Digital postage franking with coherent light velocimetry

Abstract From the Canadian patent

A postage meter (franking machine) uses a digital print head such as an ink jet or thermal transfer or dot-matrix print head for which it is necessary to know the velocity of the mail piece passing by the print head. Two collimated monochromatic beams strike the mail piece one at an angle leading the mail piece velocity and the other at an angle lagging the mail piece velocity. The beams converge yielding a sensing region filled with a diffraction pattern. The mail piece assumed to be rough at a scale that is appropriate for the velocity measurement moves at some velocity. A detector detects light intensity (photon flux) at a small region within the sensing region and the intensity signal …