The present invention relates to a reverse vending machine (10), comprising: a chamber (16) adapted to receive an object (18) returned to the reverse vending machine; a plurality of cameras (20) arranged around the perimeter (22) of the chamber for viewing said object; a transparent or translucent plate (28) arranged such that the cameras in use view the object obliquely through the transparent or translucent plate; and means (34; 60,62; 60,62'; 60,70; 60') adapted to couple light (36) into the plate such that the light undergoes total internal reflection in the plate. The present invention also relates to a method of detecting dirt in a reverse vending machine.