PTB News 1/2005 (135 kB) PTB News 1/2005, English edition, Issue May 2005The fabrication of high-precision optics and mechanical precision stages requires a demanding metrology. For this purpose, an optical profiler based on angle measurement has been developed at PTB with the aid of which a measurement uncertainty of the topographic measurement of 1.5 milli arc seconds has been achieved.
Today, the fabrication and subsequent form correction of optically polished surfaces with dimensions larger than 300 mm is possible by advanced local processing methods. The resolution of the respective procedures is better than 1 nm in the removal; laterally, it amounts to up to 1 mm. The accuracy achievable is limited by the accuracy of the correction data which must be metrologically acquired.
At PTB, a procedure and a measurement device have been developed which are in a position to work steadily in a manufacturing environment. In the measuring system, a commercial electronic autocollimator determines the angle between its optical measuring beam and a back-reflecting surface. The autocollimator was modified by the manufacturer in a way that the angle difference between elements of a reflecting surface, when segmented by apertures, can be read out. The sensor head equipped with a threefold diaphragm allows two angle differences to be simultaneously measured.For the measurement, the sensor head is guided at a short distance along the surface to be measured by a conventional stage in a way that disturbances of the ambient air cannot occur. With the aid of a procedure patented for PTB, the complete angular information can be reconstructed from the two angle differences measured along the specimen. This allows reproducible results to be achieved even under unfavourable ambient conditions such as building vibrations or air circulation which are limited only by the long-term stability of the autocollimator. This stability lies clearly below the measurement limit of single milli arc seconds as measurements performed on the angle comparator of PTB (cf. PTBnews 03.3) have shown.
Even for the measurement of free-form surfaces,this procedure does not require any form reference, but only calibration of the zero point of the angle differences which is, for example, performed by means of a flat reference plate. The topographic height profile results from the integration of the angle over the length.
The device was used to measure the central track of flat test specimens made of Zerodur with lengths up to 620 mm. Determination of the height topography in comparison with the result of an alternative instrument (cf. PTBnews 02.3) showed a maximum systematic difference of 1 · 10–9 m after
subtraction of the mean curvature. Here, the measurement
uncertainty amounts to 1.5 milli arc seconds
(7.3 · 10–9 rad).