Fresnel rhomb

The Fresnel parallelepiped (also: Fresnel rhombohedron ) is an optical prism which was introduced in 1817 by Augustin -Jean Fresnel, to convert 45 ° linearly polarized light into circularly polarized light. The function of the parallelepiped is therefore similar to that of a retardation plate, it is not based on the generation of a defined phase shift by birefringence but due to a double total internal reflection at a particular angle. It has the advantage that the phase shift in the contrast to when the retardation plate scarcely depends on the wavelength.

Design and operation

The function of the Fresnel parallelepiped defined based on a phase shift of the two components of polarized light at the total reflection on the inner surface of the prism. To a 45 ° linearly - polarized light is directed to a perpendicular end face of the prism and broken without changing direction in the prism. Then it falls on an inclined longitudinal surface of the prism. Is the angle of incidence greater than the critical angle of total reflection, it is totally internally reflected there. The phase shift thereby occurring cause is elliptically -polarized light from the original linearly - polarized light. Therefore, a second total reflection is the creation of circularly polarisertem light still necessary before the light exits through the second face of the prism.

As already mentioned, it is necessary for a defined phase shift in that the light is incident at a certain angle to the totally reflective interfaces. The angle depends on the refractive index of the material used and can be calculated from the following relationship:

The number of total reflections in the parallelepiped. The refractive index of the material flows into.

With a Fresnel parallelepiped normally be two total reflections in the prism. For a prism of crown glass with a refractive index of 1.51 the angle of incidence on the total reflective surfaces must therefore be at approximately 54.62 °.