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Fluorescent Lamp Basics

The fluorescent lamp is a form of low pressure mercury discharge lamp. It usually takes the form of a long glass tube coated on its inner surface with a fluorescent powder or phosphor. At each end of the tube is a lamp cathode. The cathode consists of a coiled tungsten heater coated with special oxides of Barium and Strontium which readily emit electrons when heated. Attached to each cathode are two protective plates which prevent the destruction of the heater coil by the bombardment of positive ions during the discharge. The glass tube is sealed at both ends and contains a small amount of mercury and an inert gas at low pressure. The gas can be argon, krypton or a mixture thereof.

To start the discharge, the cathodes are heated causing a cloud of electrons to be released. A high surge voltage is then applied between the cathodes which strikes the lamp. On striking, the heat produced by the discharge vaporises the mercury and the potential difference across the lamp falls to normal operating voltage. During operation, collisions with the stream of electrons cause the mercury vapour atoms to be excited or ionised. If ionised, positive ions and free electrons are formed which may cause more collisions. Excitation occurs when the electrons within an atom are raised to a higher energy state than normal but not sufficiently high to cause ionisation. An atom cannot remain in this excited state for long and when the electrons return to their previous energy level, ultra-violet light is released. This ultra-violet light is absorbed by the lamp's phosphor coating and re-radiated as visible light. The colour rendering properties and colour temperature of the visible light produced is dependant upon the phosphors used.

It should be noted that during each starting cycle, a quantity of the emissive material is lost from each cathode. This material tends to pollute the lamp gas and phosphor coatings and is noticeable in older lamps as a dark band around each cathode. This pollution leads to a progressive reduction in the output of the lamp (lumen depreciation). When there is no longer enough electron emissive material to provide the correct volume of free electrons during startup, the lamp will no longer strike. A break in a lamp cathode will also under normal circumstances prevent a lamp from striking.

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