Polarising filters (for example plastic films with stripes) are filters for the creation of polarised light. Natural light (as well as the light of the transmitter diodes) is unpolarised. After the light has been conducted through a polarising filter, it only remains the part of the original light that oscillates in the polarising direction of the filter. For reflections at specular surfaces, the polarisation remains the same. It is only possible that the polarising direction changes. Diffuse reflection destroys the polarisation. By choosing the right filters and arrangement, this difference can be used to eliminate the disturbing effects created by the reflecting surfaces.