A technique is described which will convert in real time the irradiance pattern resulting from two interferometry beams into phase information. The interferometer is modified to introduce a known temporal phase modulation to one beam and then to detect minima in the A.C. signal. Three instruments built on this principle will be discussed. The instruments are capable of detecting phase differences approaching λ/100 and may be used for measuring optical interference (roughness, irregularity) or transparent plase objects such as biological samples, microballoons and gradient index materials.