Optical and tactile metrology for absolute form characterisation

Short Name: Form, Project Number: IND10
I N D10

Measuring optical curved surfaces: Characterising free-form 3D surfaces

The design and manufacture of optical components is dependent on the ability to measure the full 3D form of precision curved or flat optical surfaces. These components are used in high-performance photonics and precision engineering applications, as well as consumer products, such as cameras and spectacles, and scientific research. The performance of optical components is based on the shape and quality of the surface and, while modern manufacturing techniques enable a wide range of shapes to be produced, consistent high-quality manufacturing is dependent on the ability to measure and characterise the optical components produced. Two types of measurement tools are used: imaging and single-point scanning – both of which have advantages, as well as disadvantages that limit manufacturing capability.


This project improved methods to accurately measure flat, aspheric and free-form optical surfaces. The project:

  • Improved the spatial resolution and measurement uncertainties for deflectometer measurements of flatness – 1 nm peak-to-valley uncertainty and 0.1 µm resolution was attained. A new cost-effective flatness measurement technique based on capacitive distance sensors was also developed.
  • Characterised and improved the performance of a novel prototype optical interferometry instrument (a tilted-wave interferometer), validating its performance as an alternative method to assess the form of optical components.
  • Improved the performance of tactile and optical single-point techniques for curved surfaces. Critical parameters that influence the onset of distortion during tactile probe testing of soft materials were identified and methods for reducing the sources of errors in ultra-high precision single-point optical form measuring instruments were validated.
  • Developed a range of reference standards with different optical characteristics to enable referencing of coordinate systems used to calibrate and characterise surface form measuring instruments and link industrial measurements to primary standards at NMIs.


The project team worked closely with an industry body – High Level Expert Meetings of the Competence Centre for Ultra-Precise Surface Manufacturing – to ensure the project met industrial needs and shared its results. The outputs of the project are already in use. A manufacturer is using the project developments to improve and commercialise a prototype tilted-wave interferometer, and predicts a significant market among manufacturers of optical components. In addition, a set of reference diamonds were characterised for the European diamond trading industry and improved measurement procedures are being incorporated into a new international standard for diamond classification.   


Coordinator: Michael Schulz (PTB)


For more information, please contact the EURAMET Management Support Unit:

Phone: +44 20 8943 6666

E-mail: empir.msu@euramet.org


EMPIR project 15SIB01 FreeFORM builds on this work.

  • EMRP Industry theme impact case studies
Project website
Reconstruction of freeform surfaces for metrology

Journal of Physics: Conference Series 483 (2014) 012003


Proceedings of 58th Ilmenau Scientific Colloqium

Tilted Wave Interferometer - Improved Measurement Uncertainty

Proceedings of 58th Ilmenau Scientific Colloqium

Comparison of tactile and chromatic confocal measurements of aspherical lenses for form metrology

International Journal of Precision Engineering and Manufacturing

Measurement of aspheres and free-form surfaces with the Tilted-Wave-Interferometer

Proceedings to 7th International Workshop on Advanced Optical Imaging and Metrology

High accuracy flatness metrology within the European Metrology Research Program

Nuclear Instruments and Methods in Physics Research A

The Tilted Wave Interferometer (TWI): A quick and flexible approach to measure aspheric and freeform surfaces

Proceedings to 28th Annual Meeting of the American Society for Precision Engineering

Measurement of aspheres and free-form surfaces in a non-null test interferometer: Reconstruction of high-frequency errors

Proceedings of SPIE 8788: Optical Measurement Systems for Industrial Inspection VIII

Trinano N100 3D Measurements with Nanometer Repeatability and Effects of Probe-Surface Interaction

Proceedings of the 27th Annual Meeting of the American Society for Precision Engineering

3D Measurement and Characterization of Ultra-precision Aspheric Surfaces

This paper will be published in Procedia CIRP

Other Participants
Fraunhofer-Gesellschaft zur Foerderung der angewandten Forschung e.V. (Germany)
IBS Precision Engineering bv (Netherlands)
Nederlandse Organisatie voor Toegepast Natuurwetenschappelijk Onderzoek TNO (Netherlands)
Technische Universitaet Ilmenau (Germany)
XPRESS Precision Engineering B.V. (Netherlands)