In 1875, John Kerr discovered electric fields in isotropic sub- stance generate a double refraction. This effect is called the Kerr electro-optic effect. The double refraction’s growth is proportional to the square of the electric field's strength – as opposed to the linear electro-optic Pockels effect in aniso- tropic substance. For symmetry, the double refraction's optical axis is in the field direction. The fabric's normal refractive index n without an electrical field is modified by applying a field in the parallel oszillation direction to the extraordinary refractive index ne and in the perpendicular oszillation direction to the ordinary refractive index no. The two indices have the following rela- tionship: