Patent ID: 7323085

Claim:
A method for determining the basis weight of a first ply of a fibrous web which covers a second ply of the fibrous web, comprising the steps of: positioning an electromagnetic transmitter and an electromagnetic receiver proximate to a side of the first ply; radiating the fibrous web by way of said electromagnetic transmitter with at least one measuring wavelength and at least one reference wavelength, the second ply having different optical characteristics at said measuring wavelength than at said reference wavelength, said radiating step including radiating the fibrous web in at least one of a measuring wavelength range and a reference wavelength range, said measuring wavelength range being between 570 nm and 690 nm; receiving, by said receiver, a measuring signal which is returned at said measuring wavelength and a reference signal that is returned at said reference wavelength; and determining the basis weight of the first ply by utilizing a comparison of said measuring signal and said reference signal.