Discharge Current Density

The lamp is at its most efficient when run at low current density, i.e. low wattage per unit volume and length of the discharge tube. This necessitates a long discharge tube of large diameter. There are limits to how large the discharge tube can be proportioned because of a troublesome phenomenon that sodium vapour is opaque to its own resonance radiation. Thus if the tube diameter was too large, light created at the core would never reach the surface. This would offset the beneficial effects of reduced current density by the negative aspects of increased self-absorption. For practical puproses it is necessary to make a compromise between current density and light absorption. There is an optimum discharge tube diameter for each rating of lamp, but for practical manufacturing two different diameters have been standardised upon for the modern lamps - a small diameter for lamps having a discharge current of 0.6A, and a larger diameter for the 0.9A types.

Fortunately the self-absorption effect is not so catastrophic as it might sound, due to a second beneficial process that occurs in low pressure discharge lamps. During operation, the inner wall of the discharge tube acquires a small negative electrical charge due to accumulation of free electrons at its surface. To offset this charge imbalance, the positive sodium ions are also drawn towards the wall. The process is called ionic pumping and is clearly illustrated in Figure S10 which shows part of an SO/H discharge tube during the early phases of run-up.

At this stage when the total quantity of sodium ionisation is low and the discharge is still predominately taking place in the rare gas filling, it is plain to see that the sodium ionisation is taking place mainly at the inner surface of the glass tube - irrespective of whether or not there happens to be a sodium droplet in the vicinity. When the lamp has fully run up, the result is that much of the light is not generated in the core of the discharge but instead close to the surface of the glass. As a result it has to travel through a relatively short length of sodium vapour and there is not so much self-absorption as would be the case if the ionic pumping phenomenon did not exist. In conclusion the effects of self-absorption are still very important, but thankfully they are minimised due to the benefits of ionic pumping. Certain linear lamp types employed specially dimensioned discharge tubes in order to further reduce losses due to self-absorption.

The earliest positive column sodium lamps of the SO/H family had relatively poor thermal insulation, and a very small diameter tube was required just to get the glass up to the required 260°C wall temperature. This limited lamp efficacy because of the resulting high current density in the discharge tube, but it was a necessary sacrifice to achieve the correct sodium vapour pressure.

Later developments in sodium lamps have focussed heavily on improving the luminous efficacy, and this has been accomplished principally by improving the thermal insulation. The efficacy increases that have resulted are not so much due to the fact that the superior insulation results in less power loss, moreover these improvements have facilitated the discharge tube diameter to be increased while maintaining the critical 260°C wall temperature. The greater diameter discharge tube is the key factor that resulted in improved lamp efficacy, thanks to the fact that the electrical current density could then be reduced.

This is evidenced by the earliest SOI/H lamps, having Integral vacuum jackets sealed directly around the disharge tube. In the original Philips lamps lamps, the discharge tube maintained substantially the same proportions as the SO/H predecessors, and the lamp showed almost no efficacy improvement. An example of such a lamp can be seen here. The integral outer jacket was offered only because of the improved mechanical design of the lamp. Meanwhile in the developments of Osram-GEC, the discharge tube diameter was increased in parallel with the transition to the integral outer jacket thanks to the provision of a pair of thermally insulating glass sleeves. A typical example is shown here. The GEC integral lamps offered considerably greater luminous efficacy than the Philips integral lamps. Naturally at a later date, Philips followed suit and implemented a similar improvement in its thermal insulation to permit an increase of the discharge tube diameter.

With the Osram-GEC SOI/H development of 1955, the tube diameter had been increased to the point where any further gain would have begun to reduce efficacy due to self-absorption of the light. Some very innovative work was then commenced by BTH Mazda, who created a range of linear sodium lamps having discharge tubes of non-circular cross-section. These were much larger diameter tubes delivering low current density for optimum efficacy, but their special shaped sections ensured that the glass was closer to the discharge enabling the light to be transmitted more efficiently. These unique light sources are described fully under the Linear Sodium category.