EMRP-IND10: Optical and tactile metrology for absolute form characterisation

• Forschungsstelle: METAS
• Projektnummer: F-5111.30007
• Projekttitel Englisch: EMRP-IND10: Optical and tactile metrology for absolute form characterisation

Schlüsselwörter (Englisch)

Aspherical lenses, aspheres, form metrology, micro-coordinate metrology

Kurzbeschreibung (Englisch)

Aspherical lenses allow smaller, lighter and simpler optical devices. Currently, they are used in many mass products such as digital cameras, mobile phones, projectors, and so on. In high-tech applications such as ophthalmology, endoscopy, lithography, astronomy, microscopy and particle physics the main interest is to achieve a significant reduction of the optical aberration.
A key issue for these applications is the accuracy of the surface form, which can only be further developed if the corresponding metrology is available.
Using the earlier developed micro-coordinate measuring machine, METAS has already now the possibility for ultra-precise measurements on small parts. For aspheres the measurement procedures need to be adapted and the measurement uncertainty calculation based on Monte-Carlo simulation has to be developed.
This project strengthens the European industries and METAS can offer new calibration services.

This project is part of the European Metrology Research Programme (EMRP, http://www.euramet.org/index.php?id=emrp); it is partly funded by the European Union on the basis of Decision No 912/2009/EC.

Projektziele (Englisch)

Extended calibration services in the field of micro-coordinate metrology by means of validated procedures to determine the complex lens parameters of asperical lenses. Measurement uncertainty below a few tens of nanometers.

Abstract (Englisch)

During the past decade, METAS has developed an ultra-precise micro-coordinate measuring machine (µCMM). Thanks to its design that fulfills the Abbe principle, this machine can reach an impressive accuracy of only a few tens of nanometres. This ultra-precision renders it very attractive for measuring optical components, especially for measuring components with high slopes where classical optical measuring techniques clearly show their limits.

Driven by the increasing demand and miniaturization of video imaging devices, the manufacturing of lenses with aspherical profiles or even with a truly free-form shape is now a common task. Measuring those lenses can no longer be limited to characterizing the root mean square (RMS), peak-to-valley, and local form deviation. Optical designers are interested in the values of lens design parameters with their respective uncertainty. These parameters are obtained by fitting the measured surface to a parametric representation of the lens. Because of the many combined influences from the measurement strategy, the applied fitting procedure and the different sensitivities of the different lens parameters, an analytic estimation of the measurement uncertainty for each fitted parameter is almost impossible. Analytic estimation of the measurement uncertainty is only possible for simple geometrical shapes like spheres, cylinders, or planes. The application of a Monte Carlo method is thus required to determine the measurement uncertainty for such complex measurement tasks.

A set of new commands and functions were implemented into the measurement software of our µCMM in order to support the measurement and fitting of aspheric surfaces. Based on a first physical measurement of the lens, a newly implemented virtual μ-CMM can then be used to repeat this measurement many hundred times virtually delivering thereby true insight to probability distributions and correlations between different lens parameters. Thanks to the realistic virtual model of our µ-CMM, reliable measurement uncertainties for each parameter can be delivered.

To asses the accuracy of the measurement, a measurement comparison of aspherical lenses was carried out among a number of European metrology institutes as well as several universities and a company. Two artefacts were circulated, a small polymer coated aspherical lens, with a clear aperture of 11.74 mm and a larger one, with a clear aperture of 28 mm but with larger form error. The measurements on both lenses generally showed very good agreement between the different instruments used. The differences in the Root Mean Square (RMS) values are mostly within 14 nm whereas the Peak Valley (PV) values exhibit more dispersion since this measurement is very sensitive to dust particles. In general the comparison was very successful demonstrating good comparability between the different measurement techniques within some 10s of nanometers which was the aim of the project.

Umsetzung und Anwendungen (Englisch)

METAS is now able to offer new calibration services for aspherical lenses. So far METAS is the only NMI capable of delivering also uncertainty values for the fitted model parameters of an aspherical lens. The results were published and presented at various conferences (see below).

Publikationen / Ergebnisse (Englisch)

A. Küng, F. Meli, A. Nicolet and R. Thalmann, "Application of a virtual coordinate measuring machine for measurement uncertainty estimation of aspherical lens parameters", Measurement Science and Technology, Vol.25, No. 9, 094011, August 2014.

Küng A., Nicolet A., Meli F., Thalmann R., "Calibration and uncertainty evaluation of aspherical lens parameters using a virtual CMM", Proc. 5th Internat. Conf. ASPEN, Taipei, Taiwan, 12-15 November 2013, best paper award.

A. Küng, F. Meli, and A. Nicolet, "Virtual CMM method applied to aspherical lens parameters calibration", 13th International Conference of the European Society for precision engineering and nanotechnology (Euspen'13), O2.4, pp.83V1, Berlin, D, May 29, 2013.

R.H. Bergmans, H.J. Nieuwenkamp, G.J.P. Kok, G. Blobel, H. Nouira, A. Kung, M. Baas, M. Tevoert, G. Baer & S. Stuerwald, “Comparison of asphere measurements by tactile and optical metrological instruments”, submitted to Measurement Science and Technology, December 2014.