SOX Low Pressure Sodium with IR Coating

Updated
05-VII-2015
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, on the Linear sodium range. 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 1968 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 This material was superseded in 1966-68 with a 0.32 micron coating of tin-doped indium oxide, the film still employed in today's SOX products.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.

Tin Oxide Lamps

Philips

40W

SO-X with IR coating of stannic oxide
1963

Osram-GEC

40W

SOX/H with IR coating of stannic oxide
1967
   

Indium Oxide Lamps

Philips

90W

SO-X of Philips' first indium construction
1968

Philips

35W

SO-X of Philips' first indium construction
1971

Philips

90W

SOX with improved inner support assembly
1988

Philips

35W

SOX-Plus design with increased service life
1996

Philips

35W

SOX-PSG with increased reliability and service life
2003

Osram-GEC

90W

SOX/H with Dimples and Graded IR Coating
1967

Thorn

135W

SOX lamp of Thorn's second generation
1975

Thorn EMI

35W

SOX lamp of Thorn's second generation
1986

GE-Thorn

35W

SOX lamp of Thorn's third generation
1991

Improved Indium Oxide (Economy) Lamps

Philips

26W

SOX-E Economy with improved efficacy
1984

Osram-GEC

26W

SOX-E Economy with improved efficacy
1985

GE

26W

SOX-E Economy with improved efficacy
2003

Miniature Lamps

Philips

18W

Mini-SOX lamp for security lighting
1971

Osram-GEC

10W

Miniature 10-watt lamp for security lighting
1981

Thorn

6W

Experimental 6-watt miniature lamp
c. 1985

Philips

18W

Experimental 18-watt improved efficacy lamp
c. 1985