A method of testing a device under test includes providing a beam of coherent light from a light source having a first wavelength, and imposing the beam of light on a test device over a spatial region within the test device substantially greater than the first wavelength, wherein the test device has a first state of birefringence. A second beam of light is imposed on the test device over a spatial region within the test device substantially greater than the first wavelength, wherein the test device has a second state of birefringence. Data representative of the interference of the first beam and the second beam within the bulk of the device under test is obtained representative of the voltages within the region