wheels 101 can roll down along sliding surfaces 2150 present on the carrier element 21 naturally the housing 100 a 100 b can also be embodied such that running wheels or guide surfaces guide the carrier element 21 at the longitudinal sides thereof particularly if grooved surfaces of the carrier element are used for example in the case of a v ribbed belt guidance by means of this grooved surface is expedient in another exemplary embodiment of the test device 1 the housing 100 only has a single reception unit here the reception unit is dimensioned and arranged such that it can record an image of the whole width of the carrier element 21 the reception unit can therefore contain a single appropriately dimensioned sensor element sensor array or a plurality of individual sensor elements arranged next to one another which are then connected appropriately the reception unit is arranged on the same side as the transmission unit 11 which illuminates the wide side of the carrier element 21 reflected light is then recorded by the reception unit analogously to as in fig4 the shown test device 1 therefore can be displaced along the carrier element 21 or be kept stationary while the carrier element 21 moves in any case the at least one transmission unit 11 and the reception units 12 d and 12 r are kept at a constant distance from the carrier element 21 this is how it is possible to make high quality image recordings and evaluate these with minimal effort test devices 1 according to the invention in the various embodiments are preferably installed in the vicinity of the drive pulley 24 and hence in a region through which the.