Photochromic Material

in photography, a material in which the photochromism of organic or inorganic substances is used. Photochromic materials constitute a new type of light-sensitive materials for the recording of images and for the recording and processing of optical signals; they became popular in the 1960’s. Depending on the field of application, photochromic materials may be manufactured as liquid solutions, polymer films, amorphous or polycrystalline thin films on flexible or stiff substrates, silicate or polymer glasses, or single crystals.

The most widely used photochromic materials are polymer materials based on the following: organic compounds, such as spiropyrans and dithizonates of metals; silicate photochromic glasses containing silver halide microcrystals, for example, AgBr or AgCl; activated crystals of alkali metal-halide compounds, for example, KC1, KBr, or NaF; and doped salts or oxides of alkaline-earth metals, such as CaF₂/La, Ce and SrTiO₃/Fe + Mo.

The use of a photochromic material in photography depends on the following: whether the material has an exceptionally high resolving power—in theory, the smallest size of a picture element may be of the order of the size of a molecule or of a unit cell of a crystal, that is, less than 1 nanometer; whether the material offers the possibility of producing an image immediately upon exposure to light, that is, virtually on a real-time scale (the recording time is limited by the duration of elementary photophysical processes and may be less than 10^–8 sec); whether the storage time for the recorded information varies over a wide range, for example, from 10^–6 sec to months or years; and whether the material offers the possibility of rerecording and revising an image by means of light or heat. Depending on the type of photochromic material, a negative or positive multicolor image may be obtained upon exposure to radiation in the range from X rays to microwaves.

The photosensitivity of photochromic materials is four to seven orders of magnitude lower than that of silver halide photographic materials. Therefore, photochromic materials are of special interest for use in laser systems, which provide for the recording and processing of optical information in strong radiation fluxes on a real-time scale.

Besides the traditional fields of photography, photochromic materials are used in display systems for dynamic information; in systems for high-speed optical processing of optical and electric signals; as computer memory elements, for which the fast response and repeated use of photochromic materials are especially important; in microfilming and micro recording systems; in holography, where the high resolution of photochromic materials is especially significant; and for photographic masking in color photography and printing, where the materials may be used to produce corrective spectral or outline masks at the moment of exposure or printing. Photochromic materials are also employed in optical electronics, dosimetry, and actinometry, as well as in optical shutters that automatically change the transmission of light as a function of the level of illumination.