Absolute form-interferometry for aspheres and freeform surfaces (AbsoForm)

• Applicants: Dr.-Ing. Ines Fortmeier; Professor Dr. Alois Herkommer
• Subject Area: Measurement Systems
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 496703792

Aspheres or freeform surfaces are increasingly important in modern optical systems because they allow new optical designs that meet the demanding challenges of the application (medical technology, laser optics, lithography, scientific instruments and large projects such as synchrotrons). Currently, however, measurement technology limits manufacturing accuracy. The so-called "reference-free" form interferometry, in which no physical reference surface exists, brings many advantages here, but is currently still limited in absolute form accuracy (manufacturing-related deviation from the design incl. the complete spherical portion ("best-fit radius" = base curvature)). It is far more demanding than the classical reference surface-based form measurement (e.g. flatness or sphere measurement).Therefore, the goal of this project is to explore if and how absolute methods can be introduced into reference-free form interferometry. The research-based tilted-wave interferometers (TWI) of PTB and ITO are used as a test system for reference-free form interferometry. It is investigated which additional information can be obtained for an absolute form measurement and in which way this information can be obtained as inexpensively as possible. The use of multiple wavelengths (white light interferometry or multi-wavelength interferometry (MWLI)) is particularly suitable.The optimal variant found by the simulation-based investigations is realized and verified in the experiment.The achieved accuracy of the absolute form measurement is verified by means of special test objects. However, since the targeted accuracy exceeds the limits of the currently possible absolute form measurement on aspheres and freeform surfaces (unless they are almost flat), so-called locally effective reference surfaces are to be developed here. These are surfaces that include some local surface elements or points that can be measured with high accuracy by other methods. The high-precision nanomeasurement and nanopositioning machine NPMM200 available at ITO has an accuracy in the range of a few nanometers with certain sensors, if these sensors are aligned nearly perpendicular to the surface. Aspheres and freeform surfaces will be designed that have surface elements suitable for this purpose.The basic radii of curvature of the surfaces to be examined will cover a broad spectrum. Starting point for the project will be a range between 20 mm and 300 mm.

• DFG Programme: Research Grants