Transport processes in tungsten–halogen incandescent lamps

The first practical tungsten–halogen lamp was made in 1959, based upon a principle patented in 1882. These lamps were in tubular form (known as linear lamps), having a coiled tungsten filament supported along the central axis of the envelope. They contained an Inert gas-filling and a small quantity of iodine vapour to react with the evaporated tungsten, thus keeping the bulb clean. Although similar lamps are now manufactured in large quantities, they have the disadvantage that they can be operated only within ±4° of the horizontal otherwise the iodine vapour separates within the lamp causing the transport cycle to break down at the upper end. [Continues.]