We propose a three dimensional position measurement method employing planar photo detectors to calibrate a Spatial Augmented Reality system of unknown geometry. In Spatial Augmented Reality, projectors overlay images onto an object in the physical environment. For this purpose, the alignment of the images and physical objects is required. Traditional camera based 3D position tracking systems, such as multi-camera motion capture systems, detect the positions of optical markers in two-dimensional image plane of each camera device, so those systems require multiple camera devices at known locations to obtain 3D position of the markers. We introduce a detection method of 3D position of a planar photo detector by projecting gradient patterns. The main contribution of our method is to realize an alignment of the projected images with the physical objects and measuring the geometry of the objects simultaneously for Spatial Augmented Reality applications.