Two shifters are used to move an optical receiver board with two PIN photodiodes (one for measuring the LEDs and the other equipped with a filter for the much brighter VCSELs) in the longitudinal (X) and transverse (Y) directions in steps of 0.1mm and 0.04mm, respectively. The LEDs or VCSELs are mounted on temperature controlled supports called blocks, emitting the light downwards to the PIN diodes. The shifter moving in the longitudinal (X) direction is shown both at the leftmost (shaded) and at the rightmost (dashed lines) position.