photochromic system

Photochromic systems are substances that can exist in both colorless and colored states. Exposure to specific wavelengths of light causes them to change to a colored state, and they revert to colorless either in darkness or when exposed to different wavelengths of radiation. This reversibility is a key characteristic, allowing for instant imaging without processing.1

These systems are utilized in microrecording due to their molecular-level changes, resulting in grain-free images with resolution limited only by the optical system. Photochromic materials can be negative or positive working, and some dye images can be made permanent. Glasses containing metal compounds can also be photochromic; light breaks down compounds into metal, forming a visible image. Similarly, glasses with silver halide crystals darken when light decomposes the halide, forming silver deposits. This property is used in automatic light-control devices, such as sunglasses that darken in bright light.12