Patent ID: 7280232

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; d) determining the distance between said first and second focal points in space to provide a reference thickness; 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) determining first and second net focus error signals from reflections of said first and second beams from said first and second surfaces, respectively; and g) applying said first and second net focus error signals to said reference thickness to obtain said thickness of said object between said first and second surfaces, wherein at least one of said first and second focus error determining means includes a photo cell having four phototransistor sensor quadrants, and wherein a net focus error signal (FES) for said photo cell is defined in terms of strength of respective quadrant signals A, B, C, and D by the algorithm FES=[(A+B)−(C+D)]/[A+B+C+D].