Figure 1
Conoscopic holography is a simple implementation of a
particular type of polarized light interference process, which
uses a birefringent crystal.
In ordinary holography, each object point is interferometrically recorded as a Fresnel zone plate (FZP). The interference pattern is formed between an object beam and a reference beam using a coherent light source. The object and reference beams propagate with the same velocity, but follow different geometrical paths.
In conoscopic holography, the object and reference beams of coherent holography are replaced by the ordinary and the extraordinary components of a single beam propagating in birefringent media. Therefore, the signal and reference beams have the same geometrical paths but different optical path-lengths; the two beams are naturally coherent one with the other and therefore the technique allows the creation of holograms, even with non-coherent light.
Conoscopic holography has some advantages over classical holography if a spatially limited illuminated area is involved.
The basic principle resides in considering a crystal sandwiched between two circular polarizers in order to provide an interference pattern (photo and text: Spagnolo et al. 2004 J. Opt. A. 6 869).