The split-beam optical thickness gauge (OTG) measures the height difference of two adjacent surfaces. Low-coherence light is generated by the low-coherence light source. The split-beam probe head receives the low-coherence light and splits the incoming low-coherence light into a primary beam and walk-off beam. The primary beam shines upon a first surface and is reflected back up into the split-beam probe head. The walk-off beam shines upon a second surface and is reflected back up into the split-beam probe head. Spatial separation between the primary beam and the walk-off beam ensures that each beam shines substantially on only one of the surfaces. An incorporated polarizer assures that the primary and walk-off beams interfere. The reflected light returns to the autocorrelator and is detected so that distance measurements can be determined based upon a change in the path difference between the reflected primary beam and the walk-off beam.