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in optics a vector diagram representing the dependence on direction of the characteristics of a light field (light intensity and polarization) or of the optical characteristics of the medium (indexes of refraction; reflectivity).
A particular case of the indicatrix is the scattering indicatrix, which represents the dependence of the intensity of the scattered light on the angle of scattering under the condition that the incident light is not polarized, and the optical indicatrix (in crystal optics). The optical indicatrix of an optically isotropic medium is a sphere. The indicatrix for crystals with trigonal, hexagonal, and tetragonal systems is an ellipsoid of revolution. The indicatrix for crystals with rhombic, monoclinic, and tri-clinic systems is a general ellipsoid.
The indicatrix is used when the analytical equations for the corresponding angular dependences are complex or unknown, and also for the systematization of experimental data.