Conoscopy

Conoscopy (from Ancient Greek κῶνος (konos) "cone, spinning top, pine cone" and σκοπέω (skopeo) "examine, inspect, look to or into, consider") is an optical technique to make observations of a transparent specimen in a cone of converging rays of light. The various directions of light propagation are observable simultaneously .

A conoscope is an apparatus to carry out conoscopic observations and measurements, often realized by a microscope with a Bertrand lens for observation of the direction's image. The earliest reference to the use of conoscopy (i.e., observation in convergent light with a polarization microscope with a Bertrand lens) for evaluation of the optical properties of liquid crystalline phases (i.e., orientation of the optical axes) is in 1911 when it was used by Mauging to investigate the alignment of nematic and chiral-nematic phases.

A beam of convergent (or divergent) light is known to be a linear superposition of many plane waves over a cone of solid angles. The raytracing of Figure 1 illustrates the basic concept of conoscopy: transformation of a directional distribution of rays of light in the front focal plane into a lateral distribution (directions image) appearing in the back focal plane (which is more or less curved). The incoming elementary parallel beams (illustrated by the colors blue, green and red) are converging in the back focal plane of the lens with the distance of their focal point from the optical axis being a (monotonous) function of the angle of beam inclination.

This transformation can easily be deduced from two simples rules for the thin positive lens:

The object of measurement is usually located in the front focal plane of the lens. In order to select a specific area of interest on the object (i.e., definition of a measuring spot, or field of measurement) an aperture can be placed on top of the object. In this configuration only rays from the measuring spot (aperture) hit the lens.

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