The non-contact open measuring systems are for high displacement velocities and high accuracies,
commonly used in clean environments.
Non-contact open encoders rely on the air gap between the encoder head and scale to be uniform over the measuring range. The flatness of the mounting surface and the parallelism of the machine guideway is important.
Depending on the version of the measuring system, two different measuring scales are available: 
glass (partially ROBAX glassceramic) or steel .

System resolution	Measuring scale, Special	Overall size	Type/Series
10 µm to 0,1 µm	glass scale or steel tape scale	small version	MS 20, MS 21 MS 25, MS 26
5 µm to 0,05 µm	steel tape scale	compact version	MS 30 series
10 µm to 0,5 µm	Low Cost and High Quality steel tape scale	flat version	MS 40 series
0,1 µm to 0,01 µm	glass scale	compact version	MS 82