1982 Philips IRC-GLS 60 W

In the early 1970s a new type of infrared mirror was developed at MIT in the form of a thin film of silver metal flanked by two layers of titanium dioxide, all 18-nm thick. Fann and Bachner discovered that the dielectric material present on either side of the metal film acts as anti-reflection layers which increases the optical transparency of the metal in the visible domain while silver's high reflectivity in the infrared is maintained. This discovery led to the development of highly efficient induced-transmission optical filters that were first intended for the improvement of solar collectors.

Following this development, Duro Test acquired the rights to this technology for an application in incandescent lamps so as to recycle part of the infrared emitted by tungsten filaments. Further developments in cooperation with MIT ensued in order to optimize the filter design for maximum reflectance in the IR-A domain while maintaining a high transmittance in the visible domain. To that end, the thickness of the metal film was increased to 20 nm while that of the titanium oxide layers was increased to 30 nm. This work eventually led to the introduction in 1977 of the Mi-T WattSaver lamp in the USA, the first incandescent lamp with infrared recycling, emitting as much light as a 100 W lamp while consuming 65 W only.

This technology attracted also the attention of Westinghouse and then of Philips during the 1970s, where induced-transmission filters were the subject of intense investigations. Interestingly, at that time Philips considered developing efficient incandescent lamps using infrared recycling by coating the inner surface of bulbs with the company's indium tin oxide (ITO) material that had been introduced in their SOX low-pressure sodium lamps in 1968. It soon became clear that induced-transmission filters are a much better alternative due to their greater reflectivity in the shortwave end of the IR spectrum, better suited to the spectral characteristics of incandescent tungsten filaments. While investigations at Philips began with the technology developed by the MIT/Duro Test partnership, their focus shifted to ZnS-Ag-ZnS stack structures as the higher refractive index of zinc sulfide was found to improve the reflectivity in the infrared.

Beside the usual lamp design challenges faced by Duro Test, the Europeans had also to address the issues related with the use of longer tungsten filaments suitable for 220–230 V mains. This required some extra attention on the specific aspects of filament positioning in the bulb, of the crystal structure of the tungsten radiator (i.e., mechanical stability of the filament), and of the optical coupling between the long light source and the mirror. For the last part Philips opted for an elliptical glass jacket instead of a spherical one, resulting in an better optical matching between the incandescent filament and its reflected infrared image. Lamps made following this design principle and internally coated a sulfide-silver mirror led to a gain in lumen efficacy of up to 40 % compared to uncoated lamps.

By the late 1970s the Dutch had gone as far as making pre-production IRC lamps whose design and structure were patented in 1979. These lamps (see above) were coated with silver and zinc sulfide in 20-nm and 30-nm thick layers, respectively. These materials were deposited using a vacuum evaporation technique. Work on the optical filter was carried out mostly at Philips’s research labs in Aachen, West Germany, while lamp development took place mostly at the Central Lighting Laboratory in Eindhoven, the Netherlands. The linear filament, held into place with the help of a central support wire, is aligned along the axis of the the bulb's elliptical mirror. Since the zones near the reflector’s vertices have little influence on the lamp’s energy balance, these are left open for the insertion of the filament assembly into the bulb. On the side opposite to the bulb neck a tubular dome was also formed. Since both dome and neck are the coldest regions of the lamp, they serve as a condensation zones for the tungsten evaporated from the incandescent filament. Two getters were also placed there so as to ensure that the lamp's atmosphere, a nitrogen-krypton mix, remains free of impurities, especially hydrocarbons that can cause filament sagging.

Despite the very advanced state of development works at Philips, the Dutch never released their IRC GLS lamps on the market. The strict control of the filament's position in the bulb proved just too difficult under mass production conditions. At around the same time, during the late 1980s, Duro Test was forced to abandon the production of its Mi-T WattSaver lamp due to early failures caused by the brittleness of their short 120 V filament.