Fluorescent Lamps

The hot cathode fluorescent lamp consists of a narrow glass tube with its inner surface coated with fluorescent phosphor. It is principally filled with argon gas at low pressure and contains a trace of mercury. Tungsten cathodes coated with a thermionic emitter are sealed into each end of the tub. Bipin caps are fitted to each end and the tube is silicone coated to improve starting in damp, cold environments. Visible light is produced by conversion of short wavelength radiation to visible radiation by the phosphor coating on the tube. Most of the radiation generated from the arc in mercury vapour at low pressure is in the ultra violet region at a wavelength of 253.7nm a resonance line of mercury. When converted the lines in the visible spectrum have an effect on the light quality emitted by the lamp. The most efficient temperature of the tube wall is 45^oC and designers of luminaires should take this into account. An inert gas is included in the lamp to assist starting as the vapour pressure of the mercury is low. It also acts as a buffer to keep the electrons constantly in the discharge. The gas pressure must be carefully controlled at the correct value to avoid difficulties in starting. Short lamp life with poor lumen maintenance occurs at low values of pressure.

The T12 (38mm) range of fluorescent tubes have a pure argon filling as the buffer gas, which has good all around performance.

The T8 (26mm) and T5 (16mm) range of tubes uses krypton gas with a small addition of argon or neon. The heaver atomic weight vies better discharge control and thereby higher efficiencies. Krypton however is an expensive gas and krypton filled lamps are more difficult to start in cold conditions.

Current Control

Control of the lamps current is achieved by introducing an 'impedance' which may be an inductance, capacitance or a combination of both. In some circuits capacitors are fitted across the mains supply to correct the power factor. These are various forms of control gear, switch start, SRS quick start, also high frequency electronic circuits which are more efficient by 20% where starting is instantaneous and operation is flicker free.

Phosphors and Colour Rendering

Fluorescent tubes supply a very high percentage of the artificial light used in the UK. The colour rendering characteristics of any installation can be changed by simply chancing the fluorescent tube.

Colour rendering is the ability of the light source to reveal the true colours of an object. This is quite different to colour appearance e.g. white lamps have a cooler appearance compared with warm white.

White light can be reproduced by mixing red, green and blue light in appropriate proportions.

A triphosphor fluorescent tube is internally coated with layers or mixtures of red, green and blue rare earth phosphors and filled with krypton / argon to approx 1-2 Torr. This produces a fluorescent tube of high efficacy with good colour rendering properties. The phosphors and gases are relatively expensive, consequently these tubes cost more than the equivalent halophosphate coated lamps, but are 20% more efficient. Crompton triphosphor lamps are known as SPECTRA and SPECTRA-PLUS.

Life of a Fluorescent Tube

The electrical life of a standard halophosphate fluorescent tube can be typically greater than 10,000 hours but tubes should not be run to extinction since the phosphor, and hence light output deterioration over life is considerable. Standard Wattsaver tubes should therefore be replaced at 7,500 hours.

Spectra-plus triphosphor lamp life is typically 14,000 hours. Again replacement is recommended earlier at around 11-12,000 hours, in order that the installation quality is maintained. Life can be extended by up to 25% when warm start, high frequency circuits are use (such as those in Crompton HF luminaires).

The optimum combination, therefore is Spectra plus lamps and warm start high frequency gear, providing long life and minimum reduction in lumen output.

High lamp current caused by either high mains voltages or hot running conditions (wall temperature above 45^oC) will shorten useful lamp life, as well as affecting colour and light output.

Very low mains voltages on conventional circuits will cause the cathodes to be insufficiently heated and emitter erosion will be rapid. Electronic circuits include supply voltage compensation and safe cut off circuitry. Life is reduced by very frequent switchings. Quoted lives related to the BS EN 60081 specified switching cycle of3 hours per start. As a rough guide each start reduces life by approximately 2 hours. A lamp started once an hour will last 5,000 hours if rated at 7,500 hours and if started once every 10 hours will last 10000 hours electrically.

All krypton filled tubes require starter switches, and if used at low temperature i.e. below 5^oC, striations may be noticeable. This condition is only temporary and should disappear as the lamp reaches operating temperature. Ideally they should be used in luminaires designed for them, then full advantage is taken of their narrow diameter which obstructs less light.

Colour Co-ordinates and correlated colour temperature

Silicone Coating

All tubes are coated with silicone which renders the glass surface waterproof. Droplets will not form on the tube due to condensation (or fog it used outside). A capacitor effect is set up between the discharge and the tube surface allowing high frequency oscillations to occur which ionises the gas filling for simple starting . This is most important in some of the older circuits fitted with instant start control gear.

Lumen Depreciation

Through the long electrical lives of fluorescent tubes it is inevitable that the phosphor will deteriorate in the very reactive 'plasma' of the discharge. In modern tubes the bulk of this fall off occurs in the first 100 hours and a further fall by 2000 hours at which point the light output begins to steady.

This effect has been reduced by the introduction of 'virtually no lumen depreciation' Triphosphor lamps. Crompton lamps of this type are called SPECTRA-PLUS.

Source : Crompton Lamps