T he work presented in the previous chapters was concluded in 1988 [93, 99, 100]. As one of the pioneering, physics-based com puter vision system s to exploit optical properties of m aterials, it touches on several topics th a t have since evolved into exciting research areas. Physics-based research had also been pursued before 1988. Especially the sem inal early work on shape from shading and photom etric stereo by Horn, Ikeuchi and W oodham [66, 69, 74, 76, 209] has provided the solid foundations to analyze the challenging problem s in physics-based vision in a coherent and focussed m anner.