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.
The vignetting field stop procedure uses a deflectometric approach to acquire big Optical Surfaces — DaOS — and it offers the possibility to measure nearly any shape or form using a scanning routine. 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 with a reflecting element. Thereby nearly any curvature of the specimen, is measurable. Due to the fact, that even sign changes in the curvature can be detected, also aspheres and freeform surfaces of any size can be evaluated.
In this publication the vignetting field stop procedure is discussed. Additionally the deflectometric setup is described. Because of some typical influences of beam deflection to the accuracy of angle measurement by using the vignetting principle, suitable methods of calibration for the sensor are examined and the results of these methods are presented.
Furthermore the technical principle of deflectometric measurements using an angle detecting device is explained inclusive of all random and systematic errors generated by the setup.
The last part of this publication shows the actual result of test measurements with calculated absolute deviation of errors with a large lateral dimension as well as the determination of the maximal achievable lateral resolution by detecting mid frequent structures on flat and spherical test parts with a diameter of 300 mm. These measurements are compared critically to reference results which are recorded by interferometry and further scanning methods.
A newly developed measuring procedure uses vignetting to evaluate angles and angle changes, independently from the measurement distance. Further on, the same procedure enables the transmission of a digital readout and therefore a better automation of the electronic signal evaluation, for use as an alignment telescope. The fully extended readout by a simple 3-D reflector will provide the user with a measurement result with six degrees of freedom. The vignetting field stop procedure will be described. Firstly, considering artificial vignetting, the theoretical basics from geometric-optical view are represented. Secondly, the natural vignetting with photometric effects will be considered. The distribution of intensity in the image plane light spot, the so-called V-SPOT, is analytically deduced as a function of differently measured variables. Intensity shifts within the V-Spot are examined independently from different effects by numeric simulation. On these basics, the theoretical research regarding accuracy, linearity as well as results in 2 dimensional surface reconstruction on precision optical mirrors and also three dimensional measurements in mechanical engineering are examined. Effects and deviations will be discussed. The project WiPoVi is sponsored by “Ingenieur Nachwuchs – Qualifizierung von Ingenieurnachwuchs an Fachhochschulen” by Bavarian State Ministry of Education, Science and the Arts.
The manufacturing of optical components more often requires grinding and polishing of non rotational symmetric aspheres
or freeform surfaces. Although there are measurement techniques available for small diameters of some 10th of
mm the measuring of larger surfaces is not or only by extreme efforts feasible. Based on the specification for a large
mirror in semi professional and scientific astronomy with up to 1.2 m diameter and a relative aperture of F# < 1.5 a final
measurement approach for the measurement of radius and shape deviation (parameters of code number 3 of DIN ISO
10110) is described. The combination and evaluation of different sensors and measurement methods for the measurement
of high-precision optical surfaces with concave radii of 3.000 mm to infinity is examined and presented. A reproducibility
and absolute accuracy of better l/12 (PV) and l/40 (rms) is to be achieved. The absolute maximum radius difference
should be smaller than 0,1 %. Thus, also the measurement of aspheric surfaces and free-form surfaces are investigated.
For the measurement of large surfaces, up to 100 individual sub apertures with up to 100 Million Points are recorded by
deflectometric or interferometric measurement techniques and composed algorithmically to a total surface area. A precision
granite portal with multifunctional device carrier will be presented as precise movements are crucial for all tests.
The realization of the required accuracy in the portal-measurement device is verified, documented and compared with a simulation. Results on specimens of 200 and 430 mm diameter are evaluated. The measurements were taken by deflectometry and interferometry on the described test equipment. The validation of the samples with various interferometric procedures was performed. The obtained results are presented, analyzed and discussed.