Lamp Lifetime

It is relatively complex to specify the lifetime of a fluorescent lamp, because it depends on several variables and can be defined in different ways. Contrary to other light sources they do not always operate for long enough to reach an abrupt failure point, but may gradually lose their initial performance as they age. Often the light output may decrease so much that the tube is considered useless and must be replaced even though it is still producing some light. Alternatively the radiated light spectrum can change such that they no longer deliver the desired colour. The useful lifetime of linear fluorescent lamps can vary from 6,000 hours to 60,000 hours depending on their construction, the operating conditions and type of control circuit.

The principal failure mechanism is a question of mortality and relates simply to exhaustion of the emissive coating on the electrodes at the tube ends. When this point is reached the lamp will fail to start. Lamps do not all reach this point at the same time - minute variations in manufacture lead to some achieving a longer or shorter life than others. The useful lifetime is expressed in terms of a mortality curve, which illustrates the percent surviving lamps as a function of burning hours. This is illustrated for Halophosphate and Triphosphor T8 in the diagram below. The rated lifetime is usually given as when 50% of the tubes can be expected to have failed.

The mortality curve can be influenced very greatly by the switching frequency, and by the type of control gear. Lamp ignition applies the greatest stress to the electrodes, and considerable quantities of electron emissive materials may be lost in the few seconds after starting. It is possible to reduce this degradation by preheating the cathodes prior to application of the voltage pulse to strike the discharge. With old switch-start control gear systems, the quality of the starter switch determines the extent and duration of cathode preheating. Electronic ballasts having programmed starting provide much softer ignition and can greatly extend tube life. Instant start systems are the most destructive and cause a significant reduction in lamp life.

The second measure of lamp life is that of lumen depreciation. This is quite rapid for the older halophosphate coatings, whereas modern Triphosphor materials achieve an admirable lumen maintenance. End of life is generally specified when the luminous flux has dropped below 70% of the original value.

From the 1990s onwards, as environmental regulations began to require significant reductions in the quantity of mercury dosed into each lamp, a relatively new failure mechanism of mercury starvation became apparent. This did not affect older lamps, which were always dosed to great excess with mercury.

During life mercury is consumed by various components, but above all by the glass wall. If the initial dose is sufficiently small, the lamp may run out of mercury before either of the two principal failure mechanisms has ocurred. When there is no available mercury remaining, the lamps acquire a pinkish appearance due to the colour of the discharge in the argon gas filling, and the absence of UV-C radiation for stimulation of the phosphor coating.

The rate of mercury loss has been decreased to suitably low levels only by the most advanced lamp manufacturers, which apply a pre-coating onto the glass to reduce the rate of mercury loss. In parallel, the lumen depreciation of such lamps is almost halted. The mechanism of mercury loss is due to its amalgamation with sodium ions contained within the glass. The operation of the electric discharge causes a negative charge to appear on the inner wall of the glass, which draws sodium ions to the surface. Some of these then form an amalgam with the mercury which not only binds it such that it can no longer take part in the discharge, it also causes a greyish coating to appear over the glass which reduces its light transmission.

The solution is to apply a very thin coating to the inner surface of the glass, consisting a few nanometres thickness of aluminium oxide in the form of Alon C powder. The coating must have extreme structural integrity and freedom from pinholes or cracks. This acts as a barrier between the mercury and sodium and prevents amalgamation. This pre-coating technology has allowed mercury weights to be drastically reduced, while also achieving exceptionally high lumen maintenance during life.