A noninvasive testing system using a method of testing a device under test by providing a beam of light from a light source having a first wavelength, and in a first beam instance imposing the beam of light on a test device when the test device has a first state of refractive indexes, and in a second beam instance imposing the beam of light on the test device when the test device has a second state of refractive indexes, in both instances the beam of light being imposed on the test device over a spatial region within the test device substantially greater than the first wavelength. Data resulting from the interference of the first beam instance and the second beam instance within the device under test is obtained representative of the voltages within the region. The first state of refractive indexes is at a first voltage potential, and the second state of refractive indexes is at a second voltage potential different from the first voltage potential.