Patent ID: 8569679

Claim:
A system comprising: a plurality of photo detectors, each of the plurality of photo detectors configured to generate signals proportional to incident light; an optical barrier configured to partially obstruct reflected light from reaching the plurality of photo detectors to produce a spatially dependent reflectance pattern that is dependent on a position of an object relative to a substrate; and a control circuit configured to determine a position of the object during a reflectance measurement cycle using each of the plurality of photo detectors, the control circuit to: calibrate each of the plurality of photo detectors to ambient light conditions; measure ambient plus reflected light using each of the plurality of photo detectors; correct the ambient plus reflected light based on the ambient light conditions to determine the reflected light received by each of the plurality of photo detectors; and detect the position of the object based on a ratio of the reflected light received by at least two of the plurality of photo detectors.