Highly accurate surface reconstruction for deflectometry

Deflectometry is a simple but powerful technique to test and measure specular surfaces, where the reflection angles from a test surface reveal local slopes, and thereby, optical imperfections. The slope data can be differentiated into curvature data, enabling sensitivities in the nm range [1]; on the other hand, integrating them into shape data is much more difficult, as the measured surface must have continuous slopes and the geometry of the measurement must be extremely well characterised. Even so, deflectometric surface measurements to less than 1 μm uncertainty are rare. This is one (and possibly the only) reason why deflectometry cannot yet compete with interferometry, although it is in principle much more versatile.