Patent ID: 7318882

Claim:
A method for determining a dry line of a layer of material, and for control based on the dry line, in a Fourdrinier a paper machine, wherein said layer of material including the dry line which is to be monitored is continuously carried forward on a wire, the method comprising: emitting laser radiation at one or more narrow frequency ranges; illuminating various parts along the wire from a certain distance direction with respect to the surface of the layer of material on the wire in an area of appearance of the dry line by a beam cluster produced by said laser radiation; optically observing an electromagnetic signal transmitted by the surface of said material by diffuse reflection of said beam cluster in a direction which differs from another direction of specular reflection related to an illumination direction; repeatedly converting the observed optical signal representative of brightness level data to an electrical digital signal; and transmitting a location of the dry line on a basis of the brightness level data represented by said digital signal determined as indicative of a borderline between two parts of the surface having a changing value of said brightness level data.