Mercury-cut increases efficiency
Philips recently introduced a technology that decreases the mercury content in their tri-phosphor fluorescent lamps by 80% while reducing the light depreciation of the lamps substantially. Mercury-cut increases efficiency
Although conventional linear fluorescent tri-phosphor lamps have an efficacy of ~95 lumens/watt (excluding ballast), the light output will have depreciated by ~15% after 12 000 hours of operation. The new lamps also start off at ~95 lumens/watt, but they will only have depreciated by 5% after 12 000 hours. In practice, this is a significant step upwards in the average lifetime efficiency of the lamps.
The fill of a fluorescent lamp is mainly composed of a noble gas (argon or krypton) and mercury. When the lamp is in operation, the mercury of the fill is gaseous, and the excited mercury atoms generate ultraviolet radiation (UV). When the UV hits the phosphor coating on the glass walls, the energy of the UV is turned into visible light by the phosphor atoms.
The main cause of light depreciation is that the gaseous mercury in a fluorescent lamp gradually is absorbed by the glass walls, which will reduce the transparency of the glass. The phosphors will also absorb mercury, but to a much lower extent. Of the 15 milligrams of mercury in the fill, 10 mg may be absorbed by the glass, and only one or two milligrams by the phosphors. When mercury is absorbed, the amount of available mercury in the fill is reduced. This could drastically reduce the life of the lamp, but manufacturers are "over-dosing" mercury to ensure that sufficient amounts are available in the fill even after several thousand hours.
In the new lamps, Philips has added a micro-thin protective shield between the glass and the phosphors that prevents the mercury from being absorbed by the glass. By doing so, 80% less mercury, only 3 mg, is needed in a lamp. Although not entirely new, another feature of the Philips lamps is their mercury dosing system. The mercury is kept in a small capsule that is not activated until the finished lamp has been sealed in the factory. The capsules helps ensure exact dosing of the mercury. Also, should a lamp break before being finished, the mercury remains in the capsule, thereby preventing any release to the environment.
The new tri-phosphor lamps will have replaced the old ones by early 1995. According to Philips, the price will be about 3% higher. Osram claims that they will launch similar low-mercury lamps in the near future.
Nils Borg