In an embodiment a method includes placing a wafer on a receptacle comprising a chuck base, wherein a light port for emitting light from a source of light is an opening located in a surface of the chuck base, and wherein the light port is located underneath the wafer, shining the light from the light port at an edge of the wafer so that light passes by the edge of the wafer and processing the wafer on the receptacle based on the light that passed by the edge of the wafer and that is received by a light sensitive element.