Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through, is emitted from, or is reflected from a particular area, and falls within a given solid angle.
Brightness is the term for the subjective impression of the objective luminance measurement standard (see Objectivity (science) § Objectivity in measurement for the importance of this contrast).
The SI unit for luminance is candela per square metre (cd/m2), as defined by the International System of Units (SI is from the French Système international d'unités) standard for the modern metric system. A non-SI term for the same unit is the nit. The unit in the Centimetre–gram–second system of units (CGS) (which predated the SI system) is the stilb, which is equal to one candela per square centimetre or 10 kcd/m2.
Violet is the color at the end of the visible spectrum of light between blue and the invisible ultraviolet. Violet color has a dominant wavelength of approximately 380–450 nanometers. Light with a shorter wavelength than violet but longer than X-rays and gamma rays is called ultraviolet. In the color wheel historically used by painters, it is located between blue and purple. On the screens of computer monitors and television sets, a color which looks similar to violet is made, with the RGB color model, by mixing red and blue light, with the blue twice as bright as the red. This is not true violet, for it does not match the color of a single wavelength shorter than that of blue light.
The color's name is derived from the violet flower. Violet and purple look similar, but violet is a spectral color, with its own set of wavelengths on the spectrum of visible light. Purple is a dichromatic color, made by combining blue and red. Amethyst is a notable violet crystal, its colour arising from iron and other trace elements in quartz.
In history, violet and purple have long been associated with royalty and majesty. The emperors of Rome wore purple togas, as did the Byzantine emperors. During the Middle Ages violet was worn by bishops and university professors and was often used in art as the color of the robes of the Virgin Mary. In Chinese painting, the color violet represents the "unity transcending the duality of Yin and yang" and "the ultimate harmony of the universe". In Hinduism and Buddhism violet is associated with the Crown Chakra. According to surveys in Europe and the United States, violet is the color people most often associate with extravagance and individualism, the unconventional, the artificial, and ambiguity.