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Ultraprecise flatness measurements of optical surfaces

PTB has developed a method to measure the topography of plane and weakly curved optical surfaces. With the new apparatus shape deviations from an ideal plane surface can be determined with a measurement uncertainty of a few atomic diameters. The measurements do not rely on an external flatness standard.

The new apparatus can measure specimens of up to 500 mm in diameter.

Highest quality plane surfaces are needed in several fields of industry and research. For instance, well-characterized and highly accurate flat surfaces are used to calibrate interferometers. Until now, the actual deviation of a real surface from perfect flatness is determined by comparison with a liquid mercury mirror which is the national primary standard for flatness in Germany. Since the liquid surface is perpendicular to the direction of gravity the standard is very flat. However, the spherical form of the earth and the surface tension induce slight deviations from a perfect plane.

To meet the industrial demand for improved accuracy PTB has developed a new device to perform highly accurate and traceable measurements of the topography of plane and weakly curved optical surfaces. The method is based on angle measurements. The reflection angles of a light beam are successively measured at different spots on the surface of the specimen and the differences between the angle measurements are determined. The spot positions are separated by a constant lateral displacement (shear) of a few millimetres to centimetres. The reflection angles are determined with a highly accurate electronic measuring device (an autocollimator). When repeated measurements of scans of 130 mm length across plane surfaces are performed the topography values typically deviate by merely one atomic diameter, i.e.: 0,1 nm to 0,2 nm. The measurement uncertainty of the new facility is 1 nanometre or better.

The method represents a novel approach to measure flatness deviations as it does not require an external flatness standard. Instead, the method utilizes the straight propagation of the light beam as a reference. The measureands, from wich the topography is derived by application of mathematical algorithms, can be directly traced to the SI units of angle and length.

The facility has the potential to become a highly precise primary standard for straightness and flatness with a measurement uncertainty in the sub-nanometre range and to replace the liquid mercury mirror.

Contact at PTB:

Phone: +49-531-592-0