Rolf Freimann

Absolute measurement of non-comatic aspheric surface errors

An absolute test for rotationally symmetric aspheric surfaces was developed. All non-rotationally symmetric surface errors apart from those of comatic nature were determined interferometrically. For this purpose, the asphere was measured 12 times under different angles of rotation. The difference of a single interferogram and the average of all 12 interferograms gave the non-rotationally symmetric errors in an absolute manner apart from the comatic errors. With the purpose of noise reduction, the so-determined 12 interferograms of the non-rotationally symmetric errors were appropriately rotated and subsequently averaged. The rotationally symmetric errors were detected via stylus profilometry. Repeatabilities of the so-obtained aspheric surface errors are presented.

Wavefront aberrations of rotationally symmetric CGHs fabricated by a polar coordinate laser plotter

Computer-generated holograms (CGHs) were fabricated by a polar coordinate laser plotter. The wavefront aberrations of these CGHs caused by fabrication inaccuracies were measured interferometrically. We tested Fresnel zone plates as typical examples of CGHs with focal ratios of ƒ/10 and ƒ/5 with a Fizeau interferometer. The tests comprised both binary phase holograms and binary amplitude holograms. The wavefront aberrations of the CGHs had an rms variation of about λ /50. Two types of aberrations were identified and could be referred to certain writing errors. Their size was deduced from the interferograms.

Absolute interferometric test of Fresnel zone plates

An absolute interferometric test of two-level binary Fresnel zone plates (FZPs) is presented. Five measurements with a wavefront testing interferometer are required to fully separate interferometer errors from those of the FZP. The method provides both errors, pattern errors and surface figure errors of the zone plate absolutely. The test method is suitable for zone mirrors and zone lenses. Test setups are explained, equations are derived and experimental results are presented. Further, applications of the absolute diffractive test method are described. For the estimation of the accuracy of the diffractive calibration method a comparison with the classical three-position test was carried out.