photon detection for this reason polaroid filters can be appended in front of both the detector fiber array and the illumination fiber array perpendicularly to avoid detection of specularly reflected photons c f fig7 an example of filters is shown with reference to fig6 the illumination and detector fibers are arranged in parallel and equidistantly distributed linearly in the lateral direction the number of optical fibres in the third set 29 and the fourth set 30 of optical fibres can be different from what is shown in the drawings and do not have to be equal experimental data from convex and concave polyacetal plastic surfaces are recorded in a first step a mathematical model of the sensor can also be used for simulating images of the surfaces analysed the detected image is compared in a second step with the recorded data and a shape associated to the recorded data that corresponds best to the detected image is selected an estimate of the shape characteristics of a surface is extractable from the images generated by the system in particular the system distinguishes perfectly accurate between convex and concave surfaces which e g is important when characterizing the tm the control apparatus 17 also comprises a control unit 31 operatively connected to other units of the control apparatus such as the signal processor 24 and a memory unit 46 experimental data or data created from the.