High point-density mid-spatial frequency measurement on polished and ground surfaces

Next to polished surfaces, the optical probe of the NMF machines can also measure ground surfaces. This brings the versatility, speed and accuracy to surfaces that can normally only be measured after a first polishing step. The high measurement speed associated with the motion platform and the optical probe, translate in the ability to acquire very high-density data which is ideal for characterizing mid-spatial frequency errors in aspherical and freeform surfaces.
Figure (LEFT) shows the mid-spatial content of a  Ø200 mm polished aspherical mirror, with Z36 Zernike coefficients subtracted. The pointspacing is 0.1 mm, it has 2k x 2k points (3.3M datapoints) and takes 18 minutes. Although only a few nm rms, the grinding residue radiating outwards from the center is still visible, as is the center defect. The machine can be used to zoom without loss of resolution, contrary to an interferometer. In Figure (MIDDLE) a subsequent measurement is shown where the measurement area was reduced to the central  Ø40 mm. This measurement was done with 0.025 mm pointspacing, yields 1.6k x 1.6k points (2M datapoints) and takes less than 10 minutes. Figure (RIGHT) shows a similar measurement with 0.1 mm pointspacing on a  Ø180 mm concave sphere, but this time in ground condition with Ra = 1-2 um. With Z36 removed, it shows residual grinding cusps of 36 nm rms. Being able to measure this without having to first polish the surface is obviously a valuable advantage for process optimization.

DUI optics - MidspatialsPolished
DUI optics - MidspatialsPolishedCenter
DUI optics - MidspatialsRough

Small freeform surface with large departure from best-fit sphere

This example shows a toroidal surface with 40 mm radius in one direction and 42 mm in the other giving a few mm of non-rotational symmetry. Moreover, it is convex of large surface form error with mid-spatial frequency content on top, and has small dimple markers on its surface. In this application the NMF machines are the tool of choice. The parts are already mounted so they fit right into the standard  Ø25 mm chuck. Programming is just as easy so within a few minutes the measurement is running. Even for a pointspacing as fine as 0.05 mm, the measurement is completed within 12 minutes. The NMF OS software automatically processes the data and fits the design to the obtained form data. Within 15 minutes from starting the error map with all the form and mid-spatial information is on the screen.

Very large concave sphere

The NMF600 S measurement machine can handle parts up to Ø600 x 150 mm, weighing up to 70 kg. Below example shows a 50 kg, Ø500 mm concave spherical R1.4 m test blank. Setup time was only a few minutes as accurate alignment is not required. Measurements were done at 0.25 rev/s, 0.5 rev/s and 1 rev/s, to investigate the influence of speed. A relatively coarse pointspacing of 3 mm was chosen, resulting in 7 minute measurement time. The resulting form error was around 45 nm rms, and repeated to < 1 nm rms.

Small coated IR lens with curvature sign change

Although the NMF600 capable of measuring very large optics, small optics are also no problem as they fundamentally require the same functionality. This application shows a particularly challenging Ø10 mm lens, as it is strongly aspherical with a sign change in the curvature and it is coated for infrared. The video shows how the probe is always perpendicular to the surface, and how the curvature sign change is followed. The measurement has a lateral pointspacing of 0.01 mm, giving close to 1 million datapoints. It was measured at 1 rev/s with a total measurement time of around 7 minutes. The result shows the error map, including the ring structures that are typical for diamond turning.

Freeform optics: tilted flat 0° to 7°

Traceble calibrated freeform reference artefacts are still hardly available, especially in larger sizes and with large departure from the best-fit asphere (NMF platforms can handle up to 5 mm PV, and local slopes up to 7°). Because the machine is so versatile, it can measure all sorts of commonly available reference artefacts like optical flats, precision spheres and straightedges. A good way to create a traceable freeform is by measuring a  l/20 tilted flat. Below example shows such measurements, where all results between 0° and 7° are consistently below a few nm rms, and virtually free of center defects, thanks to extensive calibration of tilt dependent errors and proprietary offset self-calibration algorithms.

Freeform optics: de-centred sphere

Besides the previous tilted flat example, a traceable freeform can also be created by measuring a spherical surface centred and de-centred on the table. The below example shows a correspondence within 1 nm rms between centred and 2 mm de-centred measurement.

Infrared optics: diamond turned lens with coating

As the confocal probe of the NMF machines has its operating wavelength in the visible spectrum it is also capable of measuring infrared optics. IR optics with anti-reflective coating are also no problem as shown in this example. Note that the coating does need to be homogeneous for optimal measurement results. Also clearly visible for this diamond turned mirror is the ring-shaped mid-spatials and central spike that are quite common to this type of machining.

Rectangular aperture: cylinder lens

The optical probe has an internal servo actuator with 5 mm range to focus the probe onto the surface. In this way, the confocal sensor is utilized as a null-sensor which gives highest accuracy. It also means that the probe cannot go off the edge of the surface, otherwise it will lose its signal. The NMF OS software comprises a trajectory generator that calculates the required machine trajectory to do this, including non-circular and off-axis apertures. In the below application, a cylindrical surface with a rectangular aperture is measured. In the corners, the spindle will rotate back and forth, while in the centre it will make continuous revolutions. The trajectory is also visualized in the software, as shown in the top left of the figure for this case. 

Off-axis surfaces: measuring off-axis asphere as a freeform centred on the table

Off-axis surfaces that are within the measurable diameter of the machine (Ø600 or Ø350 mm) can be measured at their off-axis position, i.e. de-centred on the table. If the off-axis distance is however larger than that, such a surface can also be measured centred on the spindle. An aspherical off-axis optic will then become a freeform centred surface. In this optional NMF OS module, all coordinate transformations and error fitting is done automatically. All the user has to do is entering the parameters of the off-axis formula and the desired surface shift as stated on the drawing of the part. This example shows a 600 x 300 mm lightweighted zerodur concave mirror substrate. The radius of curvature is so large that a high interferometric optical test tower with CGH would otherwise be needed to test it. A measurement, including setup, alignment and removal of the part, takes about one hour and then the error map is ready for further iterative machining. This allows for fast cycle time.