Absolute optical surface measurement with deflectometry
2012
Deflectometry utilises the deformation and displacement of a sample pattern after reflection from a test surface to infer
the surface slopes. Differentiation of the measurement data leads to a curvature map, which is very useful for surface
quality checks with sensitivity down to the nanometre range. Integration of the data allows reconstruction of the absolute
surface shape, but the procedure is very error-prone because systematic errors may add up to large shape deviations. In
addition, there are infinitely many combinations for slope and object distance that satisfy a given observation. One
solution for this ambiguity is to include information on the object’s distance. It must be known very accurately. Two
laser pointers can be used for positioning the object, and we also show how a confocal chromatic distance sensor can be
used to define a reference point on a smooth surface from which the integration can be started.
The used integration algorithm works without symmetry constraints and is therefore suitable for free-form surfaces as
well. Unlike null testing, deflectometry also determines radius of curvature (ROC) or focal lengths as a direct result of
the 3D surface reconstruction. This is shown by the example of a 200 mm diameter telescope mirror, whose ROC
measurements by coordinate measurement machine and deflectometry coincide to within 0.27 mm (or a sag error of
1.3μm). By the example of a diamond-turned off-axis parabolic mirror, we demonstrate that the figure measurement
uncertainty comes close to a well-calibrated Fizeau interferometer.
Keywords:
- Correction
- Source
- Cite
- Save
- Machine Reading By IdeaReader
2
References
16
Citations
NaN
KQI