Patent ID: 6215556
Filing Date: 2001-04-10
Classification: G01B

Abstract:
A process for measuring the thickness (e) and thickness variation (.DELTA.e) between opposite surfaces of a transparent material with refractive index (n), comprising the steps of:focusing a light beam with a modulated optical frequency on the transparent material so that the beam reflects off the surfaces of the transparent material to create an interference signal;determining the number of oscillations per modulation period of the interference signal;determining the phase shift (.DELTA..phi.) of the interference signal; andcalculating the path difference (.delta.) between the two beams and the thickness (e) of the transparent material, andcalculating the local thickness variations of the transparent material, wherein calculating the local thickness variation comprises:obtaining the interference signal on a detector;obtaining the ratio of the interference signal to the modulation of the intensity;band-pass filtering the ratio;determining the extrema of the resultant signal for the measurement at time k;determining the time between the corresponding extrema of two successive measurements (k and k+1);calculating the ratio of the preceding times to the corresponding periods and multiplying it by 2 .pi. to obtain the phase shift (.DELTA..phi.); andcalculating the thickness variation by the formula; ##EQU25##where .lambda..sub.0 is the wavelength of the light beam without modulation.