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Fiber optic
lighting systems are extremely versatile, and theoretically can
be used in place of any traditional lighting system. However,
the industry is still in its infancy. Given today’s level of technology,
it makes sense to use fiber optic lighting systems only in certain
applications. But don’t blink for too long, because the technology
is advancing rapidly and in a couple of years it will make sense
to use fiber optics even for general lighting.
Fiber optic
systems make sense today for application where you must remove
heat or UV from the systems (such as in retail displays and in
museums). Fiber optics make sense when the electricity in the
system should be remote from the light (pools and spas, e.g.).
Or when it makes sense to reduce maintenance costs of lighting
systems when lamp replacement is a major hassle (some chandeliers).
And fiber optic lighting systems make sense when you’re trying
to achieve special lighting effects based on a number of small
points of light, rather than a single large source (signs, accent
lighting, etc.).
Fiber optic
lighting systems are simply still too inefficient for general
lighting to reach uniformly high ambient levels (say, 50 footcandles).
And since it’s not currently possible to put enough lumens into
an individual fiber to carry over long throw distances, fiber
optic lighting systems don’t make sense when the throw distance
(from the output fixture to the object being illuminated) is greater
than four or five feet. So, much general room lighting is a poor
choice for fiber optic lighting—for now.
What are the
basic tenets of fiber optic lighting? How does the technology
work? What advantages can architects gain through using these
systems?
The following,
brought to you by Unison Fiber Optic Lighting Systems, will answer
those questions and much more. And by studying the learning objectives,
reading the article and answering the questions you can participate
in an AIA/ARCHITECTURAL RECORD Continu-ing Education opportunity.
Unison™
is a joint venture of Advanced Lighting Technologies, makers of
Venture Lighting metal halide lamps, and Rohm and Haas, makers
of OptiFlex™ flexible light pipe. By combining these technologies,
Unison offers a total solution for fiber optic lighting needs,
including side and end light fiber, illuminators and fixtures,
couplers, and even specially designed cutting and stripping tools.
Unison custom
designs and manufactures complete fiber optic lighting systems.
Unison’s CableLite™ and FiberImages™ Divisions offer
product design and application expertise for signs, panels, logos,
curtains, and other products that require fiber optic systems.
FIBER OPTIC
LIGHTING
In fiber optic lighting systems, a lamp transfers its light through
to the end of the fiber or linearly through the fiber’s transparent
sheathing.
The most noteworthy
advantage of using fiber optic lighting systems is that the light
is separated from the electricity that generates light. And, too,
one source can drive many fibers and produce multiple points or
lines of light.
By separating
light and electricity, fiber optic lighting can be used to light
electrically or chemically hazardous areas, such as pools, spas,
fountains, or in environmentally sensitive industrial situations.
The reason: the light produces no electrical shocks and will not
become a fire hazard. A sidelight fiber can be used in places
where the potential of breakage or of contact with a high voltage
transformer makes a neon light hazardous.
In addition,
nearly all fiber optic lighting systems use both heat (infrared
or IR) and ultraviolet (UV) filters at the light source. As a
result, the output light contains no UV and no heat. This makes
the systems especially desirable for lighting retail displays
containing products that are sensitive to heat, as well as museum
displays of temperature sensitive artifacts and art. Basically,
dyes and oil paints won’t fade, chocolate won’t melt, and fresh
flowers won’t wilt.
The use of
fiber optic lighting systems (FOLSs) can also lead to improved
energy efficiency in some cases, particularly through the use
of metal halide (MH) light sources. Because a single lamp can
illuminate many fibers, maintenance costs can be reduced and maintenance
tasks simplified, particularly for the hard-to-reach bulb. In
addition there is economy of scale, as one bulb can light an entire
chandelier or a ceiling or wall of sprinkle lights.
Overall, fiber
optic lighting systems can be used in almost any lighting situation.
The Lighting Research Center at Rensselaer Polytechnic Institute
and others have tracked the following applications of fiber optic
lighting:
- Displays
and exhibits—fiber optics replace the traditional linear fluorescent
and MR16 lighting in museum and retail displays.
- Water—fiber
optics is used in at least 10 percent of the water lighting
market and that market is growing.
- Architectural
highlights—spots of light from end-emitting fibers can dramatically
highlight the architectural features of a room or building.
Side-emitting plastic fiber and prism light guides can outline
the exterior contours of buildings.
- Signage
and visual guidance—fiber optics systems are used to light a
variety of signs and are also used in edge-lit exit signs, billboards,
and traffic signals. Fiber optics light steps and aisles in
theaters and, in turn provide a safer, more accessible environment.
Signage projects are perhaps the most dramatic application,
in that PMMA fibers can be assembled to create unique and vibrant
images ranging from random patterns to intricate designs including
logs and animated figures. White light or a variety of colors
can be used to achieve versatile and exciting design. Fiber
optic cables with larger diameters produce channel lettering,
and perimeter, backlight and outline illumination.
- Decorative—chosen
for special effects such as color changes and strobing, FOLS
can produce a starlight effect for ceilings and can mimic flickering
candlelight for historic ambiance.
- Downlight
and ambient—although the Rensselaer Lighting Research Center
found that fiber optic lighting systems are not widely used
for downlighting and ambient light in the United States, in
Europe such systems are popular for offices and restaurants.
ANATOMY
OF AN FOLS
A fiber optic lighting system (FOLS) consists of three principle
components—the illuminator, the fibers, and the output fixture.
Note that for sidelight applications, the fiber doubles as the
output fixture. The color of the emitted light can easily be changed
through the use of a color wheel and the intensity lessened with
a mechanical dimmer.
Illuminator.
The illuminator consists of a lamp, ballast or transformer, and
collection optics to channel the light into the fiber.
The lamp
is most often either halogen or metal halide (MH). The most commonly
used is a 150 W MH, although a variety of outputs are available.
The ballast
or the transformer convert the input electricity to the
current and voltage required to start and operate the lamp.
The collection
optics channel the light into the fiber and are usually based
on a glass or metal reflector, an UV filter, and if desired, color
filters. The reflector captures the light emitted by the lamp
and directs it into the fibers. Most illuminators employ a fan
to dissipate the heat generated by the lamp, although Unison has
recently introduced a fanless model.
Fibers.
Optical fibers for illumination are either glass or plastic and
at least two layers—an inner core surrounded by a thin cladding.
There may be an additional third layer or sheathing for protection
of the inner fiber. Bundling individual fibers together is a common
practice, with this assembly referred to as bundled fiber. Nonbundled
fibers are commonly called solid core fibers, or large diameter
fibers.
The core and
cladding must be of different materials for the system to function
properly. Nearly all cladding layers are made of some kind of
fluoroplymer (an example being Teflon FEP™). The core is
either glass or plastic, with the plastic being either an acrylic
or methacrylic copoloymer. The most common plastic core for bundled
fiber is PMMA (poly methyl methacrylate), known by the trade name
Plexiglas™. If present, the sheathing is PVC (vinyl) or polyethylene.
Glass
fibers. Made thin enough to bend (.05 mm or .002 inch.),
glass fibers are inherently brittle. And, because the glass fibers
are so delicate, they are always bundled and always completely
assembled at the factory. Generally, several glass bundles are
gathered together into a “common end,” which is attached to the
illuminator (the complete assembly is called a harness). When
you order a glass fiber harness, you must specify the number of
tails, the length of each tail, and the diameters of the common
end and of the tails.
Glass fibers
are not damaged by heat or UV light like plastic fibers can be—an
advantage for using glass. However, as mentioned above, unlike
plastic, exact lengths must be specified and factory prefabricated
rather than cut at site.
Bundled
Plastic Fibers. Made of extruded PMMA, individual plastic
fibers are commonly .030 inches to .060 inches, or about 20 times
the size of glass fibers. The diameter of the individual fiber
is determined by brittleness of the PMMA; if the fiber is larger
than 1 mm, the fibers may not be bent to a reasonable radius without
breaking. PMMA fibers are separated and used individually for
such applications as signs, star ceilings, and fiber optic curtains.
PMMA fibers
are considered very durable compared to other plastic fibers.
The main disadvantage of PMMA fibers is packing fraction loss,
in which light from the illuminator inevitably filters into the
gap between the fibers, thus decreasing efficiency.
For endlights,
the individual fibers are bundled very loosely in an opaque sheathing.
For sidelight, the individual fibers are tightly bundled and twisted
around each other and then covered with a tightfitting clear sheathing.
The tighter the twist, the more light escapes.
Solid
Core Plastic Fibers. Typically ranging in diameter from
3mm (1/8 inch) to 12 mm (1/2 inch), most solid core plastic fibers
are cast of copolymers of MMA (methyl methacrylate) and a cross-link/plasticizer
to add strength and flexibility. Unison’s OptiFlex is constructed
of a different, inherently flexible acrylate and a cross-linker
to provide more flexibility at room temperature. With either process,
the cladding acts to contain the core until the core “cures.”
Therefore, for solid core fibers, the cladding layer is much thicker
than the sprayed or dipped cladding layers on small diameter acrylic
fibers.
Output
fixtures. The first fixtures used for fiber optic lighting
systems were simply standard lighting fixtures with the electrical
guts ripped out. The industry has progressed and now offers fixtures
designed specifically for fiber optics; yet, most output fixtures
are still based on standard electrical lighting fixture designs.
Coupler.
A coupler allows several large core fibers to be attached to a
single illuminator. One coupling method is to split the light
at the illuminator’s main port into several small ports using
segmented lenses. Each fiber is then inserted into one of the
small ports. Another method is to gather the fibers together in
a bundle over the last several inches of their length, which is
actually a labor-intensive operation. A third alternative utilizes
a short glass fiber harness as a coupler, with the common end
joined to the illuminator and each tail joined to a separate fiber.
The coupler
is commonly used to randomize both the color and intensity across
the faces of the various light pipes attached to it. Intensity
variations occur because of the projection of the MH arc—the source
of the light—onto a circular port. The color variations result
from the chemistry inside the lamps. Basically, different colors
are emitted at different angles, especially as you move away from
the center of the arc.
Randomization
is easy to accomplish with a glass fiber harness—just mix up the
individual strands of glass so that the strands that are going
to any given tail go to several different areas of the common
end. Another method is to use a glass mixing rod that averages
out the angular effects.
HOW IT
ALL WORKS
All optical fibers work on the principal of total internal reflection,
or TIR. Whenever light traveling in one material approaches another
material (such as the movement from the core to the cladding),
the light is bent somewhat as it enters the second material. If
the light approaches the second material at a shallow enough angle
(known as the critical angle), then it is bent so much that it
never enters the second material at all. It is totally reflected
back into the first material—an amazingly efficient process.
For instance,
a typical glass mirror may reflect only about 90 percent of the
light that hits it. However, total internal reflection reflects
essentially all of the light. This allows the light to travel
far distances, undergoing many hundreds of bounces along the light
pipe without being absorbed.
How shallow
the light rays must be in order to be totally reflected is determined
by how different the core and cladding are. This is measured by
the refractive index, which is essentially a measure of the speed
of light in the material. The greater the difference between the
indices of refraction, the more light (over steeper and steeper
angles) will undergo TIR.
All optical
fibers are characterized by an acceptance angle—the maximum angle
(away from “straight on”) at which light can enter the fiber and
be totally internally reflected down its length. The acceptance
angle is determined only by what the core and cladding are made
of. For plastic optical fibers, the acceptance angle is usually
about 35º to 40º. This means that all lighting hitting the face
of the fiber in a cone of 70º to 80º (from +35º or +40º to -35º
to -40º) will be internally reflected. The rest of the light is
reflected back out the face or absorbed by the core, cladding,
or sheathing.
Another measure
of the ability of a fiber to gather light is the numerical aperture
(NA). This is defined as the square root of the difference between
the squares of the refractive indices: NA=Square roof of (n²core
– n²clad) where n is the index of
refraction. The acceptance angle is simply the arcsine of the
NA. The bigger the acceptance angle (or NA) the better, because
the fiber collects more light.
Also note
that if the fiber is bent, the angle at which light approaches
the cladding from the core changes. For this reason, bending an
optical fiber results in some light loss. Tests to measure loss
under various bending conditions are not well-defined. In general,
though, the more sweeping the bend, the less light is lost. The
tighter the bend, the more light is lost.
OPERATING
PROPERTIES
When specifying fiber optic lighting systems, the efficiencies,
luminance, and color properties should be considered. Unfortunately,
only a small amount of comparative data exists, although there
is an effort among manufacturers to increase their analyses. In
addition, output from any system depends on a number of factors
not controlled by the manufacturer, such as the number and tightness
of bends the fiber takes in the installation as well as the quality
of the fiber end cuts. The quality of installation also affects
the system’s operation.
To aid in
comparison and specification, the fiber optic industry is developing
testing standards under the auspices of the National Electrical
Manufacturers Association. Meanwhile, check with manufacturers
for existing quantitative analyses. As with most lighting applications,
a prototype or mock- up to determine system performance is suggested.
Efficiency.
A typical fiber optic lighting system is about 15 percent efficient
overall. In other words, of all the light generated by the lamp,
only about 10 percent to 15 percent is emitted at the end source.
Nearly all of the loss happens before the light is directed into
the fiber. Fortunately, most FOLS use metal halide lamps, which
are highly efficient, so overall efficiency can be higher than
with incandescent lighting sources.
The most efficient
system in the market is manufactured by Unison. The illuminator
is based on a 68 W metal halide lamp that uses no fan. By getting
rid of the fan and making the collection optics more efficient,
the total system lumens of light per watt (lpw) of electricity
used increases considerably and is similar to the efficiency of
incandescent systems.
Within a year,
Unison expects to introduce a system that will nearly double that
efficiency—the system will be in the 25 to 30 lpw range—through
a better coupling of the lamps’ arc gap to the reflector (more
efficient lamp-reflector combinations) and of the port to the
fiber (a more efficient coupling mechanism).
Illuminance.
To supply a particular quantity of light for a task, one
measures illuminance. Illuminance is defined as flux density on
a surface, expressed as lumens per square foot, or more typically
known as a foot-candle.
Unless the
system manufacturer specifies the total lumens emitted from the
various output fixtures, the design should assume a MH lamp will
produce 80 lpw. Thus, for a 250 W MH system, the designer should
have no more than 15 percent of (80x250) or 3,000 total lumens
at the fiber ends. (Remember, that’s 3,000 total divided by the
number of fibers, not just 3,000 for each fiber.) This also assumes
a 10-foot-fiber runs with gentle bends only, excellent quality
cuts, good connections, clean optics, fresh lamp, etc. Some manufacturers
specify lumens available at the port. In that case, the designer
should assume about 60 percent of those lumens can be delivered
through 10 feet of fiber and the output fixture into the area
to be lit.
It’s important
to note that output characteristics of the system change depending
on the installation details—fiber length, number of bends, and
the length of the run relative to the number and radius of the
bends.
Color.
All optical fibers change the color a bit as the light travels
through them. Look at the color of light transmitted through 40
feet or more of fiber. Assuming that you start with white light,
absorbing red will make the light appear greenish. Absorbing blue
will make the light appear yellowish.
All types
of plastic optical fibers absorb some red light. The absorption
of blue differs, however, and is not fundamental to plastic fibers.
This results from broad absorption bands in the UV that “tail”
into the visible region. These UV absorptions are generally caused
by “stuff” in the fiber left over from the polymerization. So,
the cleaner the manufacturing process, the less blue is absorbed.
Therefore, a greenish cast to the transmitted light implies a
cleaner fiber and lower overall attenuation.
THE FUTURE
OF FOLS
By all accounts the future for fiber optics lighting seems bright.
Look for:
- Higher
efficiency.
- Move toward
fiber optics for more general room lighting, as seen in Europe.
- Systems
cost decrease.
- Greater
illuminance.
It’s also
important to note that manufacturers of fiber optic lighting systems
are working to bring a standardization to the industry, which
translates into more consumer confidence in a product and more
information available to the consumer.
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