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In a broader sense, general names of colorants for fibers, etc. In a narrow sense, it means just colored matters soluble in water/oil, which can be broken down to a single molecule to be combined with molecules of fiber, etc. for coloring. It is distinguished from Pigments, which do not dissolve in water/oil, but make opaque color screen on the surface of substances as powder. Differing from each substance, same colored matters (colors) can be used as dyes or pigments. Dyes have been used for a long time and dyeing using indigo plant was done around 2,000 BC already. Until synthetic dyes were industrialized in the U.K. one year after W.H. Perkin's success in compounding Mauve, the first synthetic dyes, in 1856, natural dyes have been mainly used. Natural dyes, having low fastness, unclear tint, and complicated dyeing methods, have been gradually substituted for synthetic dyes and used for special purpose at present, e.g. crafts, etc. From the late 19th Century, the foundation of synthetic dyes was laid by Industrial Methods of Aniline black by L. Lighthood in 1866, Alizarin synthesis by Graebe in the same year, the first direct dyes "Congo Red" synthesis by P. Boetiger in 1878, Indigo synthesis by A. Bayer in 1880, Sulfur dye manufacturing by H.R. Bydal in 1893, and Anthraquinone system acid dye invention such as Alizarin Saphirol B of R.E. Schmite in 1897, etc. In between, the development of organic chemistry such as the discover of coupling reaction by P. Greece, Chromophore theory of O.N. Beat, Indigo Molecule structure settlement by Bayer, etc. contributed to the development of synthetic dyes. In view that Organic chemistry, especially, organic synthetic chemistry and synthetic dyes were developed helping each other, today's development of organic chemistry was owed to the improvement of synthetic dyes. Many Vat Dyes were improved in Germany in the 20th Century and Germany produced about 80% of dyes throughout the world. Since the technology of synthesizing dyes was applied to the manufacturing process of explosives or poison gas, with the World War I as a momentum, each country made efforts to develop dye industry and thereby dye industry was spread around the world. Insoluble Azo dyes "Naphtol AS" in 1912, Synthesized metal dyes in 1915, Acetate dyes in Anthraquinone system in 1930, etc. were developed and marketed. After the World War II, fluorescent white dyes, neutral dyes containing metal, dyes for synthetic fiber, reactive dyes, etc. were manufactured in U.S.A. and around the Europe.

Human beings had used dyes to color our body with natural colorants (iron oxide, hydroxide, manganese oxide, etc.) in order to reveal the dignity long before dyeing clothes. This custom is inherited until now. The fact can be found on the wall painting of the Old Stone Age. Human beings began to wear clothes from the New Stone Ages and it was presumed that coloring with mineral colors and plants' sap had begun about 7,000 to 8,000 years ago. Thereafter, it was said that indigotin extracted from indigo plant was in use in India, red colors in the Old Testament, tyrian purple from Murex or snail shell was in use by the Mediterranean people. It was recorded that Chinese were in use of dyes around 2,600BC. Dyes in Ancient times were mainly vegetable colors. In Korea, vegetable colors were used for Buddhist statues from the Goguryo Dynasty and the Shilla Dynasty. Alizarin for red, Catechu for yellow, Acorn for brown, Gallnut for grey, Corydalis incisa for purple, etc. were used. Alum was also used as mordant. Since, however, the kind and the quantity of natural colors gained from seashells and/or plants were limited and not enough, synthetic dyes have been requested. The first synthetic dyes, Mauve, basic dyes compounded from coal tar, by-products of coal dry distillation, were compounded by Perkin of England. Since then, numerous dyes were invented and more than 9000 kinds of dyes were registered on the Color Index as of now, however many dyes among them were not produced after registration since much superior dyes were invented. At present, it is presumed that around 1,500 to 2,000 kinds of dyes are in use in industry. Study in industrialized dyes compound were expanded in Germany and Switzerland and dyes were produced by Bayer (year 1862), MLB (year 1862, Hoechst (Dystar) at present), BASF (year 1865), Cassler (year 1870), Agfa (year 1973) in Germany and Geigy (year 1863), Ciba (year 1885), Sandoz (year 1886) in Switzerland, respectively. From Mauve, the first synthetic dyes, announced in 1856, to reactive dyes for cellulose fiber announced in 1956, just 100 years after from Mauve announced, dyes showing various performances and use have been developed and diversified.

The emission of light when a substance is stimulated by light. It reveals different tint from reflective color or transmission color of a substance and has longer wave than irradiation light in general. The green color shown on red ink as observed under the sunlight and milk-blue color of kerosene can be examples and, especially, if lights with plenty of ultra-violet ray were used, many materials show fluorescence to some extent. Fluorescent lamp is in use of fluorescent (visible radiation), which is formed by the stimulus of ultra-violet rays generated in the tube. Except light, X ray, Radiation, Cathode rays, etc. can cause to generate fluorescent. Phosphorescence continuously laminates after getting rid of irradiation light when light is irradiated, but Fluorescence disappears when irradiation light is eliminated. In many cases, they are distinguished from each other per this property. Phosphorescence can be frequently seen in solid substances, but fluorescence appears in liquid or gas. Generally, the brightness of Phosphorescence is diminished under low temperature. However, that of fluorescence is not changed and rather increased. The physical process that a substance generates fluorescent is treated as a phenomenon that a fluorescent substance absorbs solar-energy and radiates a part of the solar-energy once again. Since, in general, the shorter is the wave, the stronger is the solar-energy, the Stokes' law that the wave of fluorescence having lower energy than the stimulus light has longer wave than that of the stimulus light can be explained by this phenomenon. The spectrum of fluorescent is called as fluorescent spectrum and it generally becomes line spectrum in gas, band spectrum in liquid, and continuous spectrum in narrow range in solid. This explains that the solar-energy exchanging process is not simple in molecules and the theoretical support of this is not sufficiently achieved yet.

[How to Use] There are fluorescent pigments, which are used in fluorescent lamp or in various purposes. Fluorescent bleaching agents looks as fluorescent dyes shining in celadon color by ultra-violet ray, and they make a fiber being quite yellow look white as its complementary color, celadon. Plastics injection with fluorescent substances are widely used for rear light of cars or traffic signals, etc. Fluorescent analysis, detection of certain substances by using material itself, or its compound, or fluorescence derivatives, is in use of fluorescent, and, per these methods, microelements can also be analyzing out. In addition, it is used in wide scopes, for example, to detect a scar of material by finding out fluorescent liquid remained a surface of the materials by irradiating ultraviolet ray in the dark room, and to manufacture fluorescent microscope, etc.

In a reactive dye a chromophore contains a substituent that is activated and allowed to directly react to the surface of the substrate. Reactive dyes first appeared commercially in 1956, after their invention in 1954 by Rattee and Stephens at the Imperial Chemical Industries (ICI) Dyestuffs Division site in United Kingdom. Reactive dyes are used to dye cellulosic fibers. The dyes contain a reactive group, either a haloheterocycle or an activated double bond, that, when applied to a fiber in an alkaline dye bath, forms a chemical bond with hydroxyl group on the cellulosic fiber. Reactive dyeing is now the most important method for the coloration of cellulosic fibers. Reactive dyes can also be applied on wool and nylon; in the latter case they are applied under weakly acidic conditions. Reactive dyes have a low utilization degree compared to other types of dyestuff, since the functional group also bonds to water, creating hydrolysis.

An acid dye is a dye in which the coloring component is in the anion. They are often applied from an acidic solution in order to intensify the staining. In the laboratory, the home or art studio, the acid used in the dyebath is often vinegar (acetic acid) or citric acid. The uptake rate of the dye is controlled with the use of sodium chloride. In textiles, acid dyes are effective on protein fibers, i.e. animal hair fibers like wool, alpaca and mohair. They are also effective on silk. They are effective in dyeing the synthetic fiber nylon but of minimal interest in dyeing any other synthetic fibers. Acid dyes are generally divided into three classes which depend on fastness requirements, level dyeing properties and economy. The classes overlap and generally depend on type of fiber to be colored and also the process used. Acid dyes are thought to fix to fibers by hydrogen bonding, Van der Waals forces and ionic bonding. They are normally sold as the Sodium salt therefore they are in solution anionic. Animal protein fibers and synthetic Nylon fibers contain many cationic sites therefore there is an attraction of anionic dye molecule to a cationic site on the fiber. The strength (fastness) of this bond is related to the desire/ chemistry of the dye to remain dissolved in water over fixation to the fiber.

Anthraquinone dyes are dyes derived of anthraquinone. This type dyes are excellent dyes having good fastness and light resistance, so these dyes are very important along with synthetic dyes. Anthraquinone dyes are classified according to dyeing properties.

1) Anthraquinone mordant dye This is representative dye in mordant dye with high fastness but is not useful in these times because of hardness in dyeing process. Alizarin, Alizarin-red, alizarin-blue, alizarin-blue black, anthracene-brown, and anthracene-blue are very important in this category.
2) Anthraquinone acid dye We can get outstanding color in the range of green to blue as anthraquinone acid dye. It has desirable fastness and is important in dyeing wool, silk, and synthetic fiber. Alizarin-sapirol and alizarin-cyanine-green are representative dyes.
3) Anthraquinone vat dye It is called as indanthrene dye. It has the highest fastness degree in synthetic dyes but is very expensive because it is very difficult to synthesis. Examples are indanthrene-blue and indanthrene-briliant-green in this type dye.

A leuco dye is a dye whose molecules can acquire two forms, one of which is colorless. Triphenylmethan dye and sulfide dyes are produced leuco dye by reducing the dye and are capable of being reconverted to the original dye by oxidation. It is different from general azo dye which is decomposed to two amines after reduction. Dyes are reduced with sodium hydroxide and hydrosulfite(Na2S2O4) and absorbed in fabric. It can be reconverted to the original dye with color. Indigo type leuco compound has no color but anthraquinone type vat dye has its original color

It is not soluble in water but soluble in oils or various organic solvent. It has no hydrophilic functional group in molecule. It belongs to azo dye and is not useful in textile dyeing but in coloring of gasoline, plastic, butter, and etc.