Abb. 2. ElektronenrnikroskopischeAufnahmen phototroper silberhalogenidhaltiger Gliser. Links: Glas Nr. 2 der Tabelle 1; 2 Std. bei 610°C angelassen, Bruchoberflache geatzt 60 s in 2-proz. HF, Kohle-Platin-Direktabdruck, VergroRerung 12000-fach. Rechts: Glas Nr. 7 der Tabelle 1 ; 1 Std. bei 570 "C angelassen, ionengeatzt, Durchstrahlungsaufnahme, VergrdRerung 48 000-fach. die silberhalogenidreichen Ausscheidungen (80 bis 100 A) durch ihren Silbergehalt deutlich zu erkennen.Am Beispiel des Glases Nr. 7 der Tabelle 1 kann gezeigt werden, daB die silberhalogenidreichen Ausscheidungen nicht unbedingt makrokristallin sein mussen, um gute phototrope Eigenschaften des Glases

Phototropic glass darkens on exposure to light and returns to its original state in the absence of light. The most extensively investigated of these glasses contain grains of silver halide having sizes > 50 A. A discussion of theoretical principles is followed by a review of the very broad scope of the phenomenon. Phototropy and Phototropic MaterialsPhototropy is the reversible change between states having different absorptions that is induced by light (photons), or more generally by electromagnetic radiation. Photochromy is a special case of phototropy in which at least one of the states absorbs visible light. Phototropy, as a photochemical phenomenon, obeys the Grothus-Draper law, i.e. only the light absorbed by the phototropic substance is effective. Under continuous exposure, the color changes until an equilibrium is reached between the forward and the reverse reactions. Phototropy is reversible. When the light is switched off, the system returns to its original state:colorless exposure ' absence of lightThe wavelength of the light causing a change in the light transmission and the rate at which the process takes place depend on the nature of the substance and on the temperature. A temperature rise displaces the absorption band to longer wavelengths. Higher temperatures also accelerate the regeneration reaction, which proceeds simultaneously with the color reaction and opposes the phototropic process.Many inorganic compounds, of e.g., mercury 111, copper [21, zinc, and chromium, molybdenum, and tungsten 131, exhibit phototropic behavior in the solid state or in solution. When the mineral sodalite Na8[A16Si6024]C12 is exposed to X-rays it turns blue; the colorless state is regenerated on heating to 450 "C. (For reviews on organic and inorganic phototropic materials, see US-Pat. 2503758 (1950); US-Pat. 2503759 (1950).[21 A . G . Gulko, US-Pat. 3081 699 (1953).[3] E . Lind, German Pat. 1166795 (1964).[4] R . Exelby and R . Grinter, Chem. Reviews 65, 247 (1965). (51 G. Jackson, Optica Acta 16, 1 (1969).Several authors have described inorganic glasses having phototropic properties. Phototropic glasses must not be confused with photosensitive glasses, in which irreversible images are produced as in'photographic plates. Phototropic glasses are sometimes called "photochromic" in the American literature.Cohen and Smith studied the change in the transmission of sodium silicate glasses containing traces of europium.A more comprehensive description of these processes in silicate glasses with variable transmissions is given by Co- %hay and Arufu [lo1 observed a thermally reversible photochemical reaction in borate glasses containing arsenic or manganese on exposure t o 6OCo radiation. Sukka described the phototropy in alkaline earth metal tungstates sensitized with bismuth. S?roudI1*1 investigated the reversible formation of color centers in silicate glasses containing cerium. Swarts and Presseuu "31 reported the phototropy of reduced silicate glasses with a color center at 570 nm. Similar studies have been carried out...