International Comparison Of Noise In Areal Surface Topography Measurements

An international comparison of surface topography measurement noise was carried out. The comparison involved twelve optical surface topography instruments (focus variation instruments, confocal microscopes and coherence scanning interferometers) from six European research laboratories. The purpose of this comparison was to perform a practical test of the ISO definition and procedures for determining measurement noise on a variety of instruments, and to provide good practice guidance to the user community for quantifying, specifying and interpreting measurement noise. Despite taking steps to enable an optimal comparison by supplying a variety of specimens, where the 'best' could be selected and the measurement area was clearly defined, an appropriate comparison of the measurement noise beyond the actual measurement conditions was complex, as the measurement time, speed and lateral resolution were difficult to establish for most of the instruments. The results that could be obtained were: (i) the measurment noise N ( M ) can be reliably estimated from two measurements, (ii) noise may vary considerably between instruments, even when they have the same measurement principle, (iii) the noise in a practical measurement may be considerably higher than the noise with a levelled flat, and (iv) the 'rms repeatability' has little correlation to the noise determined according to the ISO specification standard, and may lead to values that are more than a hundred times smaller.