Fiber-optic lighting hass all the effects

April 1, 2003
In the market for creative illumination? Fiber-optic lighting may be the technology for you.Just when you thought you knew everything there is to know

In the market for creative illumination? Fiber-optic lighting may be the technology for you.

Just when you thought you knew everything there is to know about lighting, a new design comes up, a new market emerges or someone calls to demonstrate an exciting new product. Maybe, for you, fiber-optic (FO) lighting is that new thing. Continuously developed over two decades, FO lighting is generating interest among lighting designers, electrical contractors and engineers because of its challenge and complexity.

As this technology continues to mature and find more applications, efforts toward creating a common vocabulary, calculation procedure and testing methods are helping to reduce concerns about installation problems. Together, these activities are driving the use of fiber-optic lighting.

System specifics. Basically, FO lighting consists of two or three components. The first is the illuminator, or light source. The second is the fiber-optic tubing, which is made in two different types. The illuminator projects light into side-emitting tubing, which delivers light along the entire length of the tubing (giving the appearance of neon lighting), or into end-emitting fiber tubing. The third possible component is a fixture for end-emitting applications.

First, let's get down to some fundamentals. Fiber-optic tubing is one of the two methods for providing remote-source lighting (RSL). Exactly as the term describes, RSL is a technique of passing light from a light source through a medium (some sort of light carrying material) to a distant location. Besides the optical fiber, the second method for providing RSL is a prism light guide. (We will discuss the second method later.)

These transparent materials can carry light because of what is called total internal reflection. Consider that the wall of a light-carrying medium acts somewhat like a mirror. So, light (from an illuminator, which is also called a projector or light box) entering a fiber tube or strand is continually reflected as it moves down the path, until it comes out the far end. The far end of the tube, or cable, is terminated by a piece of hardware called a fitting.

Fiber-optic cabling is made with three different types of material:

*Large core plastic fiber. A solid polymer fiber, up to 20mm in diameter, is clad with a thin material having a lower refractive index than the core. It is generally made with a thermal set process to specific preset lengths.

*Small core plastic fiber. A solid polymer optical fiber, up to 2mm in diameter, is clad with a thin material having a lower refractive index than the core. It's made in any length, and can be field cut. Both types of plastic fiber perform similarly and have the same environmental limitations.

*Glass fiber bundles (GFB). A circular light guide made of glass with a diameter between 0.002 in. and 0.006 in. (about hair thickness). Glass fiber, which is usually the end-emitting type, has particular advantages including the fact that glass material does not lose its transparency (become yellow) over time, since it is unaffected by UV or IR energy. As mentioned above, the glass tails are much thinner than plastic. Unlike plastic fiber bundles, which can field cut, glass fiber harnesses are usually factory-cut and assembled.

Learning the lingo. The first task in understanding FO lighting is to know some of the terms and concepts of this technology:

Acceptance angle. The maximum beam spread of light (measured from the fiber's axis) that will enter one end of the fiber.

Attenuation. A measurement of how much reduction in light is experienced per unit length of fiber. It is measured in decibels/ft.

Color rendering index. A measure of the degree of color shift objects undergo when illuminated by the light source, as compared with the color of the same object when illuminated by a reference light source of the same color temperature. Incandescent and North-sky daylight are both rated 100 on a numbering system ranging from 1 to 100. The higher the number, the "truer" the color of objects appear.

Connectors, couplers and ferrules. These are hardware devices used to join parts of a system physically and/or optically. A connector holds a fiber to a port or a fixture, and a coupler aligns a fiber to the illuminator or two fibers to each other. A ferrule is a terminating device used to keep a fiber bundle properly positioned. Ferrules are usually factory-designed to work with particular fibers, so the installer simply inserts the ferrule into the fixture's connection sleeve.

A fiber-optic port or master ferrule is a metal or plastic connector attached to one of the fiber-optic bundles, allowing it to be readily inserted into an illuminator for maximum light output. A manufacturer can assemble the bundle (called porting) before shipping, or porting can be done in the field to suit changing job conditions.

A fixture is a hardware device placed at the termination of an end-emitting fiber to distribute the light in a specific way. Fixtures are generally sold as part of an entire FO system, but they are also available separately.

An illuminator is the power unit that houses the light source for the fiber-optic lighting system. Constructed of sheet metal or high-impact plastic, the illuminator is sometimes called the projector. In addition to the lamp, other components can include transformers or ballasts, reflectors, refractors or lenses to direct the light beam, cooling devices for the light sources, color filters and controls. They can be linked in tandem or series for powering large or complex fiber-optic installations.

The light guide is the material used to transmit the light from the light source. Typically, bundles of plastic fiber or glass fiber, which are also called tails, are used. If the light is to project from the end of the fiber (end-emitting), the plastic or glass core of each fiber is coated with a cladding. Cladding is a very thin layer of plastic or glass that has a lower index of refraction than the core to contain the light. Finally, the fiber is covered with an opaque or transparent sheathing. This can be a plastic, rubber or metal tube for protecting and supporting the fibers.

If the light is to project along the entire length of the fiber bundle (side-emitting), a clear plastic sheathing is used to continuously "leak" light along the surface. Thus, the side-emitting fiber looks like neon tube lighting. Light source. Commonly used lamps include 20W to 250W incandescent lamps and 70W to 250W metal-halide lamps. Using small and sturdy filaments, MR-16 tungsten-halogen lamps provide excellent beam control. But some recently developed compact metal-halide lamps provide similar tight optical control. One manufacturer allows the 250W tungsten-halogen lamp to be replaced with a 150W metal-halide lamp to provide much longer lamp life.

The size of the individual fibers and the required illumination level generally determines the wattage and type of light source to use. The lamp should approach being a point source (a very small area of light generation). A reflector located behind the light source and a lens in front of the light source help to direct light into the optical fiber.

Numerical Aperture (NA). The difference in the refractive indexes of the core and the cladding determine the NA of the fiber. The NA is the acceptance angle for the fiber, above which light entering the core will not be guided by the cladding (it will not be internally reflected). A smaller NA does not mean better output.

Source Isolation. A term to describe an illuminator hidden or mounted away from the light path, such as behind a wall, or in a nearby closet.

Details on fixtures. It's easy to become confused as to what type of FO fixture to use, since so many different designs exist. However, most fixtures are application specific. They include:

*Downlighting type fixtures are generally made of molded plastic or cast aluminum and come in a variety of designs and finishes.

A nonadjustable lens provides some type of beam focusing. Their small size and flexibility in relation to the possible mounting positions means that a downlight fixture can be used almost anywhere.

The newest types add decorative elements, such as acrylic molds, high- quality crystal or shaped clear, frosted or colored glass.

*Wall washer and accent fixtures. Often referred to as eyeballs, these fixtures are adjustable by means of a socket for easy aiming. Many of these fixtures include an adjustable lens that allows the light to be focused from narrow to wide beams. Accent fixtures, with a variety of beam shapes, are widely used in showcases or displays. These fixtures can contain numerous light outlets and are positioned in mounting bars or rails.

*Landscape and exterior fixtures. Units are available in a variety of designs for illuminating a landscape, sidewalk or a garden. Decorative elements are often used in landscape-type fixtures. Some outdoor fixtures have flexible arms to pinpoint aiming in a garden. Other fixtures can be buried in the ground or mounted in a concrete slab.

*Underwater applications fixtures. These are used in swimming pools, whirlpools and fountains.

*Specialty fixtures. An assortment of miniature fixtures, fitted with shaped clear, colored or frosted glass, provide tiny points of light, thus serving a decorative function in a ceiling or elsewhere.

*Custom fixtures. Since fiber-optic lighting lacks any electrical restrictions, the fiber bundles can be mounted in a variety of materials, including furniture, handrails and works of art. Thus, custom units can be created for applications where standard fixtures aren't appropriate. Optical-fibers are flexible and can withstand a wide range of temperatures.

An illuminator can be fitted with a color wheel to provide a continuous or fixed change of color as the light passes from the lamp to the fiber end. Also, the movement of a color wheel can be computerized to provide special effects, such as timed light changes or strobe-like bursts of light.

Fiber-optic lighting is best for color- changing special effects. Some illuminators use DMX 512, a theatrical industry control protocol. An acronym for digital multiplex, DMX was developed about 15 years ago by the U.S. Institute of Theatre Technology (USITT) to address dimmers from lighting consoles.

Because FO products are not standardized, installers must follow all project specifications exactly, and they shouldn't mix dissimilar components without checking with the manufacturer. For example, a manufacturer uses a particular light source for its color-rendering properties. A specific optical fiber is used based on the angle of acceptance of the fiber, the position of the fiber in relation to the source and the construction of the fiber bundle.

Of course, as with all types of lighting fixtures, adequate space around the light source must be provided to dissipate heat from the lamp. Many illuminators use an internal fan to provide air circulation. Some models do not require a fan, but maintaining some method of air circulation around the illuminator is still necessary.

Usually, spring caps, screws or adhesives hold end-emitting fixtures in place. Fixtures used in display or showcase applications are also easy to install; if mounting rails are used, they are cut to the correct length, either by the manufacturer or at the job-site. Individual fixtures either click or slide into the desired locations on the rail.