In a remote-source lighting (RSL) system, a single source distributes light evenly over a distance to several outlets. Benefits of RSL include:

  • Reducing the amount of infrared and ultraviolet energy introduced into a space
  • Improving safety in wet or explosive environments
  • Improving targeting of light
  • RSL also provides an aesthetic appeal that often can’t be achieved by other means.

    Losses in the light-distribution system make RSL less energy efficient than conventional systems; however, in some cases, the precision targeting of light can make up for those losses. In addition, RSL systems may lead to higher system efficiencies, as they enable the use of intense, efficient light sources that are difficult to use in conventional fixtures without producing too much glare.

    RSL is more expensive than conventional lighting for most applications but is appealing in certain contexts, including 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 RSL have also found a home in tunnels, warehouses, and industrial facilities. Additionally, prototype testing demonstrates RSL’s potential as 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 the light doesn’t add to the cooling load on the case compressors. Also, you can use RSL to bring daylight into a space and reduce your need for electric lighting.

    What are the options?

    RSL is distributed with either fiber-optic or light pipe systems and can be employed using a variety of light sources, 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).

    Figure 1: Alternatives for light distribution using fiber-optic cables

    A fiber-optic system can emit light at the end of the cable, continuously along the length of the cable, or at discrete points along its length (although this configuration is less common than the other two). In a side-emitting fiber, light refracts out of the fiber through deliberate imperfections at the boundary of the core and cladding. In an end-emitting fiber, a fixture projects the light. Series-source emitters have several small surfaces along their length that emit light.
    Diagram showing how end-emitting, side-emitting, and series-source-emitting lights work with the coupling system and fiber-optic cable.

    Side-emitting fibers are most often used as an alternative to neon lights, as they offer greater flexibility and energy efficiency than neon systems; however, they’re not always as efficient as LEDs, which have become a popular alternative to neon. Since fibers carry no electricity, they can be used in areas where neon can’t, such as in water.

    End-emitting fibers depend on fixtures to disburse the light. They’re most commonly used in 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. They also 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 one light source can feed 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, you might feed several hundred fixtures from one source. For more-sensitive applications, you might limit the number of fixtures 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).

    Figure 2: Two types of light pipe

    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.
    Diagram showing acrylic prismatic-film light pipes and metal mirror-surfaced light pipes.

    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 RSL 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. Adding filters to the light source can provide a variety of colorful effects.

    Sunlight can boost the value of RSL systems. Studies have shown that daylighting may reduce energy costs, improve productivity, and provide health benefits. The most common way to integrate sunlight into a remote-source configuration is with daylight pipes. Daylight pipes are reflective tubes that carry light from the roof of a building into occupied spaces. Their 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.

    Although daylight pipes were originally developed for residential applications, facility managers are also using them in commercial and industrial spaces. Enhancements to the standard daylight-pipe configuration include lenses and reflectors that enable the capture of daylight beginning earlier in the day and extending into dusk.

    How to make the best choice

    Find the right application for RSL RSL systems are expensive, but sometimes they’re the best or the only possible choice for lighting in wet areas, explosive environments, and situations where it’s important to minimize exposure to ultraviolet radiation, such as museums.

    Determine the cost-effectiveness of RSL In some situations, lower energy expenditures and reduced maintenance costs can justify the added up-front cost of implementing an RSL system.

    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’re wider. Use light pipes where you need a lot of light, such as tunnels and warehouses. For the same dollar investment, you can transmit more light via light pipes than fiber optics.

    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 you’ll have to add light sources every 60 to 70 feet. All light-transport mechanisms tend to absorb some wavelengths of light more than others do, so the color-quality output may not be as good as the color-quality input. This phenomenon is more pronounced with fiber-optic cables than with light pipes, but it’s a function of length in both cases.

    What’s on the horizon?

    Lasers, with their tightly collimated (parallel) beams, could also eventually be a good source for fiber optics. Applications are limited because none of the available laser sources generates white light, and they’re still too expensive to be practical for most applications. Light from lasers is also so intense that special measures are necessary 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. With its high intensity, small size, and long life, Ceravision’s product could make it a good choice for fiber-optic-lighting applications.

    Who are the manufacturers?

    Here is a partial list of manufacturers of RSL products:

    Fiber optics

    Light pipes

    Daylight pipes

    Neither this list nor any mention of a specific vendor or product constitutes an endorsement or recommendation by the authors, nor does any content in the Business Energy Advisor constitute an endorsement or recommendation, explicit or otherwise, of the technology-related programs mentioned herein.

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