emc considerations consequently the expense is not increased thereby and copper layer that is present in any case is used for optical shielding the copper surface 107 is also closed over a very large area on the underside 105 of the printed circuit board as well so that this too functions as a u201c mirror u201d for ir radiation in the area between compensator 103 and the photodiode receiver diode 104 consequently a kind of optical waveguide is created inside printed circuit board 101 in other words printed circuit board 101 itself forms an optical waveguide in the configuration shown here compensation led 103 does not protrude into printed circuit board 101 but lies flush against the pcb underside 105 as for example in the case of a sfh4257r type diode by osram used in tests and radiates into the fr4 material of printed circuit board 101 through a correspondingly exposed area in lower copper coating 107 the compensation thus does not require any additional installation space above printed circuit board 101 which in turn enables very thin coverings small optical emitter basis coupling u2014 high sensitivity range among other advantages moreover photodiode receiver led 104 may also be shielded optically from parasitic infiltration by scattered light if necessary by.