Lighting: Remote-Source Lighting
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In a remote-source lighting (RSL) system, light from a single source is carried over a distance to one or more light outlets, or is emitted evenly along the way. Several benefits result from this type of light delivery system, including:
- A reduction in the amount of infrared and ultraviolet energy introduced into a space.
- Improved safety in wet or explosive environments.
- Improved targeting of light.
Remote-source lighting also provides an aesthetic appeal that often can’t be achieved by other means.
Losses in the light distribution system generally make remote-source lighting less energy efficient than conventional systems, but in some cases the precision targeting of light can make up for those losses. In addition, RSL systems may eventually lead to higher system efficiencies since they enable the use of intense, efficient light sources that are difficult to use in conventional fixtures without producing too much glare.
Remote-source lighting is more expensive than conventional lighting for most applications, but it has appeal in certain markets, such as sidewalk and building outline lighting; underwater lighting in pools, fountains, and aquariums; and special effects lighting, accent lighting, wall washing, and downlighting. Some types of remote-source lighting have also found a home in tunnels, warehouses, and industrial facilities. Prototype testing has shown RSL to be an energy-efficient alternative for providing illumination in refrigerated display cases. In that application, the light source is kept outside the display case so that light can be provided without adding to the cooling load on the case compressors. In addition, RSL can be used to bring daylight into a space and reduce the need for electric lighting.
What Are the Options?
Remote-source light is distributed using either fiber optic or light pipe systems. A variety of light sources can be used, including several types of high-intensity sources and sunlight.
Fiber Optics
Fiber optic systems feature a light source; a set of reflectors, filters, and lenses to feed the light to the fiber optic cables; and a fixture to distribute the light at the point of illumination. Light sources for fiber optics need to be as small as possible to provide a tightly coupled optical system that can yield high transmission efficiencies. Most fiber optic cables are either side-emitting or end-emitting, although there are some series-source-emitting configurations as well (Figure 1).
A fiber optic system can emit light at the end of the cable, continuously along the length of the cable, or, more rarely, at discrete points along its length. In a side-emitting fiber, light refracts out of the fiber by way of deliberate imperfections at the boundary of the core and cladding. In an end-emitting fiber, a fixture is used. Series-source emitters have a number of small surfaces along their length that emit light.
Source: Lumenyte International Corp.
Side-emitting fibers are most often used as an alternative to neon lights. The fiber optics option offers greater flexibility and greater energy efficiency than neon systems, although they are not always as efficient as light-emitting diodes (LEDs), which have become a popular alternative to neon. Since fibers carry no electricity, they can be used in areas where neon would not be acceptable, such as under water.
End-emitting fibers depend on fixtures to disburse the light. The most common ones available today are downlights, wall washers, accent lights, and special fixtures for landscape and underwater applications. Major fixture manufacturers have been reluctant to put much effort into accommodating remote-source fiber lighting systems because they recognize that fiber systems mainly serve limited niche markets. In addition, they recognize that the design of fiber systems is in a state of flux, with fiber optics developers continually changing their cables and light sources.
The number of fixtures that can be fed from one light source depends on the intensity of the light source and the lighting requirements at the end of the run. For decorative applications where light distribution isn’t crucial, several hundred fixtures might be fed from one source. For more sensitive applications, the number of fixtures might be limited to single digits.
Light Pipes
Light pipes, also known as light guides, feature a hollow interior lined with a reflective inner surface that directs light within the tube. The most common linings are prismatic films and mirrored surfaces (Figure 2).
In acrylic prismatic-film light pipes, a sawtooth pattern of prisms reflects light down the hollow interior of the tube; light escapes along the length of the tube through the light-extracting outer surface material. The cross-section of these acrylic tubes may be circular, rectangular, or any other closed shape. In end-emitting, metal mirror-surfaced light pipes, the internal surface of the tube is polished to be highly reflective.
Source: Jennifer Schuman et al.
Most light-pipe applications require side-emitting tubes that can carry electric light or daylight. These pipes feature a layer of translucent or transparent material surrounding a layer of prismatic light-reflecting film. The light is released evenly along the length of the tube. Mirrored surface tubes are most often used with daylight pipes that emit light at the end of the tube.
Light Sources
Most remote-source lighting systems use metal halide lamps or sunlight. Metal halide lamps provide high-quality light with high efficiencies (around 90 lumens per watt) and high intensities (a 32-watt metal halide bulb puts out as much light as a 4-foot fluorescent tube). Metal halide lamps also produce high levels of ultraviolet radiation, but remote lighting distribution systems prevent the radiation from reaching the illuminated space. Filters can also be added to the light source to provide a variety of colorful effects.
Sunlight holds significant potential for boosting the value of remote-source lighting systems. Studies have shown that daylighting may reduce energy costs, improve productivity, and create health benefits. The most common approach to the use of sunlight in a remote-source configuration is with the use of daylight pipes. Daylight pipes feature reflective tubes that carry light from the roof of a building into living or working spaces. The basic components include a clear plastic dome that sits on the roof and lets in sunlight; a reflective tube that carries light into the interior; and a light diffuser, which looks like a ceiling light fixture and distributes light around the room. Originally developed for residential applications, daylight pipes are also being used in commercial and industrial spaces. Refinements of the standard daylight-pipe configuration include lenses and reflectors that enable the capture of daylight beginning earlier in the day and extending into dusk.
A different approach has grown out of a program called the Hybrid Lighting Partnership, sponsored by the U.S. Department of Energy. This effort has led to the development of a hybrid lighting system in which fiber optic light guides channel sunlight into the interior of commercial and industrial spaces. A spin-off company, Sunlight Direct, was formed in 2006 to commercialize the technology and now offers the SolarPoint Lighting System, which consists of a sun-tracking mechanism, a parabolic reflector that gathers sunlight, a fiber optic system that transports the sunlight to the interior of a structure, and fixtures that distribute the light at the desired locations. The systems include photosensor controls to dim artificial light sources in the presence of daylight, so that the combined output of the two will consistently meet a specified illumination setpoint.
How to Make the Best Choice
Is the application right for RSL? Remote-source lighting systems are expensive, but sometimes they are the best or the only possible choice when lighting is needed in wet areas, explosive environments, and situations where it is important to minimize exposure to ultraviolet radiation, such as museums.
Determine the cost-effectiveness of RSL. In some situations, the added cost of a remote-source lighting system can be recovered in lower energy and maintenance costs. Table 1 presents a sample calculation.
| Remote-source lighting (RSL) systems have the potential to reduce energy and maintenance costs in some situations. This cost analysis shows that for a particular set of conditions, reduced maintenance and energy costs for a downlighting application can lead to a payback period of about 3.2 years for a fiber optic system compared to a conventional system using MR16 bulbs and fixtures. The conventional system features six 35-watt (W) MR16 downlights; the fiber optic system contains a single 150-W metal halide source and six 15-foot runs of cable to six downlight fixtures. Losses in the fiber optic cable reduce the light output, so the two systems provide approximately equal levels of illumination. The analysis is based on the assumption that the RSL lamp is outside of the conditioned space and therefore eliminates most of the cooling load normally associated with lighting. | |||
| Conventional downlighting system (six 35-W MR16 downlights) | Costs ($) | Fiber optic system (one 150-W light source, six 15-foot runs of cable, and six fixtures) | Costs ($) |
|---|---|---|---|
| 6 downlights | 540.00 | 1 illuminator | 742.50 |
| 6 lamps | 36.00 | 90 feet of 7 mm optical fiber | 405.00 |
| 6 outletsa | 600.00 | 6 fixtures | 225.00 |
| 1 outlet | 100.00 | ||
| Total equipment cost | 1,176.00 | 1,472.50 | |
| Installation labor: 6 hours (at $45 per hour) | 270.00 | 270.00 | |
| Total installed cost | 1,446.00 | 1,742.50 | |
| Cost premium for fiber optic system | 296.50 | ||
| Operating parameters | Metrics | Metrics | |
| Power in watts, with ballast losses | 240 | 187.5 | |
| Hours | 3,640 | 3,640 | |
| Energy used for lighting, in kWh per year | 873.6 | 682.5 | |
| Cooling energy, in kWh per yearb | 205.6 | 60.2 | |
| Operating costs | Costs ($) | Costs ($) | |
| Lighting energy cost at $0.10/kWh | 87.36 | 68.25 | |
| Cooling energy cost at $0.10/kWh | 20.56 | 6.02 | |
| 2 lamp changes per year | 72.00 | 0.6 lamp changes per year | 121.50 |
| 3 hours labor (at $45 per hour) | 135.00 | 0.6 hours labor (at $45 per hour) | 27.00 |
| Total O&M cost | 314.86 | 222.77 | |
| Annual savings ($) | 92.09 | ||
| Simple payback, in years | 3.2 | ||
| Notes: a. Outlet costs represent parts and labor required to get power from the panel board to the fixture. | |||
| b. Assumes 80 percent of input power becomes a cooling load for the MR16 system; only 30 percent becomes a cooling load for the remote-source system. The cooling load COP equals 3.4. | |||
| © E Source; data from Lumenyte International Corp. | |||
Pick the right distribution system. Light pipes and fiber optic cables both use the same principle of total internal reflection, but light pipes can transport much more light because they are larger in diameter. Use light pipes where a lot of light is needed, such as tunnels, warehouses, and other large spaces. Light transmitted per dollar invested is also greater with light pipes.
Use distribution systems that won’t produce excessive losses. The longer a light distribution system is, the less light it can deliver. For fiber optic systems, side-emitting fibers are usually limited to 100 feet or less, and end-emitting fibers usually need to be less than 50 feet long. Light pipes might be longer, but light sources must be added every 60 to 70 feet. All light-transport mechanisms tend to absorb some wavelengths of light more than others, so the color quality at the output is not as good as the color quality input. This phenomenon is more pronounced with fiber optic cables than with light pipes, but it is a function of length in both cases.
What’s on the Horizon?
One of the biggest sources of losses in an RSL system comes in the optical coupling between the light source and the light pipe or fiber optic cable. Smaller sources make it easier to provide an efficient coupling. Enter LEDs—their small size makes them an ideal source to use in an RSL system, but in the past, LEDs have been unable to provide high enough levels of brightness. Recent developments are changing the picture—the newest, high-brightness, white LEDs offer 80 lumens of light output in a tiny package roughly 0.1 inches by 0.2 inches. They have an efficacy of about 70 lumens per watt—approaching the efficacies of metal halide and fluorescent lamps. Products in the laboratory are exceeding the efficacies of other lamp technologies. As LEDs continue to improve, they could well be employed in making more efficient RSL systems.
Lasers, with their tightly collimated beams, could also eventually be a good source for use with fiber optics. Applications are limited because none of the available laser sources generate white light, and they are still too expensive to be practical for most applications. Light from lasers is also so intense that special measures have to be taken to prevent fibers from melting. Nevertheless, leading manufacturers see lasers as a promising light source for the future of fiber optics.
A new type of light source using induction lighting technology and metal halide salts is under development by Ceravision. Its high intensity, small size, and long life could make it a good choice for fiber optic lighting applications.
Copyright 2006 - Platts, a Division of The McGraw-Hill Companies, Inc.