Reference algorithms and metrology on aspherical and freeform optical lenses (FreeFORM)
Duration: June 2016 – June 2019
Team: Richard Leach, Rong Su
Aspheres and freeform surfaces are a very challenging class of optical elements. Their application has grown considerably in the last few years in imaging systems (medical, safety, automotive, energy and defence applications), astronomy, lithography, synchrotron techniques, etc. The reason is that aspheres and freeforms are superior to classical spherical optics due to their additional degrees of freedom. Optical systems that employ aspheres have fewer optical elements (leading to less loss of light, less production costs, less weight, etc.) and higher imaging quality. Optical aspheres have become commercially manufacturable principally by modern polishing techniques for optics. The metrology for aspheres is scientifically very interesting, because of the challenges of high dynamic range of the information and establishing traceability to the SI unit metre. The strength of Europe in optics is not the mass market, but high quality optical systems, e.g. with a superior imaging quality. This means that the surface quality of the optical elements used must be better than 50 nm, and the metrology has to be much more accurate.
Because information processing is a key element of modern metrology, the first step in this project will focus on the development of advanced reference algorithms for asphere and freeform evaluation to sub-nanometre accuracy. Appropriate mathematical models for aspherical and freeform elements will be investigated and a reference model will be selected. Reference data (softgauges) will be generated and will be used for the validation and certification of the reference algorithms to guarantee traceability to NMIs for accredited laboratories, standardization organizations, research laboratories and end users. The second step is to develop innovative reference aspheres and freeform optical elements made of thermo-invariant materials to act as traceable artefacts. These artefacts must guarantee the best transfer of the reference metrology chain. The reference artefacts will be calibrated with different ultra-high precision reference instruments such as single point instruments equipped with accurate optical and/or tactile probing systems or optical imaging instruments, such as the tilted-wave interferometer (TWI). The third step deals with the improvement of the reference metrology instruments such as ultra-high precision single point and optical imaging instruments. For some systems using optical point sensors, an accuracy improvement can be achieved by a functional coating of the surface to be measured. Such advanced removable surface coatings will additionally be developed within the project.
The goal of the Manufacturing Metrology Team is to develop special coatings that can be applied to an optical surface to effectively increase the slope range of optical measuring instruments. The coatings will be made from fluorescent material that ensures diffuse reflection in all angles, therefore, giving the instrument a higher synthetic aperture. The coatings can be used at the quality control phase, then simply removed by soaking in water.
a) The optical problem – reflections are not collected by the objective lens. Here α is the angle between normal incidence and the reflected beam, NA denotes the numerical aperture of the objective lens, and light is not collected if αsin-1(NA). b) The stylus problem – β , the maximum local slope, must be less than γ. c) The confocal fluorescence microscope and the measured excitation and emission spectra of rhodamine B. The filter is chosen to pass wavelengths greater than 580 nm so as to separate the excitation and emission radiation. The peak of the emission spectrum is around 600 nm with a 552 nm