Gas Filling Types

All fluorescent lamps are essentially high vacuum lamps, filled to a very low pressure with one or more inert gases. The gas performs several key functions in lamp operation, the most important of which are the following:

1. It reduces the required starting voltage for the discharge, such that it can be easily struck on simple control gear. Different gases have different effects - for instance with Argon the starting is very easy but with Krypton it is somewhat more difficult.
2. It acts as a buffer gas to protect the electrodes, reducing the impact of ions which bombard them during the anode phase, and thereby reducing erosion of the electode by sputtering and evaporation. This very greatly extends lamp life, the gases of higher molecular weight offering the greatest degree of protection of the electrodes.
3. It is adjusted to provide the right balance between rate of ionisation of mercury atoms (required to maintain the current flow through the lamp) and the excitation of mercury atoms so as to optimise the UV generation efficiency.
4. Its presence randomises the direction of motion of free electons in the discharge to control their mean free path length and velocity.

Depending on the lamp design and its intended application, the gas employed in commercially available products may be Argon, Krypton, Neon or Xenon - or invariably a mixture of at least two of these components at various different pressures.

The earliest fluorescent tubes employed pure argon as their gas filling, and indeed this is still employed in the majority of T12 (38mm diameter) tubes for general lighting purposes. Two T8 (26mm diameter) tubes employ an argon filling, these being the 15W 15" and 30W 36" types, which are today generally considered almost obsolete. The modern family of T5 long tubes are also based on argon fillings.

Argon is a relatively low cost gas which adequately performs all of the functions indicated above and yields high lamp effiacies with good service lifetime. The optimal fill pressure lies in the range of around 2 to 5 torr. The striking voltage to initiate the discharge is very low with argon, and these tubes can be operated on all types of electrical control gear.

During the energy crisis of the 1970s considerable efforts were invested in improving the efficiency of the fluorescent lamp, owing to the enormous energy savings that could be realised on account of its tremendous popularity. Many intriguing lamp concepts emerged during this period, but by far the most successful was the krypton-filled T8 lamp which acts as a retrofit for the conventional argon-filled T12 tubes.

Krypton offers a major advantage over argon in that the electrical losses at the electrodes are dramatically reduced - from approximately 15% down to just 7% of the total power dissipated. Proportionally more energy is therefore driven into the discharge column where it goes on to generate UV and useful light - see Figures 7 and 8 for a direct comparison. The optimal filling pressure is around 1.5 to 2 torr.

One problem which affected Krypton lamps at the time of their introduction was the fact that the lamp voltage is lower than for an argon-filled tube. Therefore when operated on existing ballasts, the krypton-filled lamps achieve a lower wattage rating and thereby deliver much less light. This problem was circumvented by reducing the diameter of the glass tube from T12 (38mm) to T8 (26mm) diameter, which has the effect of increasing lamp voltage and this was successful in bringing the lamp wattage almost back up to normal levels. In fact some wattage reduction for the T8 Krypton tubes was desirable so as to offer an energy saving of the T12 Argon originals, and the new wattages were established at levels approximately 10% lower. Due to the fact that the Krypton T8 tube is more efficient than the Argon T12 tube, the luminous flux is approximately the same even though its power consumption is much reduced. The only exception to this general rule is the 8-foot 100W tube, which is also a krypton-argon product intended to replace the original Argon 8-foot 125W tube, however at the time it was considered impractical to make such a long tube in T8 size glass owing to its fragility. Therefore the energy saving associated with this tube is 25%, and the luminous flux is somewhat lower than for the argon original.

Another difficulty with the Krypton T8 tubes is that this gas requires a higher starting voltage than argon - a problem which is compounded by the smaller tube diameter. To faciliatate starting the modern Krypton T8 tube actually consists of a 75%:25% Kr:Ar mixture. Nevertheless the starting voltage is still somewhat higher, and these tubes normally work only on switch-start or electronic ballasts.

Fillings of the noble gas Neon are employed in cases where a very high power loading per unit length of the lamp is necessary. Contrary to Krypton which shows a reduced volt drop per unit length, neon can be used to increase the tube voltage. It is used most frequently in the HO and VHO high output tubes which are popular for industrial lighting, particularly in North America, and also in other highly loaded tubes for special applications.

One difficulty with Neon is its low atomic mass, which offers little protection against the bombardment of the electrodes by high velocity ions during their anode phase of operation. Neon is particularly destructive towards lamp electrodes, and it is customary to employ additional anode wires or plates to take some of the load away from the cathode coil, and thus minimise the liftime reduction when using this gas.

A relatively recent development is the addition of Xenon to the gas filling, to achieve an additional energy saving over the krypton-filled T8 tubes. This technique is employed in the latest generation of 'Eco' series T8 lamps which offer around 10% energy savings over the establised T8 Krypton versions. Xenon causes a reduction in the discharge voltage, thus decreasing the power consumed by the tube on electromagnetic ballasts. It does not however account for a significant improvement in lamp efficacy - that is achieved in the Eco tubes by phosphor improvements.

One primary drawback that has been identified with Xenon-filled Eco tubes is their extreme sensitivity to ambient temperature variations. When operated under ideal conditions they can indeed achieve approximately 10% increase in efficacy over Krypton T8 types, but in many installations the light level is reduced dramatically if the ambient temperature is not suitable.