A method for nondestructively and unambiguously measuring the thickness of a bulk dielectric material, or measuring the depth of a feature in a bulk dielectric material, or both; said method comprising the steps of: (a) generating microwaves from a microwave source, wherein the microwaves have substantially constant frequency; (b) directing a first portion of the generated microwaves to the material to produce a standing wave; wherein the standing wave is a function of the wavelength of the microwaves, of the distance from the source to the nearest surface of the material, and of the distance from the source to farthest surface of the material or the distance from the source to a feature within the material; (c) mixing the reflected microwaves, in each of at least two different detectors, with a second portion of the generated microwaves to produce an interference signal for each of the detectors; wherein the detectors are spatially displaced from one another; wherein the interference signal is a function of the thickness of the material, or a function of the depth of a feature within the material, or both; and wherein there is a phase difference between the different detectors at the frequency of the microwaves as a consequence of the spatial displacement between the detectors; (d) determining, for each of one or more locations on or in the material, a point in a phase plane space whose coordinates correspond to the magnitude and sign of the interference signals that are produced at each of the detectors for each of the one or more locations; and (e) correlating, for each of the one or more locations on or in the material, the point that is determined in the phase plane space with a unique thickness for the material at each location, or with a unique depth for a feature within the material, or both. (a) generating microwaves from a microwave source, wherein the microwaves have substantially constant frequency; (b) directing a first portion of the generated microwaves to the material to produce a standing wave; wherein the standing wave is a function of the wavelength of the microwaves, of the distance from the source to the nearest surface of the material, and of the distance from the source to farthest surface of the material or the distance from the source to a feature within the material; (c) mixing the reflected microwaves, in each of at least two different detectors, with a second portion of the generated microwaves to produce an interference signal for each of the detectors; wherein the detectors are spatially displaced from one another; wherein the interference signal is a function of the thickness of the material, or a function of the depth of a feature within the material, or both; and wherein there is a phase difference between the different detectors at the frequency of the microwaves as a consequence of the spatial displacement between the detectors; (d) determining, for each of one or more locations on or in the material, a point in a phase plane space whose coordinates correspond to the magnitude and sign of the interference signals that are produced at each of the detectors for each of the one or more locations; and (e) correlating, for each of the one or more locations on or in the material, the point that is determined in the phase plane space with a unique thickness for the material at each location, or with a unique depth for a feature within the material, or both.