Calibration of numerical aperture effects in interferometric microscope objectives

The numerical aperture (N.A.) of a microscope objective can affect the measurement of surface profiles. Large N.A. objectives measure smaller heights than the actual values. An experiment to calibrate these effects on objectives with N.A.s of 0.1-0.95 is described using four traceable step height standards and a computer-controlled interferometric optical profiler utilizing phase-measurement interferometry techniques. The measured N.A. scaling factors have good agreement with a theory developed by Ingelstam. N.A. scaling factors are determined to an uncertainty of +/- 1% for N.A.s </=0.5 and +/- 2% for N.A.s >/=0.9.