angular increment in this example the trace of the light beam initially activated can remain within the field 40 of the optical receiver means or leave it if the trace of the beam initially activated remains in the field 40 of the optical receiver means the beam in question remains active and the central unit performs a new computation to determine the three dimensional geometrical definition of the new section concerned and of the bezel d as a function of the new angular position of the read head 9 on the other hand if the trace of the light beam initially active leaves the field 40 of the optical receiver means the central unit deactivates the light beam concerned and activates another light beam whose trace on the bezel d is within the field 40 of the optical receiver means the central unit chooses the new light beam to be activated this can be done as previously by means of a procedure for sequentially activating the beams f 1 to f 8 in a predetermined order until the trace of one of the beams is within the field 40 of the optical receiver means however to speed up the overall reading procedure it may be beneficial for the central unit to make this choice by detecting the direction in which the trace of the light beam initially activated progresses before leaving the field 40 of the optical receiver means and deduce therefrom assuming that the bezel is continuous the adjacent light beam whose trace will most probably enter the field 40 of the optical receiver means for example if subject to the hypothesis previously envisaged the beam initially active is the beam f 2 that is part of the series of beams emitted by the first.