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. |
