Method and device for the measurement of linear speeds without contact and without marking

Claim 1

An optical apparatus for continuously measuring the linear speed of relative travel between said measuring apparatus and a generally planar, elongated object of great length over a zone of travel in front of said apparatus, characterized by: a single source of laser radiation, an optical separator system for dividing a laser beam from said source to form two distinct beams, optical means for each beam for focussing the two beams at two zones (21,22) on said object (11) spaced in the direction of relative travel across a gap between the apparatus and the passing object and for returning a portion of radiation reflected from said zones onto a single optical detector, means for processing signals reflected from the two zones illuminated and received at the detector by the application of an autocorrelation function to the sum of the signals to determine the relative linear speed of the object, and means for displaying or recording the measured speed, wherein the axes of the focussed and reflected beams of the optical means are coincident.

Claim 5

An apparatus as defined in one of claims 1, 2 or 3, wherein the optical means includes a pair of prisms for elongating the path of an emergent beam in order to equalize the paths of two emergent beams.