Twyman/Green Interferometry is a classical interferometry used to measure surface topography. An optically flat beam splitter delivers half the light to the specimen and the other half to a flat reference mirror.

After reflection from the specimen and reference surfaces, the beams meet again at the beam splitter and a portion of each propagates horizontally to be collected by the observer.

The interference pattern seen by the user will be a contour map of the z coordinate of the specimen surface. The interval between the fringes will be half the wavelength of the light source per fringe order.

Example: Deformation of MEMS device subjected to pressure