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Colour Rendering Index (CRI): ranging from 0 to 100, it indicates how perceived colours match actual colours. The higher the colour rendering index, the less colour shift or distortion occurs. back Colour Temperature of a source: expresses its colour appearance. The higher the temperature, the cooler appears the light source. Colour temperatures of 4000 K or higher appear white and cool; colour temperatures of less than 3000 K have a warm colour appearance e.g. incandescent lamps. back Discharge lamp: lamp in which the light is produced by an electric discharge through a gas, a metal vapour or a mixture of gases and vapours. All discharge lamps have to operate with a ballast in their electric circuit. This is to control the lamp current. back Fluorescent lamps: they consist of a sealed glass tube, coated on the inside with phosphors and filled with an inert gas and a small quantity of mercury. An electrical discharge within the tube excites the mercury atoms which emit radiation predominantly in the ultra violet. This UV radiation is converted to visible light by the phosphors. Fluorescent lamps are available with different diameters, inert gas fillings and phosphor coatings. The colour properties of a fluorescent lamps are determined by the phosphors used to coat the inside of the tube. A mixture of phosphors is used to produce a white colour appearance, but this can vary in colour temperature depending on the relative proportions of the phosphors in the mixture. The phosphore mixture also determines the colour rendering properties of the lamp. All fluorescent lamps require ballasts to provide appropriate electrical conditions for starting and control of the discharge. Linear fluorescent lamp (or tube): fluorescent lamp of straight tubular form and bipin electrical connections at either end. High pressure mercury lamp (also called MBF): a high intensity discharge lamp in which the light is produced by an electric discharge through a vapour of mercury operating at higher pressure than in fluorescent lamps. Like in fluorescent lamps, the arc tube is filled with argon and a small quantity of mercury. A fluorescent coating on the inside of the outer envelope converts the long wave UV radiation into visible light. When operating the lamp, at first a low pressure arc exists and very little light is produced; but gradually, as the lamp heats up, the mercury vapour pressure rises and a high-pressure arc is formed and more light is emitted. The time taken for the lamp to reach full light output is approximetly 5 minutes. back High pressure sodium lamp (also called SON): a high intensity discharge lamp in which the light is produced by an electric discharge through a vapour of sodium operating at high pressure. back Incandescent lamp: a lamp where a filament is heated by an electric current to produce light. Tungsten standard lamp: an incandescent lamp whose filament is made of tungsten. Luminous efficacy: is defined as the ratio of visible radiation (or luminous flux) to power input and is given in lumens per watt (lm/W). back |