Light intensity ratio adjustment filter for an interferometer, interferometer, and light interference measurement method

A light intensity ratio adjustment filter for an interferometer comprising:

• a light intensity ratio adjustment film of a multi-layered film structure comprising at least one optical reflection-absorption layer and at least one dielectric anti-reflection layer laminated in that order as seen from the reference surface and attached to a side of a transparent substrate opposite the sample or to a side of the transparent substrate opposite the reference surface;
• wherein the light intensity ratio adjustment filter is placed so as to be freely inserted and removed between a sample and a reference surface of a Fizeau interferometer inducing interference between sample light from the sample and reference light from the reference surface, and obtaining interference wave information for the sample;
• and
• wherein the light intensity ratio adjustment film reflects part of the incident light from the surface of the transparent substrate opposite the reference surface, and after absorbing part of the remaining light, transmits the remainder towards the sample, and absorbs part of the light returned from the sample and incident from the surface side opposite the sample while controlling reflection, and transmits the remainder in the direction of the reference surface as the sample light.