Updated 16-VI-2015
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

SOX/H Integral IR-Coated Design

The technique of employing glass sleeves in SOI/H lamps to minimise radiation losses is not a particularly efficient solution, since each sleeve only redirects about half the radiated heat back into the discharge tube. During the 1960s the primary focus of research on the low pressure sodium lamp was engaged in the search for new materials which could more efficiently trap infrared wavelengths while being transparent to light.

The earliest developments employed pure metallic coatings applied to the outer envelope. These form superb infra-red reflectors, but brought the drawback of absorbing much of the extra light they helped to generate. Eventually it was discovered that certain metallic oxide coatings displayed the desired property of high infrared reflection combined with high visible transmission. The heating effect of the IR coating allowed lamp wattages to be reduced while maintaining the discharge tube at the optimum temperature. The resulting reduction in electrical current density in the discharge tube is then responsible for an increase in luminous efficacy.

To illustrate the significance of the IR coating in bringing the discharge tube up to its correct operating temperature, the demonstration lamp shown in Figure S28 has been specially made up (courtesy of Philips Lighting Hamilton Factory, Scotland). One half of the outer bulb is coated with the normal indium-tin-oxide IR film, while the other half has been chemically etched to remove the coating and return it to clear glass. After running for a short period under normal conditions, it is evident that the left hand side of the lamp has fully run-up whereas the much lower temperatures on the right hand side of the lamp have resulted in only a very small amount of sodium ionisation.

Figure S28 - Demonstration lamp with half-coating showing effect on luminous flux

Incidentally, the uncoated region of the lamp is too hot to touch whereas the coated area remains relatively cool, demonstrating that heat loss is of course considerably greater without the coating. The impressive thermal insulation effects of the IR coating are revealed in the thermal image of the same lamp in Figure S29. This shows that the glass surface temperature in the coated region is about 60°C, whereas in the uncoated area temperatures in excess of 100°C are attained.

Figure S29 - Thermal Image of above showing greatest thermal loss in uncoated region, and of course in the vicinity of the electrodes

Metallic Heat-Reflection Films
The first commercial lamp to employ a thin-film heat reflector was a 60-watt Linear Sodium lamp manufactured by Osram-GEC, which employs a 15-nanometre thick film of pure gold on the inside of the outer jacket. For obvious reasons it was marketed for a few years under the trade name "GEC Golden Linear". The same coating was also applied to the company's 160W and 200W Linear Sodium, in which the gold reflected a very high proportion of heat back into the discharge tube and allowed its diameter to be increased, thus reducing discharge current density and enhancing the luminous efficacy. However the gold film absorbs a considerable amount of the extra light that it helps to generate. Applied as a full coating along the entire lamp it would have raised discharge efficacy superbly, but absorbed so much light that the total lamp efficacy would actually be reduced. Gold films were not practical for the U-shaped lamps because they absorbed more light than the extra they generated, but owing to a unique feature of the Linear Sodium lamp, covered in the next section, they were suitable for that design. The Linear Sodium discharge tube has a non-circular cross-section and it emits more light in one plane than the other. By applying the gold coating in the form of two broad stripes, the plane of the discharge tube can be aligned such that its surface which radiates most light is oriented alongside a clear stripe in the outer jacket for good transmission. The gold film was applied only alongside the less luminous sides of the discharge tube. An additional gain of about 30% light transmission could be attained with the use of an auxiliary antireflection zinc sulphide film applied over the gold, but this was too expensive to be applied to commercial lamp designs.

German Osram also manufactured 175W and 220W linear sodium lamps for a very brief period - these employed films of bismuth to circumvent Osram-GEC's patent on the use of gold, and not being quite so efficient, consumed a higher wattage than the GEC's 160W and 200W equivalent products for an equivalent luminous flux.

Semiconductor Heat-Reflection Films
Further improvements came with the use of metal oxide semiconductor films, the use of which was pioneered by Philips on the conventional U-Tube Integral lamps. The first material used was tin oxide and this had a very high light transmission while still being an excellent infra red reflector. As such, it could be applied over the whole surface of the lamp's outer jacket with only minimal absorption of the visible light. In 1964, the new super-efficient semiconductor coated lamps were placed on the marked and the SOX name was introduced at that time. SOX lamps were launched in wattages of 40W, 60W, 100W and 150W to replace the former 60W, 85W, 140W and 200W glass sleeved SOI types, specifications of which are detailed in Table S4 (Philips UK Lamp Catalogue, 1966). Curiously and for unknown reason, no SOX replacement was offered for the smallest 45W SOI lamp.

Owing to the fact that the coating thickness was quite uniform along the length of the tube, the use of sodium-retaining dimples was essential to prevent sodium migration along the lamp. Osram-GEC quickly copied the Philips development and manufactured its own range of tin-oxide coated SOX lamps. These were technically similar to the Philips models, and also employed dimpled discharge tubes. Osram-GEC wasted no time in adapting the same coating for its 60W, 160W and 200W Linear Sodium products, superseding the former metallic coatings of gold. Thorn Lighting (who had taken over AEI Mazda in the same year) also began to employ tin oxide films in its Linear Sodium range, which was offered in 60W and 200W ratings.
Type Lamp Current Lamp Voltage Initial Lumens Efficacy
40 W 0.6 A 75V 4,300 lm 107.5 lm/W
60 W 0.6 A 115 V 7,200 lm 120.0 lm/W
100 W 0.9 A 125 V 12,000 lm 120.0 lm/W
150 W 0.9 A 185 V 20,200 lm 134.7 lm/W
200 W 0.9 A 265 V 30,000 lm 150.0 lm/W
Table S4 - Specifications of SOX/H with Tin Oxide Film

Two years later in 1966, an improved coating of tin-doped indium oxide was developed by Philips, and this offered even greater transmission of the sodium yellow light. These appear to have been marketed as from 1968. On account of their superior efficacy, the SOX range was re-rated to new lower wattages, having approximately the same luminous flux. Lamp wattage was reduced to 35W, 55W, 90W, 135W and 180W. Technical specifications for Re-Rated SOX are given in Table S5. Notice that new smaller 18W and 10W sources were also introduced following the improvement in efficacy that their coatings allowed.
Type Lamp Current Lamp Voltage Initial Lumens Efficacy
10 W 0.2 A 55 V 1,000 lm 100.0 lm/W
18 W 0.35 A 55 V 1,800 lm 100.0 lm/W
35 W 0.6 A 75 V 4,300 lm 107.5 lm/W
55 W 0.6 A 115 V 7,200 lm 120.0 lm/W
90 W 0.9 A 125 V 12,000 lm 120.0 lm/W
135 W 0.9 A 185 V 20,200 lm 134.7 lm/W
180 W 0.9 A 265 V 30,000 lm 150.0 lm/W
Table S5 - Specifications of SOX/H with Indium-Tin Oxide Film

At this time, Thorn offered a High Output 200W Linear Sodium based partly on the improved coating, and this delivered 10% more light than the old design for no extra power consumption. The indium film was also employed in a new 140W rating which was introduced in 1966. 60W and standard 200W products continued to make use of the earlier tin oxide films until those products were made obsolete in 1985.

The tin-doped indium oxide film is the standard coating material which is still found on today's SOX lamps. Tin is required as it increases the number of free charge carriers in the film and makes it a better heat reflector than plain indium oxide. Figure S30 illustrates how the transmission and reflection of this film varies with wavelength. The transmission is high for sodium light (around 589nm), so about 90% of the light generated can pass through the outer bulb. Most of the infrared radiated from the discharge tube is at about 5500nm, and the graph shows that reflection of this is high, thus the coating is a very effective heat reflector.

Better IR reflection can be achieved by increasing the film thickness, but the thicker film also transmits less light, reducing efficacy. A compromise has been chosen between infrared reflection and visible transmission, to give maximum efficacy. This works out to a film thickness of about 0.32 microns. Such a coating usually imparts a greenish colour to reflections observed in the coated surface. Earlier tin oxide lamps are distinguishable by the fact that surface reflections appear yellow in the Linear style, and orange for the U-tube style

Figure S30 - IR and Visible Light Transmission and Reflection Curves for Indium Film

Improved Insulation in E Lamps
SOX-E (Economy) lamps are similar to standard types but have better thermal insulation so they are even more efficient. The efficiency can be further improved by running them on special SOX-E control gear, which operates the lamp at its optimum (lower) current loading of 0.3 amps for the low wattage sizes, and 0.6A for the larger lamps. Under these conditions, lamp efficacy can as high as 200 lm/W, but of course, total light output from the same size lamp falls because it is being operated at lower current. Technical specifications for the SOX-E series is detailed in Table S6 below.
Type Lamp Current Lamp Voltage Initial Lumens Efficacy
26 W 0.45 A 81 V 3,700 lm 137 lm/W
36 W 0.45 A 120 V 6,160 lm 160 lm/W
66 W 110 V 10,700 lm 165 lm/W
91 W 0.6 A 165 V 17,000 lm 189 lm/W
131 W 0.6 A 235 V 26,000 lm 203 lm/W
Table S6 - Specifications of SOX-E with Improved Indium-Tin Film

It is not currently practical to apply the IR coating to the domed end of the outer bulb, so in standard SOX lamps heat is lost through this 'window' of clear glass E lamps have extra insulation at the domed end of the lamp which may take the form of a highly reflective metal disc, or a heat reflecting cap which is clipped over the outside of the bend. As the heat losses are been reduced, lamp efficacy rises. The IR coating is also slightly superior on the E lamps made by Philips and Osram, and generally these have a reddish colour tint to light reflections seen in their surfaces.

In Figure S31 is a photograph of a tin oxide lamp, and indium oxide lamp, and an improved indium oxide SOX-E lamp The colour of light reflections in these coatings can be seen to be orange, green and reddish-pink respectively.

Fig. S31 - From top to bottom, Tin (orange), Indium (green), SOX-E (red) IR Coatings
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