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. NMF products were developed based on the NANOMEFOS technology from TNO, which combines features of being universal, large, non-contact, fast and accurate into one machine. The design of the motion system and metrology system together with the non-contact optical probe enables fast and highly accurate 3D form measurement.
A cylindrical coordinate measurement machine setup
This ‘giant cd-player’ setup enables one of the key characteristics of NMF products, being universal. It can measure all sorts of optics (from flat to aspherical and freeform, from convex to concave etc). The product to be measured is mounted on an air-bearing spindle, which is rotating continuously at for instance 1 rev/s. A non-contact confocal probe is mounted on a rotation axis (Ψ-axis) which positions it perpendicular to the surface to be measured or the best-fit rotational symmetric surface in case of freeform surfaces. The probe can be moved in radial (R) and vertical direction (Z) with the R and Z-stage, respectively.
Fig. 1 Coordinate measurement machine (CMM) concept
A non-contact probe
The optical probe is based on confocal principle. Its high NA combined with special dual-stage design enables fast measurement of any rotation symmetric parts such as sphere, asphere as well as for non symmetric freeform surfaces. Optical probes generally only have a focal depth of a few micrometers when nanometer resolution is required. Keeping the probe in focus by actuating the R and Z-stage would require large accelerations of these heavy stages, leading to undesirable dynamics. To avoid this, a dual stage servo probe as shown in Fig. 2 is implemented. In this way, the stages can be kept stationary which reducing the dynamically moving mass from tens of kg for the stages to only about 50 g of the probe objective lens. This improves system dynamics and effectively increases the range of the optical probe from a few micrometers to a few mm range. Furthermore, the local slope deviation from the best-fit asphere is captured by the high NA of the objective. Consequently, it enables the high speed measurement of the surface profile of freeform surfaces with a departure of +/-2.5 mm and a local slope deviation within +/- 7 deg from best-fit asphere for example, as shown in Fig. 3.
Fig. 2 Dual stage servo probe with high NA objective, enabling the fast measurement of freefrom optics
A separate metrology system with a stabilized HeNe interferometer and a Silicon Carbide reference frame
The motion system positions the probe relative to the product. A patented short metrology loop in the plane of motion of the probe is obtained by directly measuring the probe position interferometrically relative to the metrology frame. Mechanical and thermal simulations resulted in Silicon Carbide as the preferred material for this metrology frame. The error motion of the spindle is measured relative to this frame with capacitive sensors. As a result, this system can correct for motion and vibration errors and result in repeatability below 1 nm rms and traceable measurement accuracy.