Double pass interferometric encoder system

An encoder system comprising:

• an encoder head for use with a diffractive encoder scale, wherein the encoder head comprises one or more optical components arranged to:
• i) direct a first incident beam to the diffractive encoder scale at a first incident angle with respect to a normal to the encoder scale;
• ii) receive a first return beam from the diffractive encoder scale at a first return angle with respect to the normal to the encoder scale, the first return angle being different from the first incident angle;
• iii) redirect the first return beam to the diffractive encoder scale as a second incident beam at a second incident angle with respect to the normal to the encoder scale;
• and
• iv) receive a second return beam back from the diffractive encoder scale at a second return angle with respect to the normal to the encoder scale, the second return angle being different from the second incident angle,
• wherein a difference between the first incident angle and second incident angle is less than a difference between the first incident angle and the first return angle and less than a difference between the second incident angle and the second return angle.