Augmenting photometric stereo with coaxial illumination

The authors propose a new photometric stereo (PS) method that augments a conventional four-light approach with coaxial illumination. The new method is based on a four-light, leave-one-out approach: the authors argue that coaxial lighting improves robustness to shadowing and specular highlights. The authors also specify a method to detect pixels that are shadowed or that do not conform to Lambert´s law. The approach is tested on cylinders of various materials. The authors found that for dielectrics the method gave reliable results for gradients in the range -2 to 2. The results lead them to conclude that coaxial illumination makes PS more robust to shadow and specularity.