An apparatus that can measure a space between a first surface and a second surface such as the air bearing between a slider and a disk. The apparatus may include a light source that can reflect a light beam from the slider and the disk. By way of example, the light beam can be reflected off of an Al2O3 cap of a slider. A birefringent element such as a Savart plate may split the reflected light beam into an ordinary beam and an extraordinary beam. The ordinary and extraordinary beams may combine to form an interference pattern that is detected by a photodetector. A controller receives data from the photodetector. The apparatus may have a mechanism which can vary a phase between the ordinary and extraordinary beams so that the controller can calculate a phase value &phgr;. The controller then computes the space from the phase value &phgr;. The variation in phase between the beams may be created by tilting the birefringent element, or moving the reflected light beam directed into the birefringent element. The apparatus can determine the space without performing a retract calibration routine that moves the slider as is required in the prior art.