Method for determining the position and/or displacement of a mobile element with respect to a fixed frame, includes using a fixed light source emitting a light beam, arranging the source with respect to the mobile element and a sensor to induce an interaction between the beam and sensor, using a concave mirror, integral in movement with the mobile element, for reflecting the beam in direction to the sensor, arranging on the path of the beam a fixed optical mask which presents a two dimensional regular pattern interlaced with an absolute code, detecting and processing the image casted by the mask on the sensor, computing the displacement value of the image on the sensor and using the computed displacement value for computing and providing the position and/or the displacement in at least one direction of the mobile element in dependence of the image's displacement.