A reverse vending machine, including: a chamber adapted to receive an object returned to the reverse vending machine; a plurality of cameras arranged around the perimeter of the chamber for viewing said object; a transparent or translucent plate arranged such that the cameras in use view the object obliquely through the transparent or translucent plate; and means adapted to couple light into the plate such that the light undergoes total internal reflection in the plate. Also, a method of detecting dirt in a reverse vending machine.