A method and system for calibrating a three dimensional printing system (2) includes a specialized sensor (48). The three dimensional printing system forms a three dimensional article of manufacture through a layer-by-layer process. Layers are formed by the operation of a light engine (10) selectively curing photocure resin onto a face of the three dimensional article of manufacture. The sensor includes a photodetector (52) overlaid by an optical element (54, 55, 56). The optical element simulates a "dense portion" of an optical path between the light engine and the face of the three dimensional article of manufacture being formed. The "dense portion" of the optical path includes a layer of photocure resin that is disposed between the light engine and the face of the three dimensional article of manufacture.