Traditional optical flying-height testers use only the normal-incidence reflectivity of the interface between the read–write slider and a glass disk surrogate. We propose a tester that fully analyzes the complex amplitude reflectivity of the interface, including the polarization-dependent complex phase. The new approach is more accurate and repeatable and has no loss of precision at zero f lying height. Further, the same instrument directly measures the complex index of refraction for the slider material in situ, obviating the need for a separate metrology step with an ellipsometer. ã 1996 Optical Society of America