DETERMINATION OF CD AND MD VARIATIONS FROM SCANNING MEASUREMENTS OF A SHEET OF MATERIAL

摘要

The system of the present application determines cross-machine direction (CD) variations and/or machine direction (MD) variations within scan measurements of material being measured based on spectral components of power spectra of scan measurements taken using two or more scanning speeds. Dominant spectral components having the same spatial frequencies are used to identify CD variations and dominant spectral components having the same temporal frequencies are used to identify MD variations. Dominant spectral components (328) can be extracted from a power spectrum of noisy measurements by sorting all spectral components of a power spectrum to form a first ordered power spectrum. Background noise (322) of the ordered power spectrum is represented by a first polynomial with a first threshold being set with respect to the first polynomial. Spectral components of the ordered power that exceed the first threshold are removed from the ordered power spectrum to form a noise power spectrum. The noise power spectrum (322) in the power spectrum is represented by a second polynomial (324) and a second threshold (326) is set with respect to the second polynomial (324). Spectral components of the power spectrum that exceed the second threshold are identified as dominant spectral components (328) of the power spectrum.