In industry, the use of high-performance, high-precision and, at the same time, lightweight optomechanical imaging systems is becoming increasingly important. The use of aspherical surfaces is playing an increasing role as the number of lenses can be reduced and dimensions and weight can be minimized significantly. In the case of an asphere, on the other hand, decentering is possible as both a displacement and a tilting and this is completely independent of one another. Therefore, the aspherical semi-finished products must be subjected to certain centering rules during the grinding and polishing process and used on the production spindles in an optimized and adjusted manner in order to avoid rejects during production. A subsequent centering process, as is usual with spheres, is then no longer possible. The internal centering error in an asphere-sphere is an immanent offset of the center of curvature of spherical surface to the aspherical axis of the second surface. The new approach of the vignetting Field Stop Technology (V-SPOT) makes it possible to precisely record the local, meridional pitch error in the zone or at the edge of the aspherical surface and, together with the centering deviation of the vertex, to determine the aspherical axis and thus the inner centering error. A short insight into the latest application of centering measurement of double-sided aspheres from only one side using the high depth of field and the large measuring range of the V-SPOT principle is given.
Deflectometric measurements using V-SPOT technology has been proven to achieve accurate surface profiles for aspheres at moderate cost and low preparation effort. In order to extend the resolution limit, the optical and mechanical device has been improved to provide on the one side topography information in the slope domain at high accuracy (< 5 μrad) and an improved lateral resolution (< 0,2 mm) to cover surface profile errors in the mid-spatial-frequency range from 1 to 10 mm-1. Within this publication we are providing the experimental setup and the measurement procedures to achieve production relevant information about the surface quality. Slope deviations of aspheric samples (glass and metal) are analyzed in angular spectral components and the surface profile is compared with interferometric data to proof accuracy and lateral resolution of our device. As final conclusion we outlook for further improvements of the proposed device to allow full control of form deviation and mid-spatial frequency errors.
Accurate measurement of centration of aspheric lenses or even freeforms is a challenge for most devices in optical manufacturing. We are providing a new attempt by combining an autocollimation device and a Vignetting Field Stop [1, 2, 3] device to measure lens centration and sagittal surface profile in a deflectometric approach. Both devices work independently at high accuracy. This presentation explains the technical setup, consisting of an autocollimation sensor (ELWIMAT-AKF) and a Vignetting Field Stop Sensor (ELWIMAT V-SPOT), which is mounted together with an air bearing rotary table in a vertical arrangement. Secondly, we provide the results of the centration measurement and the results from the surface reconstruction and slope error from the measured sagittal angle deviations. Finally, the results from a measured asphere (High Level Expert Meeting HLEM sample #3) is critically discussed to state the accuracy and applicability of the proposed measurement attempt.
In-situ measurements of complex surfaces during the polishing process is a challenge for the production of aspheric surfaces or freeforms. We are providing a new attempt by using a scanning deflectometric device based on our recently published DaOS [1] principle, which allows in-situ measurements of large optical surfaces in realistic production environments and offers the conditions for direct intervention and correction in the polishing process. The results of insitu surface measurements after three polishing steps of a large glass substrate (320 mm in diameter) in a lever-polishing machine (NLP500 from Stock Konstruktion GmbH) are shown and critically compared with interferometric measurements on a SSI-A Interferometer. In this paper, the technical setup consisting of a highly precise scanning penta prism device and a Vignetting Field Stop (VFS) Sensor is explained. Secondly, we are discussing the mathematical algorithm to reconstruct the complete surface from angle measurements from a given number of cross-sectional cuts. The data of the surface reconstruction are transformed into a XYZ-file format to be analyzed with MetroPro®. The results are shown and discussed in terms of accuracy and reproducibility. Finally, a comparison with interferometric measurements on an SSI-A (QED) at TH Deggendorf (THD), Technology Campus Teisnach is shown to proof the degree of accuracy and applicability of our new, fast and reliable device for in-situ measurements of complex surfaces.
The basic physical measurement principle in DaOS is the vignettation of a quasi-parallel light beam emitted by an expanded light source in auto collimation arrangement. The beam is reflected by the surface under test, using invariant deflection by a moving and scanning pentaprism. Thereby nearly any curvature of the specimen is measurable. Resolution, systematic errors and random errors will be shown and explicitly discussed for the profile determination error. Measurements for a “plano-double-sombrero” device will be analyzed and reconstructed to find out the limit of resolution and errors of the reconstruction model and algorithms. These measurements are compared critically to reference results that are recorded by interferometry and Deflectometric Flatness Reference (DFR) method using a scanning penta device.
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