‚ÄúHow far that little candle throws his beams! So shines a good deed in a weary world.‚ÄĚ
‚Äē William Shakespeare, The Merchant of Venice
One could hardly deny that energy saving technologies are a major driving force of sustainable development. A great contribution to efficient energy use is made by low energy lamps and particularly metal halide lamps (MH lamps). These lamps are a type of high-pressure discharge lamps (HID lamps) and are very important light sources for visible, UV, as well as IR radiation. They have captured a major share of the markets for automotive headlight lamps, video projection, general lighting, street/industrial lighting, commercial lighting, floodlighting, sun tanning, microscopy, endoscopy, photochemistry, lithography, etc. .
How a metal halide lamp works?
In discharge lamps, light is generated by a gas discharge of particles created between two hermetically sealed electrodes in an arc tube. After ignition, the particles in the arc are partially ionized, making them electrically conductive, and a ‚Äúplasma‚ÄĚ is created. In high intensity discharge lamps, the arc tube is usually enclosed in an evacuated outer bulb which isolates the hot arc tube thermally from the surroundings, similar to the principle of a thermos flask. There is high pressure and a high temperature in a discharge tube .
Figure 1.¬† A design of metal halide lamp (source¬† www.edisontechcenter.org)
In an arc tube, gas discharge works through excitation of the luminous additives (metal halide salts) and the mercury is excited by the current flow. Visible radiation characteristic for the respective elements is emitted. The mixture of the visible radiation of the different elements results in the designed color temperature and color rendering for a particular lamp. In the operating state, the mercury evaporates completely. The other elements involved are present in saturated form at the given temperatures, i.e. they only evaporate in part; the rest is in liquid form at the coolest point in the arc tube. The fraction of the filling that has evaporated depends on the temperature of the coolest point on the arc tube wall and also varies for the different filling components. Changes to the temperature of the arc tube wall can change the composition of the metal halides in the discharge, thus also changing the color properties of the lamp .
Metal halide lamps were developed as an alternative to incandescent or mercury light sources. High-pressure mercury lamp is efficient but has a main disadvantage of unevenly distributed spectral lines which leads to medium luminous efficiencies and poor color rendering with high correlated color temperatures . To overcome this problem the addition of metals or metal halides to the discharge was proposed. In 1912 Charles P. Steinmetz was the first to use halide salts in a mercury vapor lamp. He used the halides to correct color and was successful, but he couldn‚Äôt get a consistent arc. The first reliable metal halide lamp was created in 1962 by Robert Reiling, but this type of lamps became more popular decades later as the price of the lamp became more affordable .
One of the first approaches towards commercial metal halide lamps was undertaken by Philips Lighting who created the advanced prototype known as the “Thulium Lamp”. Technically, the lamp was the predecessor of the company’s now well established rare earth metal halide chemistry. Delivering a neutral white light, it is one of the first embodiments of a light source employing thulium, which delivers improved color rendering over the earlier dose systems, and with less color variation as well. This sample was produced during 1985, still a few years before low wattage metal halides achieved popularity in the market. Over lamp life, thulium iodide was found to be particularly corrosive towards the quartz arc tube, resulting in short life and many explosions. Its color is also slightly greenish. Thulium was later tamed somewhat by employing it in conjunction with dysprosium and holmium .
Figure 2. Philips MHN ‚ÄúThulium lamp‚ÄĚ (1985). Prototype. (source http://www.lamptech.co.uk)
The choice of lanthanide halides as luminous additives in metal halide lamps is explained by the fact that rare earth species possess the multi-line atomic spectra which allow filling the visible spectrum almost completely (fig. 3). It is the basis of the good color rendering properties of lamps containing rare-earth compounds .
Figure. 3. Tasks of the metals [sodium (Na), thallium (TI), indium (In), tin (Sn), lithium (Li), rare earths: dysprosium (Dy), holmium (Ho), thulium (Tm)] (source www.osram.com)
Nowadays a blend of sodium and scandium iodides is widely used in metal halide lamps in the USA and Japan, while mixtures of rare-earth iodides (e.g., dysprosium, holmium, thulium) as well as mixtures of indium, thallium, and sodium iodides are popular in Europe .
It is important to emphasize that energy saving is only one benefit of MH lamps to sustainability. End-of-life lamps are a huge source of lanthanides considering global scale of consumption of these lamps. Recycling of MH lamps might reduce shortage in rare earths supply.
- P. Flesch. Light and Light Sources. High-Intensity Discharge Lamps. Springer, 2006. 344 p.
- Metal halide lamps. Instructions for the use and application. OSRAM. www.osram.com
- M. Haverlag. Prediction of spectra of high-pressure metal halide discharge lamps containing rare-earth fillings // Physica Scripta. Vol. T119, 67-70, 2005.