Comparison of form measurement results for optical aspheres and freeform surfaces

Abstract

Comparing form measurement data for aspheres and freeform surfaces is an important tool for ensuring the quality and functionality of the devices used to take such measurements and may also allow the underlying measurement methods to be evaluated. However, comparing the highly accurate form measurements of such complex surfaces is a demanding task. It is difficult to analyze measurement results whose accuracies are in the range of several tens of nanometers root-mean-square, especially when comparing data with different, and anisotropic distributions of the 3D measurement points on the surface under test. In this paper, we investigate eight different 3D measurement point distributions that are typical of highly accurate measurement systems currently in use and demonstrate the effects of these distributions on the comparison results by using virtually generated data and applying different evaluation strategies. The results show that, for the examples investigated, the different 3D measurement point distributions can yield different levels of accuracy for the comparison. Furthermore, an improved evaluation procedure is proposed and recommendations on how to significantly reduce the influence of the different 3D measurement point distributions on the comparison result are given. A method of employing virtually generated test data is presented that may be generalized in order to further improve and validate future comparison methods.

Results of round robin form measurements of optical aspheres and freeform surfaces

“…The comparison procedure is based on the refined procedure tested and discussed in [1]. For reader convenience, a description of the procedure is provided again here.…”

“…Comparison studies are important for ensuring the quality and functionality of measurement devices and are intended to provide an independent, and unbiased comparison of all available measurements while offering insights into the performance of the form measurement systems currently available. This presents a challenging task, since the results of different instruments and laboratories have different 3D measurement point distributions [1], leading to different grid patterns and grid point densities. Another challenge associated with such comparison studies is that the true form of the surfaces is not known as there are no absolutely measured and very well characterized reference standard surfaces available at the accuracy level of a few tens of nanometers root-mean-square (RMS).…”

“…The novelty of this paper is twofold: First, the form measurement results of new surfaces by a large variety of different measurement instruments are compared by applying a refined comparison method that had been previously tested on virtual test data [1]. Overall, 13 different instruments based on 9 different measurement methods were used at 12 different laboratories, and a total of 41 different measurements are analyzed here.…”

Precision form measurement of biconic surfaces using scanning Fizeau interferometry

“…A commonly used technique for this is to see if the results correlate with measurements from other instruments. Often researchers even perform roundrobin tests with multiple participants to study how well different measurement techniques correlate 7 .…”

Tilted Wave Fizeau Interferometer for flexible and robust asphere and freeform testing

“…exploiting the rotational symmetry of aspheres in rotation tests to determine the non-rotational SUT error part. For the determination of the complete systematic error, Greivenkamp et al 35,36 have suggested to use a raytracing model of the test setup that is using accessible setup characterization data (individual component's surface errors, position and orientation deviations) and an optimization to find a model that best explains the observed data of calibration measurements. Our approach is based on a black box model description of the wavefront aberrations of the setup.…”