Traceability of form measurements of freeform surfaces: metrological reference surfaces

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-meansquare 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.

Round robin comparison study on the form measurement of optical freeform surfaces

“…The transitions between the spherical segments are defined by cosine functions, leading to smooth transitions. The mathematical description of the surface form is complex and can be found in [11]. The specimen has a clear aperture of 40 mm and a best-fit radius of 39.75 mm.…”

Form deviations caused by lateral displacement errors in annular subaperture stitching interferometry

“…The artifact covers typical metrological properties listed in the ISO 25178-600 standard and shows good producibility and aging behavior. The national metrology institute of Germany, Physikalisch-Technische Bundesanstalt (PTB), recently proposed a multispherical artifact for traceable calibration of form measurements of freeform surfaces, 3 which may foster the standardization of uncertainty statements for the measurand of the associated form measuring instruments. In addition to the availability of traceable calibration artifacts to characterize the measurement uncertainties related to the instruments, a meaningful statement of uncertainty also requires profound knowledge about uncertainties introduced by the calculations and algorithms involved in the translation of the recorded images to the surface topography.…”

New European Metrology Network for advanced manufacturing

“…for asphere and freeform optics) and silicon-related technologies (e.g. nano-LED), both targeting to use light-over a broad spectrum range-for advanced applications [62][63][64].…”

“…for asphere and freeform optics) and silicon-related technologies (e.g. nano-LED), both targeting to use light-over a broad spectrum range-for advanced applications [62][63][64]. Includes all components and systems needed to be manufactured for transportation of people and goods in air and astronauts and instrumentation in space: the aerospace sector is characterised by stringent safety requirements in challenging and changing environments with associated special manufacturing demands [69,70].…”