Rayleigh interferometer

The two beams are recombined after traversing two optical paths, and the interference pattern after recombination allows the determination of the difference in path lengths.

[1] Light from a source (left) is collimated by a lens and split into two beams using slits.

The beams are sent through two different paths and pass through compensating plates.

They are brought to a focus by a second lens (bottom) where an interference pattern is observed to determine the optical path difference in terms of wavelengths of the light.

Its drawbacks are (i) it requires a point or line source of light for good fringe visibility, and (ii) the fringes must be viewed with high magnification.

Rayleigh interferometer at the National Bureau of Standards
Rayleigh Interferometer
Collimated coherent light passing through two tubes with different refraction index gases, then an imaging lens creates the interferogram.