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Chromic PhenomenaChromic phenomena are those phenomena in which colour is produced when light interacts with materials in a variety of ways. These can be categorised under the following five headings: - Reversible colour change
- The absorption and reflection of light
- The absorption of energy followed by the emission of light
- The absorption of light and energy transfer (or conversion)
- The manipulation of light.
Phenomena which involve the change in colour of a chemical compound take their name from the type of external influence, either chemical or physical, which is involved. Many of these phenomena are reversible. They are: - Photochromism - colour change caused by light.
- Thermochromism - colour change caused by heat.
- Electrochromism - colour change caused by an electrical current.
- Solvatochromism - colour change caused by solvent polarity.
- Ionochromism - colour change caused by ions.
- Halochromism - colour change caused by a change in pH.
- Tribochromism - colour change caused by mechanical friction.
- Piezochromism - colour change caused by mechanical pressure.
Classical dyes and pigments produce color by the absorption and reflection of light; these are the materials that make a major impact on the colour of our daily lives. In 2000, world production of organic dyes was 800,000 tonnes and of organic pigments, 250,000 tonnes. There is also a very large production of inorganic pigments. Organic dyes are used mainly to colour textile fibres, paper, hair, leather, while pigments are used largely in inks, paints and plastics. The absorption of energy followed by the emission of light is often described by the term luminescence. The list of these phenomena is long, and many are widely used in consumer products and other important outlets. Cathodoluminescence is used in cathode ray tubes, photoluminesence in fluorescent lighting and plasma display panels, phosophorescence in safety signs and low energy lighting, fluorescence in pigments, inks, optical brightners, safety clothing, and biological and medicinal analysis, chemiluminescence and bioluminescence in analysis, diagnostics and sensors, and electroluminesence in the burgeoning areas of light emitting diodes (LEDs), displays and panel lighting. Absorption of light and energy transfer (or conversion) involves coloured molecules that can transfer electromagnetic energy, usually from a laser light source, to other molecules in another form of energy, such as thermal or electrical. These laser addressable colorants are used in optical data storage, organic photoconductors, in photomedicine (such as photodynamic therapy of cancer), and in solar energy utilisation. Materials may be used to manipulate light via a variety of mechanisms. For instance, a change of orientation of molecules as in liquid crystal displays, by interference and diffraction as in holography, and by modifying the movement of light through materials by electrical means, as in opto-electronics, or by optical means in photonics.
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