Hybrid Lighting:
Illuminating Our Future

By Michael R. Cates

 n "scriptori" of the Middle Ages, monks sat beneath open windows in the monastery and meticulously copied manuscripts all day long, dipping their quills for the last time when the sun sank beneath the horizon. The natural light of day made it possible to reproduce the intricate details of their work—and allowed their eyes to survive the nearly timeless daily ordeal. So it has been throughout all of human history: to see clearly, daylight was needed. Torches, bonfires, candles, oil lamps, and the like have enabled people to see well enough to perform routine tasks; but those forms of artificial light were a burden to visual acuity and long-term eye use. Only the introduction of electric lighting, barely a century ago, posed a serious challenge to the primacy of the traditional pattern of daylight and darkness. Today, on the verge of entering a new millennium, we now live almost every waking hour in a world of artificial light. The modern world is flooded with extra light. In all developed and developing countries, the vast majority of the population performs its daily work in enclosed structures equipped with lighting systems. Many people sit at desks, spending hour after hour reading, writing, filing, and studying. We have come to expect an abundance of light because the tasks of our technological age require greater perception of detail and longer times for high-resolution viewing. As a result, it is common to see rooms in homes and businesses lit so well and so uniformly that small print can be read in any corner. Many offices today have both bright ceiling fixtures and high-intensity lamps on the desks.

In the United States alone our electric lighting bill is about $100 million a day. Lighting uses 25% of all our electricity. Because of the demand for extra illumination, our nation's use of artificial lighting, paradoxically, peaks near 40% at the time of day when the sun is highest and natural light is the most abundant.

We must also pay the price for artificial lighting's inefficiency—namely, its production of heat. Some of our cooling costs result from clearing away this heat.

Lighting costs money—more than most people realize. The costs are not just simply the prices of fixtures, replacement lighting elements, and the electricity to operate them. We must also pay the price for artificial lighting's inefficiency—namely, its production of heat. About 10% of our cooling and ventilation costs result from clearing away the heat generated from lighting. As the use of artificial illumination has grown more extensive and necessary, numerous efforts have been made to reduce cost, increase efficiency, and improve quality. Significant improvements that have resulted over the years include extensive use of fluorescent fixtures; halogen, sodium, and mercury vapor elements; and automatic turn-on and turn-off systems. Compared with standard incandescent bulbs, these improvements have been major; nevertheless, the world's light bill will continue to rise considering that the amount of office floor space is expected to double worldwide by 2020.

For many years the DOE has been aware of the national energy problems associated with lighting. So DOE has sponsored programs to develop and evaluate improved systems. One obvious way to reduce energy use for lighting rooms is to supplement artificial light with natural light. In these programs, based on available technology and practical cost consideration, emphasis has been placed on windows and associated devices to optimize use of light passing through windows. The idea of supplementing artificial light with natural light clearly is a useful step; however, the proper course would be perhaps a reversal of emphasis: to supplement natural light with artificial. Use of artificial light to add to daylight is the fundamental idea behind a system we call hybrid lighting.

What is Hybrid Lighting?

Picture a room with windows. Let the sun shine in. Sensors determine where light levels are too low, where the dark spots are. Special lamps turn on to illuminate those areas so that the room has a uniform light level. As the light levels begin to change, sensors provide feedback to the lamps, which adjust levels of artificial light to maintain constant lighting in the room.

Hybrid lighting is a combination of natural and artificial illumination to be used indoors for all lighting needs. Ideally, hybrid lighting is effectively indistinguishable from standard artificial lighting except in quality and cost, where it will likely be an improvement. Hybrid light fixtures will allow use of all available natural light and supplement it with the amount of artificial light required to bring the total level of illumination to the rated value. As shown in the graph below, the level of natural light available is quite high during working hours in most places on most days. For many hours during a typical work week, essentially all illumination could be provided by natural light. In fact, fully hybridized lighting in industrial and commercial working environments could cut artificial light requirements in half. By combining natural light and improved artificial sources available today—centralized, high-efficiency light sources—energy costs for lighting could be reduced by one-third.

Relative solar illuminance vs time of day in the United States. Note that use of artificial lighting peaks at the time of day when the sun is highest and natural light is the most abundant. In hybrid lighting systems, sunlight is redirected rather than wasted or converted into heat and is also conveniently available in midday, when the need for energy conservation is the most critical.

Why Is Hybrid Lighting Needed?

Lighting costs are an issue because they include not only fixture prices but also installation, retrofitting, and maintenance. When new construction is planned, modern lighting costs that are higher than those for standard, low-efficiency lighting can be justified (unless they are exceptionally high), because of long-term efficiency. For retrofitting, the same argument holds, again, as long as the cost is not extraordinary. For maintenance it is important that hybrid systems cost little if any more than today's standard systems. Long-term efficiency improvements can pay for only so much. Despite the massive benefit to the whole society that accrues when large amounts of electric power are saved, individual investors will not cooperate unless their individual "bottom line" is in the black.

As for user friendliness, we must not underrate its importance. Consider fast food in the United States today. Is it a multibillion dollar industry because the food is cheaper or even tastier than similar food you could prepare at home? No, the enormous success of the industry arises mostly because of the user-friendliness factor. We don't have to pay too much, don't have to cook, don't have to clean up, don't have far to go, and can buy a meal that has the quality we expect. We aren't involved in the details; we simply go get (or call for, or fax for!) fast food whenever and wherever we want it. Similarly, when we throw the switch to light a room, we normally expect to have no further involvement with the process. Without the user friendliness factor, cost arguments are usually abandoned because our time and effort often mean more to us than cost. Of course, we can attach a kind of virtual cost to our time and effort: with such a cost added in, the pure monetary savings of some innovation could be washed away in a flood of the red ink of inconvenience.

Mike Cates examines the flow of light through a large "light pipe."

Related to user friendliness, and a factor of extreme importance, is aesthetic appeal. Lighting engineers all over the industrialized world spend considerable effort in designing lighting systems that are attractive, interesting, unobtrusive, and highly decorative; they are often major points of focus or part of an overall theme. Any successful hybrid system must be able to meet most of the criteria for aesthetic appeal. Ideally, hybrid systems should be able to take virtually any form, especially since standard light bulbs, fluorescent tubes, and the like may not be needed. Despite the convenience, low cost, and all the other more tangible factors, attractiveness will often be the final decision maker in the selection of a lighting system, even in some commercial enterprises.

Given these caveats, the idea of passively redirecting as much natural light as possible, combined with artificial light as needed, has obvious appeal. Hybrid lighting would already be a fact of everyday life if the realization were as obvious as the concept. The importance of energy conservation and illumination quality have been understood for a long time, but little changed in business and home lighting systems from the end of World War II until a few years ago. Recently, however, political and financial incentives have arisen that urge improvements in these areas. Heightened concerns about environmental quality and waste disposal within developed countries have led to more interest in efficiency and quality. Increased information-based economic activity has brought more and more workers indoors, requiring them to have improved illumination. After hours, too, average citizens spend longer times indoors, both at home and in businesses that stay open later and later. The opening up of previously closed Third World societies has revealed numerous environmental problems related to energy use as well as untapped resources and economic opportunities. In response to the perceived need to reduce growth in energy consumption, major producers of lighting equipment have begun widespread marketing of higher-efficiency bulbs and fluorescent tubes, improved ballasts, and new designs for various replacement components. Many of the technical capabilities have been available a long time but have only recently been commercialized. With these improvements in artificial lighting, however, little has been done to emphasize natural light. In homes, taking advantage of the sun and sky has been limited mostly to skylights and picture windows. For commercial buildings large windows are often used, but primarily for the view afforded rather than the extra light provided. Stores like Wal Mart, however, are making greater use of natural light to attract customers.

In recent innovations of the lighting industry, no extensive emphasis has been placed on combining natural and artificial lighting in any quantitative, integrated way. Hybrid lighting has recently begun to make sense economically because it has begun to make sense technically. With the former energizing the latter, we have entered a period in which hybrid lighting is finally practicable. So, how do we make it happen? What are the remaining technical hurdles? What stands in the way of our taking advantage of a clearly beneficial concept?

Components of Hybrid Lighting

The solar collector on the roof gathers sunlight to illuminate the building; inside the building are light pipes to transport the light to each room.

Enough light strikes the roof on a sunny day to light every room in the building. The problem is collecting the light, then getting it where you want it to go.

Light collection and concentration. All natural light comes, of course, from the sun. On a cloudless day, with the sun high in the sky, the amount of sunlight falling on the surface of the earth is more than 1000 watts per square meter, so we do have a lot of light to work with. We are instinctively aware of the abundance of light when we step outside and squint briefly as our eyes make the transition to the difference from the adequate but much less intense light in our home or office. Keep in mind that 1000 watts of light per square meter is the value in the visible wavelengths. As compared with incandescent or fluorescent light bulbs, it is pure light. A 100-watt light bulb, for example, uses 100 watts of electricity but produces only about 17 or 18 watts of light. The rest is heat. One square meter of bright sunlight, then, is equivalent to turning on about 55 100-watt light bulbs. Most offices or small rooms can be very well lit by two or three light bulbs; consequently, a square meter of sunlight could theoretically light about 20 rooms or offices. If the roof area of a building is taken to be approximately the floor area of any one of its floors, enough light strikes the roof on a sunny day to light every room in the building even if it's more than a hundred stories high! The problem is collecting the light, then getting it where you want it to go.

Yes, the sun is a wonderful giant light source, but we on the earth are constantly moving with respect to it. Depending on the time of year and time of day, the sun's light arrives from different angles. The most efficient way to gather the light is to constantly track the sun's position and focus the maximum amount of available light all the time. This sort of thing is done in some experimental solar energy plants and solar furnaces. For lighting, however, this technique is probably too cumbersome and expensive. Trackers are electromechanical devices that have moving parts. Moving parts wear out much more often than static systems. They require energy and some sort of microprocessor to keep them going. They can fail or get out of alignment. They can be blown over or away in a storm; be damaged by hail; or get their mechanisms clogged with leaves, dirt, twigs, pine needles, or whatever. Fortunately, as we have seen, for a large part of a sunny day, considerably more sunlight hits the roof of a building than is needed inside to light it; consequently, it makes sense to develop and place on roofs fixed, passive collectors and concentrators that look good and require little or no maintenance. A number of designs have been worked on, often, however, with the goal of optimizing collection efficiency. The goal should be rather to optimize cost, maintainability, attractiveness, and efficiency, probably in that order.

A representative illustration of this kind of optimization is the use of skylights. In many cases they add a useful amount of light that is both attractive and essentially free of maintenance. Though skylights can and will play an important role in hybrid systems, in themselves they are not efficient enough, nor do they distribute light to remote locations. Collectors, which are required for distributing light, though necessarily more complex than a passive skylight, can be designed with optimization in mind. For example, a collector could have three fixed parts, one looking straight up to catch the sun in the middle of the day, and two angled parts, to catch the sun in the morning and afternoon. Such a collector might be very inefficient compared with an active tracker and relatively inefficient compared with a multicomponent passive system; however, it could easily be much cheaper and require less maintenance, even if its collection area had to be scaled up by comparison.

Concentrators can be lenses or mirrors. Modern plastic materials can probably be used, cast, molded, or extruded rather than finished with any kind of precision. It is not necessary to have image-quality optics. Plastic materials may also be able to filter out naturally occurring ultraviolet light or to allow the transport of infrared wavelengths into different optical circuits for nonlighting purposes.

Schematic of light-coupling components in a typical hybrid lighting arrangement.

Generating artificial light. Incandescent and fluorescent lamps are designed to produce a spectrum of illumination that serves a particular purpose. If exactly duplicating sunlight were our purpose, artificial lights would have to look like a 5750 K blackbody shining through several miles of atmosphere, made up mostly of nitrogen, oxygen, and water vapor. Clearly, the challenge is formidable. Few incandescent lamps can operate either efficiently or for any length of time at extremely high temperature, so a compromise has been worked out. Standard 100-watt bulbs, for example, operate at about 2850 K. Of course, this kind of light makes things look a little yellowish compared with the vivid clarity and whiteness of sunlight. Fluorescent fixtures, which were designed partly to deal with that spectral problem, have the added benefit of being more electrically efficient. Fluorescent lamps operate by producing a high-voltage discharge through a gas volume that, in turn, emits light well up into the ultraviolet portion of the spectrum. The ultraviolet light stimulates fluorescence from a powder layer deposited on the inside walls of the glass envelope or tube. The particular phosphor on the tube walls determines the visible spectrum it emits. The most common fluorescent lamps, in use in virtually all commercial buildings, emit a white spectrum that is reasonably close to that of natural light. Other phosphors are used to make yellower light or to approximate the optimum distribution of wavelengths for growing plants (as in grow-lamps).

Halogen lamps and bulbs containing metal vapors produce well-defined spectra that are effective sources of illumination. But like the incandescent and fluorescent sources, they also are no real match for sunlight. Because no artificial source is just right, practicality takes over. We have gotten used to artificial lights that are not just like sunlight. Besides, sunlight itself varies enormously. Sunset and noon are equally valid times of the day, after all. And human beings, struggling to survive and thrive over the millennia, learned to use red light as well as blue. We have also learned to open our eyes wide to the dim illumination at sunset and at night and peer carefully through shaded eyes in the squintingly bright middle of the day. Because we don't need precisely tailored spectra in artificial illumination, cost and convenience become the overriding factors. Nevertheless, the more closely lighting approximates the solar spectrum of a clear day, the better our visual acuity.

There are also psychological factors relating to illumination spectra. Lighting that is lower in intensity and redder in color "feels" quite different to the human psyche than brighter and bluer light.

Most people would probably be satisfied if indoor lighting approximated that experienced outside.

Hybrid lighting can, in principle, generate any of a variety of spectra and intensity levels, but most people would probably be satisfied if indoor lighting approximated that experienced outside. In theory, if cost is no major consideration, the user could dial a spectrum, very much like dialing the frequency equalizer of a stereo. Microprocessors would activate various filtering and monitoring instrumentation and, voila, out would come the dialed-in spectrum. But, let's save that for the next decade of hybrid lighting. Let's assume that people will be content with sunlight of the given day mixed with a good standard artificial spectrum, perhaps with some blue enhancement in the early morning and later as the afternoon wears on.

Perhaps the most important new consideration in the production of artificial light is the use of a centralized source. Gone would be the need for separate light bulbs and the maintenance associated with them. Gone would be the heating in each lit room that comes from the wasted energy of operation. Centralized sources, taking advantage of the most efficient processes available having staggering luminous efficiencies that dwarf those of existing fluorescent lights, can be locally cooled and ventilated. They can be isolated but accessible, like a water heater or washing machine. It is also possible to design centralized systems where one or more spare sources are wired into the system, coming on automatically when the active one fails. Fortunately, this idea is more than just a dream. Companies like GE Lighting and Fusion Lighting, Inc., are making legitimate headway on a variety of new concentrated light sources.

Light distribution. Imagine street lights without light bulbs; the illumination from the top of the lamp posts is piped from light sources at the sidewalk level—making it easier to change burned-out light sources (see drawing below). Imagine a tunnel subway having only 5% the present number of light bulbs interconnected with passive systems bringing in outside light.

In this concept for a street light, the source of light is at the bottom rather than the top of the pole, making it easy to replace the light bulb. The light is transported up the pole and out the lamp at top.

The secret to light distribution is fiber optics. Optical fibers have already brought about a revolution in the technology of data transport and communication. Their entry into our homes is imminent—to carry television, computer, and telephone signals. Yet, these little bundles of single-mode high-purity silica strands are unlikely to be used for lighting. That will become the job of their flabby, less sophisticated cousins—fibers of plastic, perhaps filled with a clear gel—that are approximately the size and weight of the electric wiring now ubiquitous in modern construction. The photograph below shows a gel-filled fiber with a frosted bulb attached to the end. The bulb glows from the sunlight piped through the fiber. In new construction and in many types of retrofitting, plastic "wires" for light can be handled and routed like electric wiring.

Large-core optical fiber conducting sunlight into a diffusing bulb.

Large optical fibers like this, often called optical light guides, are capable of transporting large amounts of light. For example, bright sunlight passing through a square-meter area—enough to light several rooms—can be focused into and transported by a guide one square centimeter in cross section or less. This guide can lead into a number of small guides, in much the same way electric current is distributed into different outlets of a particular circuit.

Optical fibers of any cross section are more efficient when they are fabricated with a highly transparent central core surrounded by a transparent cladding of material with a lower index of refraction than the core. At the interface of core and cladding, light impinging at less than a certain angle (called the critical angle), is perfectly reflected off the optical interface with the cladding and continues down the core to its ultimate destination. Light guided in this way will have a very low leakage rate; consequently, little heating will occur along the guide. Light guides, then, can be handled without special equipment; they will not produce an electric shock of course, although looking at the bright concentration of sunlight could be hard on the eyes. Light coming out of an optical wave guide, however, is not generally as dangerous as that from a laser, because the waveguide light exits in an expanding cone (determined by the numerical aperture of the guide) as opposed to the more dangerous, narrow collimation of a laser beam.

Schematic of an optical fiber having a highly transparent control core surrounded by a transparent cladding whose index of refraction is lower than that of the core.

Controlling illumination. Because sunlight is so variable, both in intensity and in location, hybrid lighting must have active control of light levels at all times. Fortunately, current technology has provided a number of inexpensive, compact sensors and electronic components that will help with the problem. A typical day for most of us goes something like this: the sun rises on a clear morning, clouds develop at midday leaving the early afternoon partly cloudy, giving way perhaps to late afternoon clearing before sunset. Of course, other days are cloudy from dawn to dusk and a few are continuously clear. Whatever the conditions, hybrid lighting sunlight collectors will be taking what is available and routing it to the various fixtures in various rooms. Most of the time artificial light must be added to what nature provides if we are to achieve a uniform level adequate for modern activity.

Light sensors will almost surely be an integral part of every hybrid lighting fixture. It is possible to sample several wavelength bands in the visible spectrum by using filtering techniques or sensors with sensitivity peaked in certain wavelength bands. Thin-film photodiodes and similar devices can be located in unobtrusive, hidden, or decorative places. Circuit chips can be similarly configured. In mass production these controlling elements will not make up a large fraction of the lighting cost despite the fact that they will make the difference between a curiosity or decorative system and a full-time, hands-off lighting package.

An example of a hybrid lighting fixture in which sunlight is routed through an optical fiber to two of the four 40-watt tubes. The fixture's sensors constantly monitor the room light level.

To illustrate the idea, consider the light fixture pictured here. For clarity, we have attempted to simulate a typical fluorescent light fixture containing four 40-watt tubes. In this case, however, two of the tubes are replaced by diffusers into which sunlight is routed from an optical fiber system something like those previously mentioned. Built into the fixture's structure are a couple of illumination sensors that constantly monitor the room light level. Part of the electronic package taking the place of the usual ballast is an active controller, laid out in a rugged solid-state circuit design. The current and voltage to the second pair of tubes, true fluorescent lamps, are varied to produce a total level satisfactory to the constantly vigilant room sensors. If the fluorescent tube has sufficient brightness capacity to illuminate the room fully when no sunlight is available, such an arrangement can provide a room with a level of light that always seems about the same as far as the user is concerned.

Looking ahead

As the human population grows and the global economy expands, drawing in more and more of the previously excluded portions of society, demand for energy is certain to increase at a precipitous rate. Growth in the production of goods and services will naturally follow in a similarly skyrocketing fashion. Enormous increases in pollution, waste, and resource depletion will go hand in hand with all these increases in production. In the United States and in other industrial societies, we may continue to believe that the costs and availability of resources and energy will remain about the same. Realistically, however, most of us know this is a foolish assumption, especially because of its near-certain negative impact on our children and grandchildren. If for no other reason, population growth and demand for more energy justify development and deployment of hybrid lighting systems.

Hybrid lighting sunlight collectors will be taking what is available and routing it to the various fixtures in various rooms. As technology develops in the coming decades, some of its improvements can be important to hybrid lighting. There will probably be major gains in the quality of the components previously discussed, along with decreases in their costs. For example, more easily manageable optical fiber bundles with better optical transmission, improved collectors and control circuits, and better spectral selection are very likely to come about for fewer dollars. As solar conversion to electricity improves in efficiency and drops in cost, the same sunlight that is piped inside buildings and homes can be used to produce electricity to control illumination (as well as other numerous—and perhaps new—uses for electricity). Hybrid lighting systems will give us a head start in designing collectors and appropriate architectural configurations that make best use of our ever-present fusion energy source eight light-minutes away. With the development of light storage systems—perhaps batteries that store light chemically, like high-energy fireflies—sunlight can be saved during bright days and used at night and on dreary gray afternoons. In the home of the future, the latest hybrid system would, upon your departure for work in the morning, switch off the light distribution and put the system into a light "storage" mode. The stored solar energy would then be used at some time when you are at home. With reasonable technological luck and a few years time, our children's or grandchildren's homes will likely be self-contained energy systems, providing light and power for themselves, with perhaps some left over for the community grid.

No matter what the future holds, lighting will continue to demand a large fraction of our energy needs. And, as technology grows all over the world, the requirement for higher-quality lighting will grow with it. Now, in an era of relative plenty, it is wise to advance the technology of lighting so that the future will run no risk of going dark.

B I O G R A P H I C A L Sketch

Mike Cates is a physicist in ORNL's Engineering Technology Division. Previously he was in the Centrifuge Division at the former Oak Ridge Gaseous Diffusion Plant and in ORNL's Applied Technology Divisions, where he established the Photonics and Laser Applications Group. A pioneer in developing phosphor-based thermometry, he has extensive experience in fluorescence applications, imaging, fiber-optics sensors, and related areas of photonics. His first contact with Oak Ridge was as an Oak Ridge Associated Universities fellow during his doctoral program. After graduation he worked at the Los Alamos National Laboratory for 12 years before joining the ORNL staff. Collaborating with Cates on the hybrid lighting project is Jeff Muhs, also of the Engineering Technology Division (shown with Cates in the photograph). Two other researchers in the same division, Steve Allison and Bart Smith, are becoming more involved in this project.

Mike Cates (left) and Jeff Muhs show components that should make possible hybrid lighting in which sunlight and artificial lighting are blended to save energy.

Suggested Reading Building Technologies Program 1993 Annual Report, Lawrence Berkeley Laboratory, LBL-35244, June 1994. Joseph B. Murdoch, Illumination Engineering, McMillan, 1985. "Plastic Optics Shine in High Volume Production," Photonics Spectra, March 1995.

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