An imaging method creates a two-dimensional image of a voltage distribution or a capacitance distribution across a surface of a substrate under test using an electro-optic modulator which is positioned and biased with respect to the surface of the substrate. The method involves a first coarse offsite calibrating step to compensate for nonuniformities in the light emerging from the modulator. Then, for each successive portion of the substrate over which the modulator is to detect characteristics of the substrate, the system undergoes a modulator relocating step, a modulator levelling step, a modulator gapping step, a fine onsite calibrating step, and a measuring step. An apparatus is disclosed for levelling the bottom surface of a modulator into a substantially coplanar spacial relationship with a portion of top surface of the underlying substrate being tested monitors light emerging from at least three regions on the top surface of the modulator in order to determine the relative distances of three corresponding regions of the bottom surface of the modulator from the underlying top surface of the substrate being tested. In one embodiment, three transducers are used to control the spacial orientation of the modulator. The method and apparatus see special application in detecting defects in LCD display panels.