OPTICAL ELEMENT

摘要

An optical element for generating diffraction orders for an encoder apparatus, in which the optical element includes an array of diffraction features arranged such that the spacing between the centres of adjacent diffraction features varies irregularly from one pair of adjacent features to the next.

主权项

1. 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 a signal varying with movement between the two parts, 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, such that said 1st and selected mth diffraction orders contribute to the production at the detector of a signal varying with movement between the two parts.