Pigment

A pigment is a material that changes the color of reflected or transmitted light as the result of wavelength-selective absorption. This physical process differs from fluorescence, phosphorescence, and other forms of luminescence, in which a material emits light. Most materials selectively absorb certain wavelengths of light. Materials that humans have chosen and developed for use as pigments usually have special properties that make them useful for coloring other materials. A pigment must have a high tinting strength relative to the materials it colors. It must be stable in solid form at ambient temperatures.... For industrial applications, as well as in the arts, permanence and stability are desirable properties. Pigments that are not permanent are called fugitive. Fugitive pigments fade over time, or with exposure to light, while some eventually blacken. Pigments are used for coloring paint, ink, plastic, fabric, cosmetics, food, and other materials. Most pigments used in manufacturing and the visual arts are dry colorants, usually ground into a fine powder. For use in paint, this powder is added to a binder (or vehicle), a relatively neutral or colorless material that suspends the pigment and gives the paint its adhesion. A distinction is usually made between a pigment, which is insoluble in its vehicle (resulting in a suspension), and a dye, which either is itself a liquid or is soluble in its vehicle (resulting in a solution). A colorant can act as either a pigment or a dye depending on the vehicle involved. In some cases, a pigment can be manufactured from a dye by precipitating a soluble dye with a metallic salt. The resulting pigment is called a lake pigment. The term biological pigment is used for all colored substances independent of their solubility. In 2006, around 7.4 million tons of inorganic, organic and special pigments were marketed worldwide. Asia has the highest rate on a quantity basis followed by Europe and North America. The global demand on pigments was roughly US$20.5 billion in 2009. Pigments appear colored because they selectively reflect and absorb certain wavelengths of visible light. White light is a roughly equal mixture of the entire spectrum of visible light with a wavelength in a range from about 375 or 400 nanometers to about 760 or 780 nm. When this light encounters a pigment, parts of the spectrum are absorbed by the pigment. Organic pigments such as diazo or phthalocyanine compounds feature conjugated systems of double bonds. Some inorganic pigments, such as vermilion (mercury sulfide) or cadmium yellow (cadmium sulfide), absorb light by transferring an electron from the negative ion (S2−) to the positive ion (Hg2+ or Cd2+). The other wavelengths or parts of the spectrum are reflected or scattered. The new reflected light spectrum creates the appearance of a color. Pigments, unlike fluorescent or phosphorescent substances, can only subtract wavelengths from the source light, never add new ones. The appearance of pigments is intimately connected to the color of the source light. Sunlight has a high color temperature and a fairly uniform spectrum and is considered a standard for white light, while artificial light sources tend to have strong peaks in parts of their spectra. Viewed under different lights, pigments will appear different colors. Color spaces used to represent colors numerically must specify their light source. Lab color measurements, unless otherwise noted, assume that the measurement was taken under a D65 light source, or 'Daylight 6500 K', which is roughly the color temperature of sunlight.