Interferometric measuring apparatus employing fixed non-zero path length difference

Interferometric measuring apparatus for measuring movements of an object in a fluid medium, the apparatus comprising:

• means for producing two collimated, coherent light beams,
• beam splitting means for splitting each of the two light beams into two components to provide two measuring component beams and two reference component beams, the measuring component beams being directed in parallel through the fluid medium towards reflecting surfaces on the object the movement of which is to be measured, the beam splitting means re-combining the respective measuring and reference component beams after reflection of the measuring component beams from said surfaces to produce two output combined beams,
• detector means for receiving each of the two output combined beams,
• wherein a fixed non-zero path length difference is established in the fluid medium between one of a) the two reference component beams and b) the two measuring component beams, and the detector means includes means for providing from the two output combined beams electrical signals indicative of both the relative phase difference of the two output combined beams, and the temporal phase changes of at least one of the output combined beams.