Fresnel rhomb
![A diagram of a Fresnel rhomb (blue). Incoming light, if linearly polarized at 45° with respect to the plane of incidence (along the page) has the relative phase of its s and p polarization components altered at two reflections, outputting a circularly polarized beam.](/Images/godic/202501/14/Fresnel_rhomb.svg3500.png")
A Fresnel rhomb is a prism designed in 1817 by Augustin-Jean Fresnel for converting linearly polarized light into circularly polarized light. While the device performs the same function as a quarter wave plate, the rhomb does not depend on a material's birefringence. Rather, the relative phase shift between the s and p polarizations is a result of total internal reflection, two of which each contribute a nominal retardance of 45°. This achieves a wide spectral bandwidth, which is difficult using birefringent waveplates. The precision of the 90° phase shift is compromised only due to changes in the refractive index of the material due to dispersion; this change can be very small over a wide spectral range. The material (typically glass) is itself specifically not birefringent.