An optimized design and a complete inspection of the surfaces of optics are crucial for modern optical systems, because even the smallest imperfections lead to stray light and thus to a loss of contrast and a lower light yield. The Fraunhofer Institute for Applied Optics and Precision Engineering IOF in Jena is developing stray light measurement methods that can detect unwanted stray light components.
No surface is free of imperfections; even crystals that come very close to an ideal solid-state image have imperfections. In order to find the right compromises when designing an optical system, detailed specifications are needed, such as what degree of imperfections is acceptable and how these values are influenced by a necessary coating, for example.
This data is provided by Fraunhofer IOF, which develops a wide range of scattered light measurement systems and sensors as well as the associated analysis methods and scattered light models. This makes it possible to predict the scattered light with a virtual coating even before the actual production process. In addition, complex optics can be characterized during the process, i.e. the surfaces can be automatically checked and fully assessed based on their design and construction data sets. This enables an optimized ratio of production costs and benefits.
Traditionally, surface imperfections are measured using microscopy, interferometry or tactile methods in which the surface is scanned with a diamond needle. However, these methods are very complex and time-consuming. For smooth surfaces, on the other hand, scattered light measurement technology offers a proven alternative due to its high sensitivity, fast and non-contact recording of measured values and high robustness, e.g. it is insensitive to vibrations. For example, it would take more than 40 years to inspect the entire surface of a 60 cm diameter mirror with an atomic force microscope. With scattered light methods, on the other hand, this can be done in just a few hours.
Due to its insensitivity to vibrations, this method can also be easily integrated into the production process, unlike conventional measuring methods. The illustration shows the integration of a compact scattered light sensor in an ultra-precision diamond turning machine, which was specially developed for fast and flexible roughness and defect characterization. This means that the surface roughness can be characterized directly during the manufacturing process and process parameters can be adjusted if necessary.
