1.      Overview

Aspheric and freeform surfaces are a very challenging class of optical element. Their application has grown considerably in the last few years in areas such as imaging systems, lithography, automotive, etc. The reason for this growth is that aspheres and freeform surfaces are superior to classical spherical optics due to their additional degrees of freedom of geometry. Optical systems that employ aspheric or freeform surfaces have fewer optical elements and higher imaging quality.

The strength of Europe in optics is not the mass market, but high quality optical systems, with superior imaging quality. This means that the surface quality of the optical elements must be better than 30 nm, and this requires the accuracy of the metrology to be even better than this.


2.      Need

The use of aspheres and freeforms is growing rapidly and metrology is the limiting factor. Thus, there is an urgent need to strengthen and harmonize the metrology for optical surfaces, which result in progress in the photonics research and industry sectors.

Within the EMRP JRP IND10, an uncertainty below 100 nm was achieved for aspheres and, in some selected samples an uncertainty of 50 nm could be obtained. Nevertheless, for the metrology of high quality optical surfaces, optics manufacturers and manufacturers of optics metrology instruments or polishing machines require enhancement of the metrological capabilities in all typical cases and need to address the lack of high-accuracy traceability chains at NMIs and DIs.

The most critical and immediate need of research institutes and industry is the capability of form metrology for optical aspheres and freeforms, below the 30 nm uncertainty level. This need to develop accurate form metrology for asphere and freeform optics was strongly emphasized during the ongoing discussions conducted at the High Level Expert Meetings and workshops of the Competence Centre for Ultra-Precise Surface Manufacturing (CC UPOB, www.upob.de) and the European Optical Society.

Although the infinite norm (Min-Max (L¥)) method works for simple geometries, it still faces major challenges when used on complex geometries such as aspheres and freeform surfaces. New developments are urgently needed to realize a reference metrology chain for asphere and freeforms.

To realize reference surfaces suitable for low uncertainty traceability, these reference surfaces must be thermo-invariant to be less sensitive to thermal drift. Furthermore, a large variety of surface forms is necessary, either with rotational symmetry or without rotational symmetry, because there are several applications and the measurement range is not the same for different machines at different NMIs.


3.      Objectives

a.        Development of robust reference least-squares and Min-Max minimisation algorithms including the generation of reference data and recommendations on the reference mathematical model for aspheres and freeforms optical elements. The algorithms will allow asphere and freeform evaluation to sub-nanometre accuracy.

b.        Advanced techniques for data analysis (alignment/registration techniques, stitching algorithms, data fusion, interpolation methods and improved filtering methods) will be developed to support the experiments. These algorithms are necessary for comparison measurements to be performed by improved reference instruments on selected reference aspherical and freeform optical elements made from thermo-invariant materials. An estimation of the uncertainty of the reconstruction results will be established and applied for the determination of the uncertainty of measurement for the calibration of aspheres and freeforms.

c.        Design, manufacturing and characterisation of innovative aspherical and freeform optical reference elements, made of thermo-invariant materials to develop a reference calibration chain at European NMIs, and to provide transfer of traceability between NMIs, standardisation organizations, research laboratories and end users.

d.        Improvement of measurement capabilities of NMIs and DIs on aspherical and freeform standards for high level areal and single point scanning reference measurement systems, achieving an uncertainty of less than 30 nm. This will involve the improvement of reference metrology instruments ensuring tactile and/or optical measurements such as ultra-high precision single point and optical imaging instruments.

e.        Development of a strategy for the long-term operation of the capability developed including the take up of the technology and measurement infrastructure developed by the project. Two case studies on the application of the developed reference fitting algorithms and on the improvement of the metrology chain for innovative 3D printed precision freeform optics will be performed. This will also include the development of a strategy for offering calibration services from the established facilities especially to the European countries.


4.      Progress beyond the state of the art

a.      Robust reference algorithms and softgauges

To realize a reference metrology chain for aspheres and freeforms, robust and deterministic reference algorithms (least squares and min-max) will be developed, which have the calculation uncertainty at the sub-nanometre level. Existing approaches do not offer the determinism required to guarantee a calculation uncertainty at the sub-nanometre level. Furthermore several reference softgauges, also not existing at present, will be developed to ensure the traceability of the developed algorithms for asphere and freeform analyses at NMIs. Therefore, an ultra-precise analysis of the form errors of aspheric and freeform optical surfaces, usually calculated using a complex mathematical model will become possible at a low level of uncertainty.

To combine measurements made by different instruments, robust algorithms for data fusion, stitching, interpolation and filtering will be developed. The investigation of such approaches creates the capability of using ultra-high precision CMMs with limited working range for scanning/probing of large asphere and freeform surfaces, which will go further beyond the state-of-the-art.

b.     Thermo-invariant asphere and freeform reference standards

Innovative reference surfaces from thermo-invariant materials will be realized for the first time. Surface forms suitable for new traceability routes will be investigated and their design will be optimised for ease of manufacturability. This will lead to suitable and affordable reference surfaces for different types of aspheres and freeforms that will be useful to enhance the traceability of metrology instrumentation manufacturers and optics manufacturers to below 30 nm.

c.      High accuracy reference measurement systems

The uncertainty for form measurements will be reduced to less than 30 nm for aspheres and freeform surfaces with dimensions between 10 mm and 200 mm and slopes of up to 20°. This objective will be achieved by improvements to specific measurement instruments and statistical methods for the calibration of measurement devices, where appropriate.


5.      Results

a.      Robust reference least-squares (L2) and Min-Max (L¥) fitting algorithms

This project will deliver robust reference algorithms to generate reference softgauges for aspheres and freeforms, combining an asphere/freeform profile and both systematic and random errors. Robust reference least-squares and Min-Max fitting algorithms will be also be developed and validated on generated reference softgauges to enable the traceable and accurate characterisation of asphere and freeform surfaces. The mathematical models described in ISO 10110, Forbes or Zernike polynomials and NLLS will be investigated to select the most appropriated one for aspherical and freeform surfaces that will be used as a fitting model.

b.     Techniques for data analysis

Advanced techniques for data analysis including alignment/registration techniques, stitching algorithms, data fusion, interpolation methods, improved filtering methods, etc. will be developed to support the experiments. These tools are necessary for the comparison measurements that will be carried out using improved reference instruments on selected aspherical and freeform optical reference elements made from thermo-invariant materials.

c.      Innovative aspherical and freeform reference elements

Innovative aspherical and freeform optical reference standards made with thermo-invariant materials will be designed, manufactured and calibrated using improved tactile and optical reference instruments. These thermo-invariant standards will be made available to overcome the current lack of traceable measurement tools and methods for reliability assessment of aspheres and freeforms.

d.     Measurement capabilities of NMIs and DIs on aspherical and freeform standards

There are several ultra-high precision measuring machines at NMIs, universities and industries which allow the measurement of flat surfaces accurate at the nanometre level. These instruments are equipped with optical or tactile probes accurate at the same level. To reach a measurement uncertainty below 30 nm on asphere and freeform optical surfaces, the traceability of these machines will be investigated and improved, including the optimization of the probing systems, the calibration procedures, the alignment of the artefact along the vertical axis, etc. Furthermore, innovative coatings will be developed for optical artefacts to increase the slope range of measurement technology based on coherent and incoherent optical sensors to more than 20°, rather than the less than 10° at present.

e.      Long-term operation of the capability developed (case studies)

To ensure the take up of the technology and measurement infrastructure developed within this project, two case studies about the application of the developed reference fitting algorithms (least-squares and min-max) and the use of the improved metrology chain for innovative 3D printed precision freeform optics will be performed in close collaboration with industrial and academic partners. New calibration services based on the established facilities at NMIs and DIs will be offered to industry and research laboratories in the photonic sector to ensure the traceability chain to the SI metre definition.


6.      Impact

a.      Impact on relevant standards

For asphere and freeform surfaces, metrology, calibration chains and standardisation are still not sufficiently developed for scientific and industrial applications. This is especially true for freeform surfaces, which will be improved by the outputs of this project. The partners will have an active participation in metrological activities of key international and European length committees: BIPM-CCL (17th CCL meeting at BIPM in Paris, September 2018), EURAMET TC-Length (regular reporting at the annual meeting), CIRP (keynote and publication) and standards organisations such as ISO (ISO TC 172 “Optics and photonics”: DIN NA 027-01-02 and ISO TC 213).

b.     Impact on industrial and other user communities

The project will increase metrology research capabilities and expertise of European member countries by the provision of reliable traceability of asphere and freeform element measurements. To reach this challenging goal, a number of stakeholders will be selected from metrology institutions and industry which will provide assistance to the project via the Stakeholder Committee. The Stakeholder Committee will ensure that the results and expertise will have a strong impact on users of asphere and freeform optical elements. The consortium will disseminate knowledge and results through the project’s web page, presentations at relevant stakeholder meetings and at international conferences, input to standardisation committees and publications in trade journals. In addition, the main results, methods and developments will be easily accessible through scientific publications in open access journals and reports/guides that will be disseminated via the website.

An effective traceability route will be a key result of this project and will strengthen the capabilities for the metrology and production of asphere and freeform optical elements within the European optics and precision engineering industries. As a consequence, manufacturing and testing of accurate optical systems will progress significantly in Europe accelerating novel developments in new and existing areas of the optics sector. Furthermore, the worldwide harmonization of metrology for aspheres and freeforms will be highly fostered.

c.      Impact on the metrology and scientific communities

The significant improvement of the measurement capabilities will have immediate metrological impact through international comparison measurements between NMIs. These international comparison measurements ensure that other NMIs and recognised research laboratories in metrology will also benefit from the achieved results by strengthening their scientific knowledge of asphere and freeform metrology and unifying the international metrology methods for such surfaces.

The calibration capabilities at the NMI level will be significantly enhanced which will benefit the wider metrological and scientific communities by enabling NMIs and DIs to offer new and enhanced measurement services in asphere and freeform calibration. These developments will also enable improved R&D for nanometre level production of optical surfaces, and will open the door for many applied sciences, such as medical engineering, automotive, synchrotron techniques and defence applications.

As a result of this project a network of experts will be established that will be able to carry on the developed research. For this purpose, CC-UPOB is an excellent platform.