Metrology of complex-shaped surfaces such as freeform optics is tricky. Techniques such as interferometry require an optical null and suffer from retrace errors. Deflectometry, on the other hand, does not have these limitations. This paper highlights one challenge in deflectometry: the LCD screen serves as an extended light source, but only a small portion of the rays reach the camera after reflection by the specular measurement sample. The key challenge is to cover the whole sample surface with those rays. The authors were inspired by cavlectometry, which is a technique that aims to surround a measurement sample over a 2π-steradian measurement range with extended light sources. The idea is appealing, no surface slope is too steep, and no shape is too complex, and there is always a ray for a surface point reaching the camera(s). This paper proposes a rotational stage within a deflectometry system, thereby generating a series of virtual extended light-sources. Gerd Häusler’s group reported a similar approach for in-situ metrology using the c-axis of a lathe (C. Röttinger, C. Faber, M. Kurz, E. Olesch, G. Häusler, and E. Uhlmann, "Deflectometry for ultra-precision machining: measuring without rechucking,"in DGaO Proceedings (DGaO, 2011), urn:nbn:de:0287-2011-P028-0). The punchline of this story? The angular dynamic range in deflectometry is directly related to the spatial size of the light source, and the authors propose sample rotations to extend these capabilities.

---

---

 Add Comment

You must log in to add comments.