SOI/H Integral Low Pressure Sodium

Updated
04-XI-2004
In 1955 the thermal insulation (and hence luminous efficacy) of the 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 as a result the efficacy of the so-called Integral lamp rose from 72 lm/W to 87 lm/W. This increase is purely because with better thermal insulation, the discharge tube diameter could be increased while maintaining 260C at the wall. Lower current density in the larger tube increased the efficacy.

A further advantage was that lumen maintenance during life was greatly enhanced primarily by keeping the lamp clean. When SO/H lamps cool, dusty air from outside can be drawn in and the jackets rapidly became quite dirty in use. This deterioration was, of course, eliminated with the advent of the Integral lamp. But the integral design brought with it the problem of sodium migration. Vacuum does not proved a uniform temperature along the tube length, which was previously ensured by the turbulent air movement around SO/H discharge tubes. 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 which reduce light output and efficacy.

Sodium migration was combated in some lamps 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 also employ a metal heat reflecting cap over the bend.

Further gains in efficacy were realised in 1956 by reflecting some radiated heat back onto the discharge tube. A simple glass sleeve redirects roughly half of the radiated heat back, while absorbing only some 3% of the light. Typically two sleeves were found to be adequate (this includes the outer jacket). Greater numbers of sleeves resulted in heavy and fragile lamps with a subsequent loss of light due to absorption.

Philips

85W

SOI 1st Generation with narrow diameter outer
1958

Philips

45W

SOI 2nd Generation with single glass sleeve
1963

Philips

140W

SOI 2nd Generation with single glass sleeve
1962

GEC

45W

SOI/H with twin glass sleeves over each limb
1972

GEC

85W

SOI/H with twin glass sleeves over each limb
1973

GEC

140W

SOI/H with twin glass sleeves over each limb
1972

Osram

280W

Double discharge tube for dimming applications
1958

Osram

140W

Experimental design with holes in glass sleeves
1958

Philips

45W

SOI/H with orange darkroom safelight filter
1966