Photosensitive refers to the ability of certain materials to change their physical or chemical properties when exposed to light. This effect has been exploited in a variety of ways, from photographic film and solar cells to optical fiber communications and medical diagnostics. In general, light exposure causes molecules in the material to become excited, leading to changes in properties such as color, conductivity, or fluorescence.
The term “photosensitive” is often used interchangeably with “photochemical,” but there is a distinction between the two terms. A photosensitive material is one that changes its properties upon exposure to light, while a photochemical material is one whose chemical composition is changed by light exposure. For example, many photochromic glasses are photosensitive (their color changes upon exposure to sunlight), but they are not photochemical (the composition of the glass does not change). Likewise, some photographic films are photochemical (the film negatives are chemically altered by exposure to light), but they are not necessarily photosensitive (the photographs themselves do not change color).
There are two main types of photosensitivity: intrinsic and extrinsic. Intrinsic photosensitivity refers to materials that naturally exhibit this property, while extrinsic photosensitivity refers to materials that have been artificially treated to make them sensitive to light. Many common materials can be made extrinsically photosensitive through a process called sensitization, which usually involves coating the material with a light-sensitive compound known as a sensitizer. For example, glass can be sensitised with silver halide compounds to produce photographic film; similarly, silicon wafers can be sensitised with dopants such as boron or phosphorus for use in solar cells.
One of the most important applications ofphotosensitivityis in photography. Early photographers experimented with various techniques for capturing images on sensitized surfaces such as metal plates or glass slides coated with collodion (a mixture of gun cotton dissolved in ether and alcohol). These methods were eventually replaced by the more convenient roll film introduced by George Eastman in 1884; this was essentially a long strip of paper coated with silver halide emulsion and wound onto a spool for easy transport and storage. Today’s modern cameras still use this basic principle, although the Film Negative has largely been replaced by digital image sensors which capture pictures electronically instead of on an exposed piece of film.