axes of the two beams 26 and 24 coming from the PDI 10, a half-wave plate 30 rotating at an angular frequency of .omega., and a linear polarizer which orients the polarization vectors of the two beams in the same direction along the propagation axis. The output of the frequency shifter 12 is a moving interference pattern consisting of alternate light and dark lines. This pattern is projected upon a phase-measuring means 14 comprising an array of photodetectors 34, 36 connected to a plurality of phase-to-voltage converters 38. There is one reference photodetector 34, the rest being test photodetectors. The reference photodetector 34 is connected to all phase-to-voltage converters 38, but each test photodetector 36 is connected to a different phase-to-voltage converter 38. The output of each converter 38 is the phase difference between the light at the point viewed by its associated test photodetector 36 and the light at the point viewed by the reference photodetector 34.