The projection diagram.
We describe a diagram, named the projection diagram (PD), that can be used for the interpretation of fringe projection operations in a similar way as the Holodiagram is used in holography and other branches in optics. It is obtained as a Moiré pattern between two spoke targets that mimic central projections, the same as those in a projection and observation system or two projection systems. N. Abramson [Academic, London (1981)] has already proposed its use in systems with two observation points (i.e., stereoscopic observation). By using this PD, several interesting features dealing with fringe projection are highlighted. Information on the sensitivity vector and the geometry of the contouring surfaces can be straightforwardly obtained in a graphical way. The effect of defocusing can also be included in the diagram.