Lighting up the World
As your eye ages, the proteins in your lens starts to yellow, colouring your world to a darker, redder hue, with often a resulting depressive effect in your mood. What if a change in household lighting could make your world, not just a brighter place, but one which gave you back the colours of your youth?
It might sound far-fetched, but that´s just one of the possible outcomes of research going on at the University of Canterbury where the underlying physics of fluoresence is being closely examined, with support from the MacDairmid Institute.
Fluorescent lights have been around for over a century – the enthusiastic Thomas Edison and his rival Nikolai Tesla both had a go at producing them in the 1890s, and the mercury-vapour lamp was patented in 1901. The general principle hasn´t changed a great deal since then. Ahost material and activator element, commonly made of a blend of metallic and rare-earth phosphor salts, is milled into fine powder and used to coat the inside of a glass tube. The tube is filled with low pressure mercury vapour. When the vapour is excited by an electrical current, it emits ultraviolet light which in turns causes the phosphor coating to fluoresce, re-radiating visible light.
Although fluorescent lighting is considered very efficient, the conversion from ultraviolet to visible light is where the greatest loss of useful energy occurs. And the presence of mercury vapour, minute though it may be, raises concerns about environmental contamination, disposal issue and possible health effects. Clearly there is room for improvement.
If Professor Roger Reeves, head of physics at Canterbury University, has his way, the light you get will be produced more efficiently in future and in a more environmentally friendly fashion. Roger is part of a team which is to receive $900,000 from the Marsden Fund over the next three years to investigate this area. While a fluorescent light may seem a small thing, lighting accounts for a significant amount of energy production and consumption around the world, and there is global interest in finding better ways of lighting our world.
One way to improve things is to replace the mercury vapour with xenon. At an emission wavelength of 154 nanometres, xenon packs quite a bit more energy than mercury at 254nm, and doesn´t have the environmental disadvantages of the toxic metal. Making use of that additional energy is proving tricky, however.
With mercury, the loss in the shift from ultraviolet to visible light is not too bad — the difference between 4.8 electron volts and around 4 eV. But with xenon-based alternatives, almost half is lost as the energy drops from 8 eV down to 4eV. And it’s that loss that Roger is hoping to mitigate. “The world is trying to think about making phosphors that will do quantum cutting.”
By “cutting” that 8eV drop in half and getting the involved ions to interact and share the energy in two loads, the full energy output could be used, effectively doubling it. To investigate this possible approach, Roger and his team propose to “reverse-engineer” the process by running it backwards, pushing two ions of 4 eV together to find out more about the energy states in the 8 eV region.
“[Doubling the efficiency] would be really good,” says Roger, “but it’s really hard work. I don´t believe anyone else will be trying this approach.”
The high energy states need special techniques and special equipment to be able to be investigated. That´s brought a collaboration that reaches from Christchurch to Europe. Associate Professor Mike Reid, the principal investigator for the project, provides a strong theoretical framework for experimentalists Reeves and colleague Dr Jon-Paul Wells, who is handling part of the laser investigation side of things. Roger has been looking at purchasing equipment that will allow the team to map out the high energy states of the phosphor ions, which can then be modelled by Reid to provide a clearer idea of what is happening and how to utilise it.
“We want an understanding of the physics to enable us to work out why some systems are more efficient than others. Better modelling will give us an idea of whether we´re designing the right phosphors to improve the efficiency”.
Different phosphor blends produce different emission colours over different periods at different efficiencies. Changes in blends can push light to a bluer end of the spectrum, as in the age-related “tuned” lighting mentioned here. The recipe for the phosphor blends used in commercial lighting tends to be highly protected by the companies involved, with the likes of Philips and GE undertaking their own research in this area.
“Minor changes in concentrations in the host material can make a dramatic change in efficiency,” explains Roger. That can have a dramatic effect on the bottom line where commercial returns are required, hence the global interest. That´s also something of which the university professor is aware – “we need to protect our ideas for New Zealand´s economic benefit”.
Roger credits the support of the MacDiarmid Institute and its provision for capital purchases as a key element in the team going on to win Marsden funding. Ultraviolet lasers are very hard to get and the ability to have one available means that the team can take their unusual path to investigating the high energy states.
In some cases, the search for equipment has to head further afield, and other experimentation is planned at DESY(Deutsches Elektronen Synchrotron), in Germany. Reeves is keen to send graduate students to Europe for training to build up the necessary human capital to keep up the research. This also means that when the Australian synchrotron in Melbourne is available in a few years, New Zealand researchers will be ready to use it.
Reeves admits to a strong vested interest in what may seem, on the surface, an esoteric problem in applied physics. “I worry about what my grandchildren will use to heat and light their homes,” he says. “Probably not oil-based electricity, so where´s that energy going to come from?”