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 .