Differential measurement system

A system comprising:

• first and second coaxial members configured to have an axis of rotation substantially parallel to a light path,
• wherein:
• the first member comprises a first pattern and is arranged to rotate with a first speed of rotation, wherein the first speed of rotation of the first member is configured to be based on a flow rate of a first target,
• the second member comprises a second pattern and is arranged to rotate with a second speed of rotation, wherein the second speed of rotation of the second member is configured to be based on a flow rate of a second target, and
• the first and second patterns have an angular variation and are configured to generate an interference pattern from a light beam traveling along the light path based on their interaction with the light beam and in response to the first and second members having a relative difference in their rotational speeds, the interference pattern being indicative of a magnitude of this difference;
• and
• a fluid circulation system having a fluid inlet and a fluid outlet,
• wherein the system is configured to measure a difference between flow rates of the fluid inlet and the fluid outlet.