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 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, 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.
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, 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.
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.