Conoscopic Holography is a simple implementation of a particular type of polarized light interference process based on crystal optics. In the basic interference set-up, a point of light is projected on a diffuse object. This point creates a light point, which diffuses light in every direction. In a Conoscopic system a complete solid angle of the diffused light is analyzed by the system. The measurement process retrieves the distance of the light point from a fixed reference plane.

To better understand the system functionality, the behavior of a single ray needs first to be comprehended.

A single ray, at a given angle, emitted by a light source point, impinges on the first face of the crystal Fig 1. It is split into two rays propagating inside the crystal at different velocities along almost the same geometrical path. The velocity of one is isotropic and so this ray is called ordinary, while the other has an anisotropic velocity, i.e., it is extraordinary. Thus two superimposed rays emerge from the crystal with a phase difference at orthogonal polarizations. In order for both rays to interfere, an analyzer (polarizer) aligns the directions of the electrical fields.

Fig. 1 Conoscopic basic set-up

Fig. 2 Complementary Fresnel zone plates

For a complete solid angle, each emerging ray will have a different phase difference between the ordinary and extraordinary rays. At a given plane, the collections of all the rays will create an intensity figure – the Conoscopic figure, the parameters of this figure being dependent on the angular distribution of the rays inside the crystal. These in turn are dependent on the position of the point in space. Recording and analyzing the figure reveal the geometrical parameters of the point.
Several types of figures can be observed depending on the polarizer and analyzer, and on the crystal optical axis direction.

This new technique has many advantages over classical holography:

1. Uses concentric optics and is insensitive to the position of the key optical components.
2. An interfringe distance adjustable to common CCD sensors, which facilitates the interface with computer system.
3. Light need be only quasi-monochromatic (in practice, a 10nm spectral bandwidth is enough) but not spatially coherent.

Additionally Conoscopic systems offer specific benefits:

4. Simplicity of the set-up.
5. Increased stability, as the geometrical paths of both split wavefronts are almost the same.
6. The possibility of obtaining complementary interferograms with no moving parts so that part of the optical noise due to speckle can be cancelled.

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Last Update: October 28, 2007      Site Map