SOI/H Integral Low Pressure Sodium

Updated
16-VI-2015
In 1955 the thermal insulation (and hence luminous efficacy) of the low pressure sodium lamp was improved through an Osram-GEC innovation, which dispensed with the separate dewar jacket and replaced it with a permanent one piece evacuated outer bulb. The vacuum directly surrounding the discharge tube minimised conducted and convected heat losses, and allowed the diameter of the discharge tube to be increased while maintaining the same wall temperature. Owing to the resulting reduction in electrical current density in the discharge the efficacy of the so-called 'Integral' lamp was increased.

Further gains in efficacy were realised in 1956 via another GEC development, which involved adding a glass heat-reflection sleeve around each limb of the discharge tube. This provided additional thermal insuation while only absorbing about 3% light. It took a further four years for Philips to catch up, and its lamps are of slightly different design - employing one large single sleeve around both limbs of the discharge tube.

Another advantage of the Integral design is that lumen maintenance during life is enhanced by keeping the inside of the lamp clean. When the earlier SO/H Dewar Jacket lamps cooled, dusty air from outside would be drawn in and the jackets rapidly became quite dirty. However the Integral design brought with it the problem of sodium migration. Vacuum does not proved a uniform temperature gradient along the tube length, which was previously ensured by the turbulent air movement within the dewar jacket. Thus sodium migrates (in fact it is distilled) from hot areas to cold, namely from the electrodes to the U-bend. It accumulates in the cold regions where it forms light-blocking mirrors, while leaving other parts of the tube devoid of sodium.

Sodium migration was combated by adjusting the gas composition and pressure, such that when a region of the discharge became depleted of sodium the temperature at that point would fall and encourage sodium to migrate back. In addition it became customary to constrict the tube at the bend to marginally increase the temperature at this point. Philips lamps were less susceptible to this problem, owing to the use of sodium-retaining dimples formed into the discharge tube surface.

Osram-GEC Lamps

Osram-GEC

45W

SOI/H with twin glass sleeved discharge tube
1973

Osram-GEC

85W

SOI/H with twin glass sleeved discharge tube
1973

Osram-GEC

140W

SOI/H with twin glass sleeved discharge tube
1972

Osram-GEC

280W

Double discharge tube for dimming applications
1958

Osram-GEC

140W

Experimental design with holes in glass sleeves
1958
   

Philips Lamps

Philips

85W

SOI prototype without sleeved discharge tube
1958

Philips

45W

SOI with single glass sleeved discharge tube
1963

Philips

60W

SOI with single glass sleeved discharge tube
1965

Philips

85W

SOI with single glass sleeved discharge tube
1965

Philips

140W

SOI with single glass sleeved discharge tube
1962

Philips

200W

SOI with single glass sleeved discharge tube
1960

Philips

45W

SOI/H with orange darkroom safelight filter
1966