Conoscopy

Conoscopy is an optical technique to make observations of a transparent specimen in a cone of converging rays of light.

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 Charles-Victor Mauguin to investigate the alignment of nematic and chiral-nematic phases.

[1] 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).

Modern advanced conoscopic devices are used for rapid measurement and evaluation of the electro-optical properties of LCD-screens (e.g., variation of luminance, contrast and chromaticity with viewing direction).

raytracing to illustrate the formation of a directions image in the back focal plane of a positive thin lens
raytracing to illustrate the formation of a directions image in the back focal plane of a positive thin lens
formation of an image of the object (aperture) by addition of a second lens. The field of measurement is determined by the aperture located in the image of the object.
formation of an image of the object (aperture) by addition of a second lens. The field of measurement is determined by the aperture located in the image of the object.
schematic raytracing of a complete conoscope: formation of the directions image and imaging of the object
schematic raytracing of a complete conoscope: formation of the directions image and imaging of the object