OBLIQUE INCIDENT INTERFEROMETER

PROBLEM TO BE SOLVED: To obtain an interference fringe image having good contrast by shortening an optical path difference of a reference light and an object beam shorter than a coherence length of the reference light by a light quantity attenuation path matching member, and reducing a light quantity difference of the reference light and the object beam, thereby generating a light interference of the reference light and the object beam even in the case of using a light source having a short coherence length. SOLUTION: The oblique incident interferometer comprises a light source 1 for emitting a light having a short coherence length, a collimator lens 2, a first diffraction grating 3 for wave surface dividing its parallel beam, a second diffraction grating 7 incident with a reference light 4 of a zero-order diffraction light from the grating 3 and an object beam 6 from a + primary diffraction light from the grating 3 via a surface 5a to be detected to interfere a - primary diffraction light of the light 4 with the zero-order light of the beam 6, a converging lens 8, and a television camera 10. A light quantity attenuation path matching filter 11 for attenuating a light quantity of the light 4 and reducing an optical path difference of the light 4 and the beam 6 is arranged between the two gratings 3 and 7.