Substrate holder for use with interferometer

A method comprising:

• positioning a substrate within a substrate holder, the substrate holder comprising:
• a first support comprising a first bearing land and a first bearing base arranged to form a first bearing pocket and a first gas inlet coupled to the first bearing pocket;
• and
• a second support comprising a second bearing land and a second bearing base arranged to form a second bearing pocket and a second gas inlet fluidly coupled to the second bearing pocket;
• wherein:
• the first support and the second support are positioned relative to one another such that the first bearing pocket is opposed to the second bearing pocket thereby forming a measurement cavity between the first support and the second support;
• at least one of the first support and the second support comprises reference optics through which one or more interferometric or optical measurements can be taken;
• supplying gas to the first bearing pocket and the second bearing pocket to form an air bearing in the measurement cavity for supporting the substrate in the measurement cavity without contact between the substrate, the first support, and the second support;
• and
• making one or more interferometric or optical measurements through the first support, the second support, or the first and second supports.