Lamps with Multilayer Interference-Film Reflectors
Incandescent lamps have always been efficient producers of infrared energy. In typical lamps of wattages used in general lighting service, about 11 percent of input energy is converted to light and 69 percent to infrared, while the remaining 20 percent is dissipated in losses in the bulb. In some cases the heating capability of incandescent lamps has been utilized effectively for industrial heating and comfort heating. However, in many lighting applications the radiant heat accompanying light from the filament has been thought of as a necessary evil. Indeed, the heat content of concentrated beams has placed some severe limitations on the illumination levels that can be used in certain types of displays.
Removing the radiant heat from incandescent light by conventional filtering has been limited in the past because most heat-absorbing glasses were almost equally low in light transmission and/or produced a significant shift in the color of the light. Newer glass filters are improved in both these respects and they are expected to find considerable use in lighting practice.
The extensive recent development of vacuum-deposited multilayer interference films has offered a new approach to the separation of light from infrared at substantially higher efficiencies than has been possible before, with good control over color characteristics of the light. After considerable study of coatings and manufacturing techniques, 300-watt PAR56 lamps have been produced, utilizing interference films in place of aluminum as a reflecting medium.