Pictures of the Future      Fall 2003

The Wavelength of Change

In 20 years or so, the most advanced light sources will blur the distinction between light and information. Long before that happens, existing light sources will merge into intelligent networks that automatically alter their brightness and color in harmony with each other and the environment. Figuring out and controlling the processes that produce light is the first step down a long, bright path.

Strange things are happening in the lamps that light stores and factories – things that even lighting experts are hard pressed to explain. High above racks of dark suits and production lines pumping out everything from servers to frozen soufflés, so-called high intensity discharge lamps or HIDs, produce a symphony of wavelengths approaching the spectral richness of sunlight. HIDs produce more and better light per unit of power than any other commercially available light source. Yet scientists do not yet fully understand the reactions that take place inside these workhorses of the workplace. Figuring out their underlying physics – and learning how to modulate the complex patterns of gases that make these light sources so effective – could improve the environment, the economy and the way we see and feel.

Like fluorescent lamps, HIDs produce light by exciting a gas and various metal compounds (see table at the end). But unlike fluorescents, they produce light directly, and thus more efficiently. HIDs operate at much higher pressure than fluorescent sources and generally employ no phosphor coatings, making them much brighter than necessary for residential use, but ideal wherever a great deal of high quality light is needed.

When the power is switched on in an HID lamp, an electric arc – essentially a flow of charged particles – is struck between two tungsten electrodes inside a sphere within an outer bulb. As the gas within the sphere heats up and becomes a plasma, the lamp body gradually reaches an operating temperature of 1,200 °C, – enough to evaporate metal compounds. The energized metal atoms, and other substances in the gas, such as metal halides, collide with the electrons in the plasma causing them to give off photons, which we perceive as visible light.

And HIDs produce plenty of white light – up to 120 lm/W. For the sake of comparison, conventional incandescent bulbs (including tungsten halogen) produce only 6 to 24 lm/W, while low pressure discharge lamps – such as fluorescents – produce up to 104. Thus, the energy efficiency of discharge lamps makes them more environmentally friendly than incandescent bulbs. But HIDs, like fluorescents, have a significant shortcoming: they rely on mercury, a dangerous, non-biodegradable poison. In fluorescent lamps, for instance, excited mercury vapor efficiently transforms nearly 75 % of the electrical energy consumed by the lamp into ultraviolet radiation, which is received by the lamp's phosphor coating and converted into visible light. In HIDs, on the other hand, mercury is essential in order to cause metal atoms to produce photons under high pressure and to stabilize the resulting gas. In these light sources, up to 35 % of the electrical energy is converted into visible light. Efficient though it is in lamps, no one wants mercury in the environment. That's why OSRAM GmbH, Siemens' fully owned lighting subsidiary and one of the world's leading lighting products manufacturers, is determined to find alternatives.

Mercury-Free Future? "If we can develop the technology to run these lamps mercury-free without sacrificing lumens, we will be doing the environment – as well as manufacturers and users – a big favor because the lamps will not have to be recycled," says Scott Butler, who manages the HID Systems Lab at OSRAM's U.S. R&D headquarters in Beverly, Massachusetts. What's more, since HIDs and fluorescents share much of the same technology, the road to a mercury-free future may not be that far away.

Interestingly, the goal of developing mercury-free products in the lighting industry intersects with another major trend – namely the development of electronic systems to control processes inside lamps. Ask Dr. John Gustafson, who is in charge of OSRAM's U.S. research activities, what the most exciting development in his field is and he says, "Electronics. With a view to kicking the mercury habit, we are developing electronic systems that can control the flow patterns of gases and change the way photon-emitting substances in the gas mix. Fine tuning the electronics is one possible way of doing without mercury and still having an efficient white light source."

To accomplish this, OSRAM researchers have replaced the magnetic ballast – basically copper wire wrapped around an iron core that limits lamp current – with an electronic equivalent weighing only one third as much. Inside is a microchip that can, says Butler, "change the temperature profile of the gas, make the temperature more uniform, and increase efficiency because the gas flow can be controlled by pulses. We believe that this research has the potential to lead to mercury-free HID lamps in the mid term that are comparable in efficiency to today's (mercury-based) lamps, yet have the same wattage and color rendition."

But getting HID lamps to work without mercury is like trying to start a fire without oxygen. In fact, if trial and error were the only route to an answer, such lamps might still be decades away. But steadily increasing computational speed has shortened the wavelength of change. "We can now understand things that we were only guessing at in the past," says David Bay, manager of the Fluorescent Systems Lab in Beverly. "Our ability to simulate the complex temperature, chemical and flow properties inside a discharge lamp are improving steadily."

Transparent Ceramics. Indeed, simulation has already paid off for the OSRAM Powerball HID metal halide lamp. The lamp's ceramic burner – an inner sphere that contains the electric arc and gases – is the first in the industry to be round instead of cylindrical. The new shape allows much higher temperatures to be attained than with older quartz burners because ceramic materials can operate at temperatures that exceed the capabilities of quartz. Furthermore, the burner's round shape provides a much more uniform temperature than can be obtained with a cylindrical shape. "The higher temperature means that we get better color rendering and more light for the same amount of power throughout the life of the lamp," explains Butler.

OSRAM researchers in Beverly, Berlin and Munich would like to do even more with Powerball. For instance, right now, the burner is a translucent, milky white, which is excellent for illumination. But if it were transparent, the product could also be used in the rapidly growing beamer and automotive headlight markets. The question is: how do you make a ceramic object transparent? Today, the burners are made of particles that are fused together. But each particle tends to scatter light. "So what we need to do," says Gustafson, "is either make the particles so small they can't scatter light, or make them so large they act like sapphire crystals." In either case it will not be a question of getting more light out of an HID, but of being able to focus it exactly where it's needed.

Converging Beams. Like the light from today's ceramic burners, research developments in the lighting products market may appear to be scattered; but a number of trends are bringing developments together into a sharp, new focus. Spurred by the quest for mercury-free products and the resulting research in electronic ballasts, lamps are becoming smaller, lighter, more energy-efficient, more environmentally friendly, more dimmable and more capable of producing white light over a longer lifetime.

Converging Beams. Like the light from today's ceramic burners, research developments in the lighting products market may appear to be scattered; but a number of trends are bringing developments together into a sharp, new focus. Spurred by the quest for mercury-free products and the resulting research in electronic ballasts, lamps are becoming smaller, lighter, more energy-efficient, more environmentally friendly, more dimmable and more capable of producing white light over a longer lifetime.

What these trends add up to is that lamps are likely to become responsive, networked objects. Combined with sensors that measure light on the work surface, and standards such as DALI (Digitally Addressable Lighting Interface), that allow lighting products from different manufacturers to communicate with a building's management system, a new vision of lighting is emerging.

"Until now, interior lighting was not expected to look particularly natural. But thanks to the convergence of a number of technologies, this is changing and we are moving toward a new concept called adap-tive lighting," says Dr Reinhard Weitzel, head of light sources research at OSRAM, Munich.

The idea is that artificial lighting will adapt to and merge with natural light and that these combined sources will change in harmony with daily and seasonal light levels. If broadly implemented, such a technology would save energy while improving worker productivity (see article Architecture of Light). In the U.S. alone, lighting-related energy consumption amounts to some 60 GW per year, with each GW being the energy equivalent of four million tons of coal. By automatically dimming lighting in response to sunlight, a huge amount of lighting energy would be saved.

Flat Light. Already, the quest for mercury-free products is paying off. PLANON, a revolutionary, flat fluorescent white light source recently introduced by OSRAM, is 100 % mercury-free. Although not as bright as conventional fluorescent sources, the new lamp's patented pulsed excitation technology delivers an extraordinary 100,000 hours of service. Furthermore, to enhance its bright-ness, OSRAM researchers, in conjunction with Germany's Federal Ministry of Education and Research, are studying the development of new phosphors that could produce two visible light photons in response to each ultraviolet photon they are struck by. "This has been a dream in the lighting industry for years," says research director Weitzel. "But we are still very far away from realizing it."

Meanwhile, OSRAM researchers are mulling over the possibility of building LEDs into conventional lamps. The idea is to allow them to alter their color rendition as naturally as dimming alters their light output. "The problem is that LEDs cannot withstand the temperatures in some areas within fluorescent lamps," explains Weitzel.

Inside or out, LEDs are set to revolutionize the lighting landscape. Already in vehicles and traffic lights around the world, they mark a fundamental transition in the evolution of lighting. "Instead of burning a tungsten filament or a gas, LEDs produce light directly from electrons," says Dr. Makarand H. Chipalkatti, Director, Lamp Modules, North America, at OSRAM Opto Semiconductors in Danvers, Massachusetts. Chipalkatti foresees that in the next decades, in addition to traditional lighting fixtures, there will be more and more LED-based systems and that they will be integrated into walls and ceilings. "Some LEDs already have a 100,000-hour life span. And LEDs can be combined to create any imaginable color. Furthermore, they have the potential to reach 100 lm/W and more," says Chipalkatti. "If we can get the price per lumen down, we'll be able to have our cake and eat it too."

Like the computer industry of the 1970s, today's lighting products industry is heading in the direction of smaller, more versatile, more efficient, hybrid and, ultimately, networked products. Eventually, OLEDs – paper-thin electro phosphorescent sheets sandwiched between conductors (see Brilliant Plastics) – will blur the distinction between light and information, allowing us to live in virtual worlds in which walls of light effortlessly give way to video telephony, television and much more. Understanding the arcane physics of the lights on factory and department store ceilings is simply the first step down that long, bright path.
                                                                                                                 Arthur F. Pease