SOX/H Integral IR-Coated Design

The technique of employing glass sleeves 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. The other half is still wasted and escapes from the lamp. New materials were investigated which could increase this ratio while still being transparent to visible light.

The first commercial lamp of this type was the Golden Linear, 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. The same technique 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 absorbed a considerable amount of the extra light which it helped 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 could be aligned such that its surface which radiated most light was oriented alongside a clear stripe in the outer jacket for good transmission, and the gold film was only applied to the area 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.

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. In 1966, the new super-efficient semiconductor coated lamps were placed on the marked and the SOX name was introduced at this time. The first material used was tin oxide and this had a very high light transmission while still being an excellent infra red reflector. 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. SOX lamps of this type were also manufactured by Osram-GEC. That company wasted no time in adapting the same coating for its 60W, 160W and 200W Linear Sodium products. 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.

Two years later in 1968, an improved coating of tin-doped indium oxide was adopted and this offered even greater transmission of the sodium yellow light. On account of its superior efficacy, the SOX range was re-rated to new lower wattages, having substantially the same luminous flux. Lamp wattage was reduced to 35W, 55W, 90W, 135W and a new 180W lamp was introduced which effectively replaced the 300W SOI, for which no tin oxide SOX retrofit had been marketed. 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.

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 at the same time. 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 S26 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

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.

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 S28 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.