Optical element

An optical element for generating diffraction orders comprising at least one series of consecutive diffraction features for diffracting electromagnetic radiation from a source into diffraction orders, wherein the diffraction features in said at least one series are shifted from a nominally regular spacing, each feature being shifted by a selected predetermined amount so as to substantially eliminate at least one nth diffraction order whilst maintaining the existence of the 1st and at least one selected mth diffraction order.

A readhead for an encoder apparatus comprising a detector and a diffraction grating comprising at least one series of consecutive diffraction features for diffracting electromagnetic radiation from a source into diffraction orders, wherein the diffraction features in said at least one series are shifted from a nominally regular spacing, each feature being shifted by a selected predetermined amount so as to substantially eliminate at least one nth diffraction order whilst maintain the existence of the 1st and at least one selected mth diffraction order, such that said 1st and selected mth diffraction orders can contribute to the production of an interference signal at the detector.

A method of manufacturing a diffraction grating for an encoder apparatus comprising determining a select arrangement of diffraction features such that the spacing between the centres of adjacent diffraction features varies such that at least one nth diffraction order is substantially eliminated whilst the existence of the 1st and at least one selected mth diffraction order is maintained to such a level that said 1st and selected mth diffraction orders can contribute to the production of an interference signal at a detector of an encoder apparatus, and forming diffraction features on a substrate according to said select arrangement.