Object tilt - a source of systematic error in transmission deflectometry

Phase measuring Deflectometry (PMD) utilises the deflection of rays impinging at an object, to acquire information about the local gradient of an object surface (in reflection) or the local gradient of the optical path length (in transmission). PMD in transmission displays an interesting difficulty: in order to get the correct wedge angle from the deflected rays, we need to know the local tilt of the object, which, in general, cannot be determined. In other words: a planar glass window will appear as a wedge, if it is measured in a tilted orientation. Although the systematic error introduced by a tilt is commonly small, it severely disturbs the ultra-precise measurement of planar objects (car window glasses) or lenses. We will discuss the origin of the systematic error and present a measuring scheme to overcome the problem.