This paper presents a new hardware and software calibration technology to improve the process of automatic multiple projector calibration using photo detectors to support Spatial Augmented Reality. This research builds on existing calibration methods that employ point-based photo detectors by advancing the hardware with a larger surface area photo detector to locate sub-pixel position with structured light. This data is used to expand the Gray-coding algorithm with an additional step that improves the measurement. The novel hardware allows a sub-pixel position to be calculated and leveraged to improve alignment of multiple-projector environments. The results show the new approach improves position measurement from pixel accuracy using a point photo detector with Gray-code by an order of magnitude providing sub-pixel accuracy by leveraging the planar photo detector and additional algorithm steps. Refereed/Peer-reviewed