Patent ID: 7593106

Claim:
A method for measuring properties of a moving web in which sensor heads, each comprising at least one source for providing energy and an associated detector system for detecting energy reflected from said web to be measured, are installed on both of sides of said web to measure the same location on said web, wherein each detector system comprises a measurement detector, a reference detector, a synchronous detector and a phase detector, said method comprising: providing from said at least one source on one side of said web energy modulated at a first frequency; simultaneously providing from said at least one source on the other side of said web energy modulated at a second frequency which is different than said first frequency; using said detector system on said one side of said web to detect only energy reflected from said web modulated at said first frequency; and using said detector system on said other side of said web to detect only energy reflected from said web modulated at said second frequency; and wherein the steps of using the detector systems to detect energy reflected from the web each comprise using the phase detector to detect zero crossings of a signal from the reference detector and using the synchronous detector to detect energy reflected from the web using zero crossings and a signal from the measurement detector.