Determination of cd and/or md variations from scanning measurements of a sheet of material

Abstract

CD variations and/or MD variations in scan measurements are determined from spectral components of power spectra of scan measurements taken using two or more scanning speeds. Dominant spectral components having the same spatial frequencies identify CD variations and dominant spectral components having the same temporal frequencies identify MD variations. Dominant spectral components are extracted from a noisy power spectrum (PS) by sorting all spectral components into an ordered PS. A first polynomial representing background noise of the ordered PS is used to set a first threshold. Spectral components of the ordered PS that exceed the first threshold are removed to form a noise PS. A second polynomial representing the noise PS is used to set a second threshold. Spectral components of the PS that exceed the second threshold are identified as dominant spectral components of the PS.

What is claimed is: 
     
         1 .- 12 . (canceled) 
     
     
         13 . A process for extracting dominant spectral components from a power spectrum of noisy measurements comprising:
 sorting all spectral components from a power spectrum in order of magnitude to form an ordered power spectrum;   representing background noise of said ordered power spectrum with a first polynomial;   setting a first threshold with respect to said first polynomial;   comparing spectral components of said ordered power spectrum to said first threshold;   removing spectral components of said power spectrum that exceed said first threshold from said ordered power spectrum to form a noise power spectrum;   representing said noise power spectrum in said power spectrum with a second polynomial;   setting a second threshold with respect to said second polynomial; and   identifying spectral components of said power spectrum that exceed said second threshold as dominant spectral components of said power spectrum.   
     
     
         14 . The process as claimed in  claim 13  wherein said first and second polynomials are low-order polynomials.