This set of hidden messages use a birefringent
material that is visible when placed between two cross polarized filters. The polarizing table provides a light source, a diffusing layer and two layers of linearly polarized
The two layers of linearly polarized filters are aligned at 90 degrees from each other to block the table illumination. Only objects placed between the filters that effect the polarization of light (optically anisotropic
materials) will allow the light to escape the top filter. This is the same principal used in LCD displays, gem and mineral identification, stress imaging and some medical diagnostics.