Optical element

An encoder apparatus for determining movement between two relatively moveable parts of an apparatus, the encoder apparatus comprising:

• a detector;
• and
• at least one optical element comprising at least one series of consecutive diffraction features for diffracting electromagnetic radiation from a source into diffraction orders and which are for contributing to the production at the detector of an interference fringe varying with movement between the two parts,
• wherein the diffraction features in said at least one series are shifted from a nominally regular spacing such that the spacing between the centres of adjacent diffraction features selectively varies irregularly from one pair of adjacent diffraction features to the next so that harmonics in the interference fringe at the detector are reduced compared to a like encoder apparatus with a diffraction grating having a constant spacing between diffraction features.