||The first double envelope halogen lamps were offered only in high wattage ratings - such as for instance the linear IDE types of 500W to 2000W. It was not until the development of compact universal burning capsules that the concept was extended to offer a retrofit for ordinary GLS lamps employed in domestic service. The first embodiment of the sub-150W versions was the Halogèna family launched by Philips France in 1987. They feature double lifetime, around 10% higher lumen output and a whiter colour light than GLS lamps.
This particular version employs the typical bulged tubular design and was offered in both clear and opal finishes. The BT bulb shape has no technical purpose, it simply introuced a distinctive new decorative appearance for these halogen retrofits, and remains the preferred format for domestic lighting markets. For the professional sector a similar lamp was introduced with a more compact tubular envelope, available in either clear or pearl finish.
The filament is a compact coiled-coil type made suitable for universal burning by a novel method of dimpling the quartz tube into the double-spiral support wires. Linear halogen lamps of low wattages were previously limited to horizontal burning, to prevent collapse of the filament in the axial direction. The Philips lamp also makes use of a thin-walled soft glass outer bulb, whereas competitors employed a heavy borisilicate bulb designed to withstand a possible capsule explosion. This was achieved firstly by introducing a low pressure gas filling, and secondly by double fusing. The moly foil seals of the capsule penetrate into a small void above the pinch, which acts as an internal fuse. There are also two secondary fuses inside the stem of the outer bulb.