Patent ID: 6917428

Claim:
A method for determining a refractive index difference between two samples, comprising: illuminating a first sample at an oblique incidence angle with a polarized incident light beam, said first sample reflecting at least some of said incident light beam to form a reflected light beam; modulating at a modulation frequency the polarization of at least one of said incident light beam and said reflected light beam; nulling said reflected light beam by interacting said incident light beam or said reflected light beam with a phase shifter and adjusting a phase difference between an s-polarized component and a p-polarized component of said reflected light beam, wherein said adjustment minimizes an odd modulation frequency harmonic of said reflected light beam intensity; without further adjustment of said phase difference, illuminating a second sample at said oblique incidence angle with said incident light beam, said second sample, reflecting at least some of said incident light beam to form a second reflected light beam; and determining a magnitude of an odd modulation frequency harmonic of said second reflected light beam intensity, wherein said magnitude corresponds to a refractive index difference between said first and said second samples.