Abstract: The detector comprises an optoelectronic sensing unit (3) and a transparent rule (2) having two parallel graduation tracks (4, 5). One track (5) has incremental graduation of constant step and the other track (4) has pseudo-random graduation forming binary coding bits constituting a continuous sequence of words which are different from each other, the width of a bit being a function of the step of the incremental graduation. The incremental track (5) is read through a sensing graticule (9) by photodetectors (8) which are 90 electrical degrees out of phase in order to permit an interpolation in the step of said track. The track (4) with pseudo-random graduation is read by a CCD detector (11) through an image-enlarging lens (13). The signals generated are processed by circuits (15a, 15b) which assure the combining of the information taken from the two tracks of the rule.