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.