||Until the mid 1960s tungsten-halogen lamps universally employed Iodine fillings. One of its drawbacks is its purple colouration leading to the absorption of some 5% lumens. Iodine lamps must also be operated horizontally ±4° to prevent separation of the gas phases along the tube. Philips was the first to overcome these limitations, by replacing the iodine with bromine compounds. These are substantially colourless and being much lighter, permit universal burning.
Unfortunately bromine is very corrosive towards tungsten and moly parts operating below about 1700°C - i.e. support wires and filament tails. It was found possible to limit the rate of attack by introducing a small quantity of hydrogen into the gas filling, but halogen lamps generally require the use of a refractory quartz bulb and this is substantially porous towards the tiny hydrogen molecules. Any H2 dosed into a quartz lamp quickly diffuses out through the wall.
Philips solved the problem by sealing its linear bromine lamps into a second outer envelope of from borosilicate glass, which is not permeable to hydrogen. By filling the outer with a similar concentration of hydrogen, it is possible to ensure that hydrogen diffuses back into the inner burner as quickly as it escapes. This sealed system has proven to be very effective in allowing long life, high efficiency bromine lamps to be realised. The IODE lamp featured here attains an efficacy of 24.0lm/W with a 2000-hour life, substantially better than the 22.0lm/W of the manufacturer's equivalent 2000-hour unjacketed lamp. Note also the black layer of graphite painted over the pinch-seals of the burner. This serves to raise their temperature and further slow the rate of bromine attack on the colder filament regions.