FIELD: physics.
SUBSTANCE: method comprises steps for: directing a first and a second optical beam perpendicular the extrudate and reception thereof by a first receiving surface of a first image sensor at a given first point on the opposite side of the extrudate; analysing, at given moments in time, first diffraction patterns displayed on shadow boundaries on the first receiving surface in order to determine at least one characteristic feature the surface profile, followed by storage thereof; directing a second optical beam perpendicular the extrudate onto a second receiving surface of the first or second image sensor lying in the direction of feeding the extrudate at a second point located away from the first point of the first receiving surface, on the same side as the first receiving surface; analysing second diffraction patterns in order to determine at least one characteristic feature of the surface profile; determining over which period of time the characteristic feature of the first diffraction pattern will approximately correlate with the characteristic feature of the second diffraction pattern; determining the rate of feeding the extrudate and the length of the stretched extrudate.
EFFECT: high accuracy of measurement results.
10 cl, 4 dwg
