A technique is described which converts in real time the fringe pattern resulting from two interfering beams into a wavefront map. The interferometer incorporates, as part of a servo system, a piezoelectrically driven mirror that is capable of applying both a known optical phase offset and a periodic optical phase modulation to one beam. The ac signal detected by a single photodiode is processed to generate an error signal for the servo system and a signal proportional to the optical phase difference between the two beams. Three instruments built on this principle will be discussed. The instruments are capable of detecting phase differences with a precision approaching A/100 and may be used for measuring surface topography (roughness, irregularity) or transparent phase objects such as biological samples, microballoons and gradient index materials.