J. Eur. Opt. Society-Rapid Publ. 2025, 21, 19 Ó The Author( s), published by EDP Sciences, 2025 https:// doi. org / 10.1051 / jeos / 2025016 Available online at: https:// jeos. edpsciences. org
Journal of the European Optical Society-Rapid Publications
EOSAM 2024 Guest editors: Luca De Stefano and Raffaele Velotta
RESEARCH ARTICLE
Investigation of the positioning accuracy of the Cat’ s Eye as a reference position in asphere-measuring interferometry
Gregor Scholz *, Daniel Evers, and Ines Fortmeier Physikalisch-Technische Bundesanstalt( PTB), Bundesallee 100, 38116 Braunschweig, Germany
Received 29 January 2025 / Accepted 31 March 2025
Abstract. The increasing interest of the optics manufacturing industry in apshere and freeform optics leads to a high demand in accurate form measurement of such surfaces. The tilted-wave interferometer offers a fast and flexible solution to the needs. However, similar to other interferometric methods, ambiguities between specimen misalignment and certain form errors exist. To ensure the accuracy of the form measurement, the accuracy of the specimen positioning is crucial. This work presents a multi-step specimen positioning procedure that uses the Cat’ s Eye reference position as a fixed reference position. Different alignment criteria are evaluated by simulation and implemented into an alignment algorithm for specimen alignment into the Cat’ s Eye position. The procedure, investigated with multiple test specimens, shows a good short-term repeatability with a standard deviation below 30 nm. The use of this specimen alignment procedure will significantly reduce surface form measurement errors associated with axial misalignment and improve the overall measurement accuracy of the tilted-wave interferometer.
Keywords: Interferometric form measurement, Asphere metrology, Surface positioning.
1 Introduction
Modern optical technology utilizes aspherical and freeform optics more and more for their inherent capability of correcting optical aberrations, which gives these optical elements an advantage over classic spherical optics [ 1, 2 ]. This has led to more compact, yet high-quality, optical systems both in professional and consumer devices. The advancements in usage and design of optical aspheres and freeforms have led to an increasing demand in measurement technology for these surfaces, since highly accurate manufacturing is fundamentally limited by the accuracy with which the surfaces to be manufactured can be characterized. Measurement methods for such surfaces can be divided into point-based and area-based approaches. Point-based approaches use either an optical [ 3 ] or a tactile probe [ 4 ] to scan the surface under test. These methods are highly adaptable in regards to the surface form that can be measured. However, the serial nature of the approach leads to a trade-off between measurement time and lateral resolution. Area-based methods on the other hand can achieve both short measurement times and high lateral resolution due to their parallel measurement approach. They are usually based on interferometry [ 5 ] or deflectometry [ 6 ] and have typically more limitations in regards of the surface
* Corresponding author: gregor. scholz @ ptb. de form that can be measured. In contrast to spherical optics, which can be measured easily by interferometric null-test, the measurement of aspherical or freeform optics is much more challenging [ 7 ]. Here, an interferometric null-test would require a custom-made reference for every desired specimen, which is often done utilizing a computergenerated hologram( CGH) that has to be calculated and fabricated with lithographic methods. Measurement with CGHs requires meticulous effort in specimen alignment or else can lead to large measurement errors. For small series or individual specimens, this approach is too costly and time-consuming to be feasible. Therefore, for this use-case interest has shifted away from interferometric null-test to non-null-test measurement approaches such as subaperture stitching interferometry [ 8 ], axial scanning interferometry [ 9 ], or sub-Nyquist interferometry [ 10 ].
A very promising optical method for such measurements is tilted-wave interferometry [ 11 – 13 ]. A tilted-wave interferometer( TWI) combines a special interferometer setup with model-based data evaluation methods. The setup is a modified Twyman-Green interferometer, in which a microlens array is added to the objective arm such that the specimen is illuminated by differently tilted wavefronts instead of a single wavefront. In contrast to the null-test scenario of spherical form measurement, a common path between incident and reflected wavefront is not necessary and the imaged interference patches just represent
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