SOX Low Pressure Sodium with IR Coating

Updated
19-IX-2003
The next major step forward in low pressure sodium came in 1964, when thin heat-reflecting films of exotic materials were introduced. A microscopically thin coating of these materials on the inside of the outer jacket successfully dispensed with the cumbersome heat-reflecting glass sleeves of the SOI design. Film composition and thickness is arranged such that they reflect the infrared while still allowing the visible light to pass through. Initially a 50-nanometre film of pure gold was employed - this remains the best heat reflector, but it also absorbs a lot of light. Bismuth was tried in some German lamps, but the biggest improvements came with the use of metal oxide semiconductor films and the name SOX was introduced with these lamps. These materials can be doped so that they have high infrared reflectivity, while still being transparent to visible light.

Between 1964 and 1966 a range of SOX lamps having stannic oxide coatings was marketed. Because of the great increase in efficacy, lamp wattages had to be reduced as outlined in the table below. This material was superseded in 1966 with a 0.32 micron coating of tin-doped indium oxide, the film still employed in today's SOX products. The corresponding increase in efficacy saw the SOX lamps re-rated once again.

A further improvement was made in 1983 with so-called SOX-E lamps. These employ a film of modified composition and also have improved insulation around the U-bend. An advanced platinum 'getter' in the outer jacket serves to crack hydrdocarbon vapours and further reduce conducted heat losses. Efficacy has exceeded 200 lm/W with this range, but the luminous flux has to be lower than SOX lamps of the same physical size.

Incidentally it is easy to tell the coating type from the colour of surface reflections in the film. Stannic oxide lamps produce a yellow/orange colour, indium oxide takes on a greenish hue, while SOX-E coatings impart a reddish colour to reflected light.

 SOI Integral. . . . . .45w60w85w140w200w300w
 SOX Original . . . . . . . . .40w60w100w150w . . .
 SOX Re-rated10w18w . . .35w55w90w135w180w
 SOX Economy . . . . . . . . .26w36w66w91w131w

Philips

40W

SO-X with IR coating of stannic oxide
c. 1966

Philips

35W

SOX with IR coating of indium tin oxide
1971

GEC

26W

SOX-E Economy with improved efficacy
c. 1980

Philips

35W

SOX-Plus design with increased service life
1996

Philips

35W

SOX-PSG with increased reliability and service life
2003

GEC

10W

Miniature 10-watt lamp for security lighting
1986

Thorn

6W

Experimental 6-watt miniature lamp
c. 1985