In a first aspect according to the invention there is provided a sensing system 100, suitable for sensing the stage of processing of a wafer 200, said sensing system 100 comprising receiving means 110 in the form of a first photosensitive device 110a and a second photosensitive device 110b, detector 120, a comparator 130 and a control system in the form of a programmable logic controller (PLC) 140. The first photosensitive device 110a receives light from the wafer 200, while the second photosensitive device 110b receives ambient light. The light received by the first photosensitive device 110a can be incident ambient light reflected off the surface of the wafer 200, refracted light radiating through the wafer 200, filtered light radiating through the wafer 200 or translucent light radiating through the wafer 200. It is further envisaged that the received light may be filtered through filters (not shown) before being received by the photosensitive devices 110a&b. The detector 120 then generates a wafer control signal associated with the color(s) of the light received from the wafer 200. The wafer control signal produced by the detector 120 is then compared by a comparator 130. Once the comparator 130 has compared the wafer control signal to the predetermined values, this indirectly indicates the level of processing of the wafer 200. The PLC 140 then controls the further processing of the wafer 200 according to the result of the comparison.