Encoder apparatus

A position measurement encoder apparatus, comprising:

• a scale comprising a series of features extending along a measuring dimension and which diffract light into multiple diffraction orders;
• and
• a readhead configured to detect said scale features and to output one or more signals which can be used to determine the relative position of the scale and readhead, the readhead comprising:
• a light source for illuminating the scale,
• a primary position information sensor and a secondary position information sensor, from which the signals which can be used to determine the relative position of the scale,
• a primary-sensor filter configured to filter light before it falls on the primary position information sensor, and/or
• a secondary-sensor filter configured to filter light before it falls on the secondary position information sensor;
• wherein the readhead further comprises an optical diffraction order encoder located in the path of the diffracted light relayed to the primary and secondary position sensors, which encodes different diffraction orders of the light with different optical states, which are subsequently selectively filtered by the at least one primary-sensor and/or at least one secondary-sensor filters, such that the light that falls on the primary position information sensor has been filtered differently to the light that falls on the secondary position information sensor.