Photochromic glasses are mainly used for selfÃ¢â‚¬â€adjusting sunglasses. Thus, the photochromic materials have to fulfil several specifications in order to be useful for the human eye. At low light intensity they should be transparent enough, whereas at full solar power their transmittance should be reduced to about those values known for conventional sunglasses, e. g., values in the range between 20 and 30 %. The transmittance should be reduced in the whole visible spectral region uniformly as much as possible in order to avoid too large a shift of the color stimulus. Since these glasses should darken especially under solar irradiation, the darkening mechanism must be induced in the UV range of the solar spectrum which is not seen by the human eye. The IR is not to be used for that purpose, since incandescent lamps are emitting in that spectral range and the photochromic glasses are to remain in the transparent state under that illumination. The time constants for the darkening and regeneration kinetics should be fast enough to make these glasses useful. There are additional prerequisites, such as chemical durability, optical homogeneity, low light scattering and low sensitivity of the optical properties to variations of the ambient temperature.