Small freeform surface with large departure from best-fit sphere
Very large concave sphere
Small coated IR lens with curvature sign change
Freeform optics: tilted flat 0° to 7°
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
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 (Æ600 or Æ350 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.