Patent ID: 7283256

Claim:
A method for measuring the thickness of an object having first and second surfaces, comprising the steps of: a) providing first and second opposed light beams; b) providing first and second focus error determining means for said first and second opposed light beams; c) focusing said first and second opposed light beams at first and second focal points in space along an optic axis; d) determining the distance between said first and second focal points in space; e) inserting said object into said first and second opposed light beams such that said beams impinge on said first and second surfaces, respectively; f) passing first and second beams reflected from said first and second surfaces, respectively, through respective pinhole apertures; g) impinging said first and second reflected beams on respective first and second photodiode sensors; h) varying focus on said first and second surfaces by varying respective focal points along said optic axis to determine respective first and second maximum photodiode sensor outputs and corresponding distances of said respective focal points at said maxima; and g) applying said first and second distances to said distance determined between said first and second focal points in space to obtain the thickness of said object between said first and second surfaces.