Abstract
Due to the complexity of measurement systems for asphere and freeform surfaces, well-known artifacts are required to characterize the accuracy of the results of their form measurements. We present advancements in manufacturing and characterization of metrological multispherical freeform artifacts. The strong cooperation between the manufacturing and measurement units of Physikalisch–Technische Bundesanstalt (PTB) allows the form of the artifacts to be manufactured very accurately and the root-mean-square deviation from the desired design form to be improved by a factor of >20 compared to former results (e.g., from a value >500 nm to about 20 nm). Furthermore, a nickel/phosphorous coating is added to the copper surface, resulting in greater hardness and allowing the coated surface to be used as a reference for low-force tactile measurement systems. Four Gaussian peak fiducial marker structures are added to the design to improve the evaluation of comparison measurements. In addition to characterizing the radii in the spherical segments using PTB’s radius measurement bench, we also characterize the sphericity of the spherical segments using a Fizeau interferometer. We show form measurement results for a full-field measuring tilted-wave interferometer and compare the form measurement results in the spherical segments with measurement results obtained with the Fizeau interferometer.
Highlights
Asphere and freeform surfaces play a central role in modern optical systems of all kinds.[1,2] accurate manufacturing is still a challenging task that depends on how well such forms can be measured
Based on the assumption that every deviation of the surface under test (SUT) leads to a characteristic change in the optical path length differences (OPDs) between a source and a pixel, the measured OPDs are compared to the simulated OPDs using the design function of the SUT.[13,14,15,16,17]
mail: ines.fortmeier@ptb.de surfaces” (MRSes) have characteristic features that are measurable with a traceable reference measurement technique
Summary
Asphere and freeform surfaces play a central role in modern optical systems of all kinds.[1,2] accurate manufacturing is still a challenging task that depends on how well such forms can be measured. Several standards for testing optical and tactile measuring machines have already been published.[5,6,7] The drawback of these artifacts is that their form does not match the most common asphere and freeform lenses used in the optical industry. Simple geometries such as spheres and flats are unsuitable because the dynamic range of asphere and freeform metrology systems cannot be tested with them in an easy manner. Since the forms of typical asphere (and freeform) surfaces are not wellknown,[3] these surfaces are unsuitable for use in highly accurate calibrations of such measurement systems
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