A Holographic Interferometer Using An Oil Surface As A Reference Plane

When the measurement of a surface flatness over a large circular area of an object is made with interferometers, it is imperative to use a highly polished large surface area as a reflecting reference plane. However, supplying such a plane is a difficult thing. To overcome this difficulty, an oil surface on the object to be studied has been used as its reference plane. In this method, the object under investigation was completely immersed in a tilted container filled with oil, and thus the arrangement of a Fizeautype interferometer was able to produce the interference fringes between two beams; one reflecting from the oil and the other from the object surface 1).