One can only make what one can measure, so metrology of aspherical and freeform optics is a key enabler to unlock the potential of these complex surfaces. TNO, TU/e and VSL therefore developed the NANOMEFOS measurement machine. A prototype that combines universal, large, non-contact, fast and accurate into one machine was completed in 2009. It has been in use at the TNO optical workshop ever since, enabling numerous innovative instruments for space, astronomy and lithography applications.
Three basic principles
A cylindrical coordinate measurement machine setup. This ‘giant cd-player’ setup enables universal measurement of all sorts of optics (small to large, convex to concave), and fast measurement with minimal fast moving axes.
A non-contact probe with large range captures the freeform departure of the surface, enabling high measurement speeds even for highly freeform surfaces (up to 1.5 m/s at the probe). This in turn gives a very high data point density making it especially suitable for mid-spatial frequency characterization.
A separate metrology system with a stabilized HeNe interferometer and a Silicon Carbide reference frame. This system corrects for motion and vibration errors resulting in repeatability below 1 nm rms and traceable measurement accuracy.
Benefits of the technology
- Measuring all sorts of optics without setup changes. Just load the product, program the measurement and press run.
- Iterative loops in corrective machining. The machine measures absolute form (so including the radius of curvature) and the error map can be easily exported. As a measurement only takes minutes (especially with the quick product mounting interface, iterative runs can be performed quickly.
- The high point density provides good insight in mid-spatial frequency content of a surface, which is often a problem in aspherical and freeform optics.
- The ability to measure smaller portions of a surface with super high point spacing allows to zoom in on for instance centre defects.
- Line scans with micrometer point spacing provide even more details on mid-patials, especially for diamond turned optics.
- As the machine operates like a Coordinate Measuring Machine, it can measure multiple surfaces in one part, giving not only the form error of each surface, but also the relative position.
- Powerful fitting and post-processing software for aspheres and freeforms allow for accurate calculation and export of error maps, and reduce setup times due to alignment insensitivity.
Benefits of the technology
Ø50 mm convex asphere,
0.1 mm point spacing @ 1 rev/s, 6:25 min
Ø500 mm concave sphere
3 mm point spacing @ 1 rev/s, 7 min
Ø30 mm 7⁰ titled flat (freeform measurement),
0.125 mm point spacing @ 1 rev/s, 3:45 min