Updated 28-XI-2011
Sodium Vapour
Spectral Properties
Lamp Technology
Vapour Pressure
Current Density
Gas Filling
Sodium Migration
Failure Mechanisms
Lamp Designs
Low Voltage Style
     Compton's Lamp
     Philora DC
     GE NA-9
High Voltage Style
     Philora AC
     SO/H U-Tube
     SOI/H Integral
     SOX/H Coated
     SLI/H Linear
Self-Starting Style
     Double Ended
     Single Ended
Control Gear
Series Operation
Autoleak Reactance
Ballast-Ignitor System
High Frequency Electronic

SOI/H Integral Design

To combat some of the disadvantages of the SO/H lamp which were mentioned previously, a range of so-called 'Integral' SOI/H lamps was invented by Osram-GEC and put on the market in 1955. The dewar jacket which had accompanied all low pressure sodium lamps up to this time was dispensed with, and instead a one-piece sealed on evacuated outer bulb enclosed the discharge tube. This was expected to reduce thermal conduction and convection losses from the discharge tube to the inner wall of the former dewar jacket, and bring about an increase in lamp efficacy.

However contrary to popular belief, this change in fact did very little to improve thermal insulation and did not account for any significant increase in initial lamp efficacy. The performance of the dewar jacket was so good that depsite the fact there was indeed some heat from the discharge tube to the air surrounding it, the system quickly reached equilibrium and after a few tens of minutes there was no further significant heat loss. In fact as explained earlier in the section concerning sodium migration, this sheath of air was quite advantageous and its replacement with vacuum brought with it a major new problem. Nevertheless, the one-piece outer jacket justified its use by dramatically attacking the rate of lumen depreciation throughout lamp life, through eliminating the progressive ingress of dust into the lamp. The occurrence of troublesome, conductive films of moisture on the discharge tube surface was also prevented, resulting in much more reliable lamp ignition. Lastly the design of course also resulted in a more compact, cheaper, and lightweight product which had commercial advantages. An example of the first slimline integral lamp is illustrated in Figure S24.

Figure S24 - The Earliest Integral SOI/H Lamp with Narrow Diameter Outer Jacket

Improvements in the thermal insulation of sodium lamps had thus far concentrated only on reducing the conducted and convected thermal losses. In 1956 the problem of radiated heat loss was addressed by Osram-GEC, who placed an ordinary glass sleeve around each limb of the discharge tube of its 140W lamps - the lower wattages receiving the same treatment in 1958. Figure S23a on this page shows an end-view of the arrangement in this lamp. The glass sleeves direct roughly half the radiated heat back onto the discharge tube while only absorbing about 3% light. The crucial effect of this was that the wall temperature increased, and thus to bring it back to the optimum level of 260°C, the tube diameter had to be increased. This resulted in a corresponding fall in current density within the discharge tube, and it has already been explained that sodium lamps are most efficient when operated with a lower discharge current density.

The introduction by GEC of the large bore discharge tube raised the efficacy of the 140W lamp to a new all-time high of 93 lm/W. The specifications of typical SOI/H lamps can be found in Table S3 below(Philips UK Catalogue, 1965 and Light & Lighting p.248). New 200W and 300W lamps also joined the low pressure sodium range at this time. The outer jacket diameter was necessarily increased back up to the original sizes employed with SO/H lamps, to accommodate the wide bore discharge tube and extra glass insulation sleeves.
Type Lamp Current Lamp Voltage Initial Lumens Efficacy
45 W 0.6 A 80 V 3,300 lm 73 lm/W
60 W 0.6 A 105 V 4,900 lm 82 lm/W
85 W 0.6 A 160 V 7,900 lm 93 lm/W
140 W 0.9 A 160 V 13,000 lm 93 lm/W
200 W 0.9 A 260 V 21,500 lm 108 lm/W
300 W 1.6 A 215 V 34,000 lm 113 lm/W
Table S3 - Specifications of Typical SOI/H Integral Style Lamps

Osram's patent on the wide bore glass sleeved design prevented competitors from copying it directly. However Philips managed to circumvent the patent by mounting a single large diameter heat-reflecting sleeve around both limbs of the discharge tube. This was an improvement over the former dewar-style lamp, but it did not offer such good insulation as in Osram's lamps. The larger bore discharge tube could not be employed, so the efficacy of its lamps was not so high. The use of double and triple sleeves was trialled, however no more than two were used in production since as more sleeves are added, more light is absorbed and there comes a point when efficacy beings to fall off again despite the thermal insulation being better. An end-view of the Philips construction is detailed in Figure S25. BTH Mazda was not active at all in the production of Integral lamps, since the company had at that time decided to focus its efforts entirely on a new concept, the Linear Sodium lamp, which offered still further advantages over both of these integral developments.

Figure S25 - Arrangement of Glass Sleeves in the Osram and Philips SOI Lamps

Sodium Migration
Unfortunately, the integral design brought with it the new problem of sodium migration in both company's lamps. The vacuum around the discharge tube does not provide a uniform temperature distribution along its length, which was previously ensured by the turbulent air movement around SO/H discharge tubes. The electrode end of the lamp always runs slightly hotter than the bend end due to power losses ahead of the electrodes, thus sodium migrates (in fact it is distilled) along the temperature gradient from hot areas to cold. It accumulates near the U-bend, with the hotter areas becoming depleted of sodium vapour and the discharge in these regions can revert to a pure neon discharge. The neon discharge generates more heat than the sodium discharge, which further heats the already sodium-depleted regions and exacerbates the problem.

To counteract the trend for sodium to migrate to the U-bend, in many SOI lamps it will be seen that the glass is constricted here to bring it closer to the discharge and increase wall temperature. In certain Philips lamps, a layer of heat-reflective platinum paint was applied to the outside of the bend instead. Both techniques were partially successful in increasing the bend temperature to reduce migration effects. However it was not until the development of the IR-coated SOX type lamp that this sodium migration issue was effectively solved.

A further design feature which had to be addressed was that in SO lamps, ignition was facilitated by the long enamelled metal fork which supported the discharge tube and acted as a third auxiliary electrode. This support was not required in SOI, but to ensure reliable ignition, GEC replaced it with a pair of nickel wires spiralled around each limb of the U-tube and connected to the cathode of the adjacent limb. Once again this design was protected by the company's patents, and Philips instead used a short length of enamelled wire which ran alongside one of the limbs of the discharge tube for just a few centimetres. Figure S26 illustrates the eventual construction which was employed in the mass-produced lamps of both the Osram-GEC and Philips SOI styles.

Figure S26 - Arrangement of Sleeves and Ignition Aids in the Osram and Philips Lamps

Sodium Migration
It is not easy to achieve a high vacuum in the outer jacket of the lamp, and it is even more difficult to keep once it has been obtained, because the internal components will continue to outgas over time. It is for this reason that a chemical 'getter' is employed in the outer jacket of all intergral-type sodium lamps. This not only helps to initially achieve the required vacuum, but maintains it throughout lamp life. Sodium lamps employ a barium getter which consists of a mirror of that metal on the inside of the outer bulb (normally near the cap where it doesn’t interfere with light output). Barium is a highly reactive metal which chemically removes many impurity gases from the atmosphere in the lamp. Disappearance or tarnishing of the mirror means that the outer bulb no longer has the required vacuum. Figure S27 illustrates how the efficiency of the lamp varies with the quality of the outer bulb vacuum.

Figure S27 - Effect of argon pressure in the outer bulb on lamp efficacy