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Specifications on Our Sideglow Cable
As with any type of
illumination, lighting via optical fiber requires the answers to a few
questions before a successful installation can be executed. Fiber optics can
distribute and project high quality illumination that can supplement
traditional methods, often to an advantage. Additionally, fiber optics can
perform dazzling tricks that no other form of lighting can touch. They are
often used purely for their unique aesthetic, often referred to as
"flexible neon". Generally,
the first question to answer is "How much light is required?" For
task and area lighting, various groups and agencies have established light
levels as standards and guidelines. Or, experience dictates what is appropriate
for a given application. In either case, the need will be to project light into
an area via fiber optics and knowing what needs to come out will determine what
needs to go in. For the "flexible neon" or, what is referred to as
"side-illuminated" effects, it is still necessary to determine how
bright the glow will need to be to provide the desired effect. In many cases,
particularly for side-illuminated fiber, experimentation may be the only way to
determine what looks best. At first blush, it would seem that to evaluate fiber
optics for the purposes of illumination, it would be good to start with
photometric measurements. These might be a foot-candle (lux) measurement for
task lighting; a lumen measurement for raw output; and in the case of
edge-illuminated fiber, a foot-lambert (nit) for "brightness". But unless
you already have the fiber, the light source, luminaries (for end-light),
tracking (for sidelight), and all the various other bits, you will not be able
to make these measurements. And neither can any of the manufacturers of optical
fiber. There are just too many variables in installations that affect these
three measurements for any manufacturer to be able to reasonably duplicate them
all. When it comes to optical fiber, any raw output data for fiber optic
illumination are meaningless... period. A fiber optic illumination system is a
little different. When the light source and fiber (at set lengths), along with
all the other accessories are evaluated together, then manufacturers and
suppliers can provide useful photometric data. But you must still be careful to
follow the installation instructions very closely. Optical fiber
is a passive conductor of light; the measure of ultimate "brightness"
will be largely a function of the light source powering the fiber. Most Tec's
consider loss or attenuation to be the most important evaluation parameter. All
fibers exhibit attenuation that will prevent 100% transmission. The amount of
loss will depend on many factors; the type of media used in the fiber core, the
surface geometry of the core/clad interface, mechanical stresses imposed on the
fiber, and the finish quality on the input and output ends of the fibers, among
others. The second
most important criteria are numerical aperture, which affects the light
gathering ability of the fiber. And the
third, unique to side-illuminated fiber, is the evenness of the glow effect.
Scattering effects within the fiber core and cladding force light to be
directed out of the fiber, something typically avoided with most optical fiber. ATTENUATION All fiber
experiences losses, and this shows up as two distinct but related forms. The
gross attenuation of a fiber is concerned with broad band losses that affect
the transmission of light. This figure of merit for a fiber is the loss or
attenuation value presented in manufacturer's literature. It is most often
given as "%/foot", "%/meter", "dB/foot", or
dB/meter". The other form of attenuation is often the most important
however. Fiber losses will affect certain portions of the visible spectrum more
than others. Color shifting results from the selective transmission and
attenuation of various wavelengths of light passing through the fiber. These
losses are minimized by using extremely pure base materials, by designing
polymers that are will better carry the visible wavelengths, and by incorporating
high-finesse fiber geometry. FOP takes advantage of all three of these to
produce the best fiber available with the least amount of spectral attenuation
(color shifting). It is
important when evaluating fiber (ours or anyone else’s) on the basis of loss
and color shift to make sure the same "white balance" is used, or
that the data has been "normalized". This means that the data are
adjusted in reference to the spectral content of the light source to eliminate
fluctuations in the source colors. The light source has a dramatic affect on
the measurement of fiber performance. Two sources rated identically in terms of
wattage can yield vastly different results. Even if both units were rated the
same in terms of optical power, there can be huge differences if the white
balance is not the same or the data not normalized. MEASURING LOSSES This is how
most manufacturers measure gross loss in a fiber: A relatively long sample of
fiber is illuminated and the output is measured. A pre-determined section of
the fiber is then removed from the output end, and another measurement is
taken. The same length section is then again removed from the output end of the
fiber, and another measurement taken. This continues until the remaining sample
can no longer be cut back by the same amount. From these measurements, it is
possible to calculate a loss factor. This is known as the "cut-back"
method. LOGARITHMIC SCALE As mentioned
above, loss factors are presented as a logarithmic value such as a percentage
of loss per unit length such as "%/foot", or in decibels as
"dB/meter", or "dB/ft". The use of decibels (dB) to measure
light may be confusing at first, but in this case, "decibel" simply
refers to the logarithmic nature of loss in the fiber, exactly as the term
relates to loss or attenuation in sound and radio signals. Coincidentally,
logarithmic values are also appropriate for measuring light and sound because
of the way our senses work. In order to deal with the huge variation in energy
levels we encounter in nature, both the eye and the ear exhibit the same
logarithmic sensitivity to their respective stimuli. Understanding the
logarithmic nature of light is extremely important when evaluating the visual
performance of fiber optics prior to installation, because it is the one figure
that relates to subjective brightness. The
logarithmic sensitivity of our eyes requires a doubling of optical power to
perceive an increase in "brightness". This doubling requires a 3dB
gain. Conversely, a drop of 3dB would bring perceived brightness down to the
next perceptible level. Our hearing is the same (as any car-stereo installer
will tell you!). Double the power of an amplifier, and you get just a bit more
sound. In both cases, this logarithmic nature allows us to safely observe
variations in light and sound over a 10,000:1 ratio. (When verifying this fact
by experiment, it is necessary to maintain a constant spectral content as power
is being reduced, a difficult task in practice when producing both aural or
visual stimuli. While insensitive to gross power differences, the eye and the
ear are both highly sensitive to changes in spectral balance.) All things being
equal (and that is saying a lot), a length of fiber with an attenuation factor
of .2 dB/ft. will have dropped to the next level of perceived brightness after
15 feet. (.2 x 15= 3dB). LIGHT
"QUALITY" AND COLOR BALANCE Many factors
such as contrast ratio, color, viewing angle, and ambient light conditions will
affect the observed quality of light from either type fiber. When using end-illuminated
fiber for task and/or area lighting, be prepared to adjust the spectral content
of the output to match standard illumination sources. Very typically, for
polymer based fibers, there will be a shift towards the green or yellow-green
part of the spectrum after several feet. Correction filters used in photography
to correct the color temperature of various lights may or may not work,
depending on the light source spectral content, and the length of fiber used.
Ideally, the proper correction filter would display the inverse (or
"opposite") function of the fiber spectral attenuation curve. This
would "balance" the various hues in exact proportion to each other by
filtering out those portions of the spectrum shifting the color away from white
light. Most manufacturers supply data concerning the spectral attenuation of
their fiber products, and these can give you a good idea of what to expect in
the field. NUMERICAL APERATURE, F#, AND ACCEPTANCE ANGLES After
attenuation, the numerical aperture is the next important consideration. Bear
in mind that higher or lower NA’s (wider or narrower acceptance angles) does
not make a fiber "better" or "worse". In some applications,
there may be an advantage to the wider spread of light possible from larger NA
fibers, but there are practical trade-offs that may cancel out any gains.
Similarly, the narrow angles of low NA fiber can improve light source coupling,
but may impose other constraints, such as higher cost. It should be noted that
currently, there are no manufacturers of large core (6mm and up), polymer fiber
manufacturing small NA fiber, though there are some smaller diameter fibers.
(We consider NA’s under .45 to be small, those over .45 to be large.) The way fiber
optics work (dictated by physics) imposes limits on the angles through which
light can enter the fiber. This limit is called the Numerical Aperture (NA) of
the fiber, and has the same affect as the aperture in a camera lens. That
effect is to limit the angles of light rays passing into the system. Both can
be evaluated in terms of F#, NA, or acceptance angles. Like camera apertures,
the "faster" the fiber, the more light it can collect. A camera lens
of F#1.0 is considered very fast. A fiber at F#1.0 is about average. This is
equivalent to a numerical aperture of .50; an acceptance angle of 45 degrees
(full angle). Here is the relationship: F#= distance to target/ diameter of spot at
target NA= 0.5/F# Acceptance angle (full) = 2 x sin e-1 [NA] Just how the
NA affects performance can be illustrated in the following: Imagine yourself in
a dark room with a window that has been painted over so as to be opaque. You
want to look outside, so you start to scratch the paint off the window but make
the smallest of holes so your activity won’t be noticed. You have to bring your
eye right up to the hole in order to see out and you really can’t look around
because to hole is too small to see much more than straight ahead. So you open
the hole a little more (at great risk of being caught!) and look again. Now
that the hole is larger, you can see over a wider range of angles. By
increasing the whole diameter- the aperture, you allow light rays from more
angles to pass into your eye. Not only that, but the room gets brighter as the
hole becomes larger. The aperture for optical fiber is a little different in
terms of how it is formed, but the effect is exactly the same. The larger the
aperture, the more light you can "couple" into the fiber. But rather
than being a simple hole in a surface, the aperture of a fiber is formed by
what is called the "critical angle". Fiber Optic
Products fiber has a NA of .66 which calculates to an acceptance angle of 82.59
degrees, and an F# of .33. What is actually useful though, is usually somewhat
less. Practical limits on the perfection of fiber geometry and chemical
composition, as well as installation-specific effects, all work to decrease the
useful angle. So most designers don’t feel the need to run light out to the
maximum acceptance angle. The half power points in the angle vs. throughput
graph are often used to set what we call the "working acceptance
angle". Additionally, finding light sources that efficiently operate at
the extreme wide angles is also difficult... try going to a light source
designer and tell them you want a light cone converging at F#=.33 and watch
them squirm! CRITICAL ANGLES Contrary to
popular belief, fiber optics are almost never "silvered on the
inside", or hollow, though some exotic fibers are either or both. The vast
majority of optical fiber relies on the phenomenon of total internal
reflection, to conduct light from end to end. In the same manner as the sky is
reflected from hot pavement, light traveling through a fiber is re-directed
back into the core whenever it begins to wander out. And just like a hot
pavement mirage, the effect only works at certain angles. Exceed the critical
angle and you see pavement and not the reflection of the sky. Or, in the case
of fiber, the light passes through the side instead of getting a nudge back
into the core. What
determines this critical angle is the relationship between the fiber core
(equivalent to the relatively cool air several inches above the pavement), and
the fiber cladding (equivalent to the layer of hot air hugging the road
surface). It is this relative difference in "optical density" (better
known as "refractive index") between the hot and cold air over the
pavement, and the core and cladding of the fiber that provides the mechanism
for reflection. Light travels faster in denser mediums. When a ray of light
encounters the "interface"- the boundary between more dense and less
dense media. The laws governing the conservation of energy dictate that the
energy present in the ray will have to undergo a transformation if it is to
pass through into a different density medium- a process that cannot occur
without loss. Re-direction incurs less of a loss penalty than transmission. And
like so many things in nature, light will tend to follow the path of least
resistance, and that path is back into the core of the fiber, the lowest
energy-loss option. MODES Up to a point
then, larger NA’s mean the fiber can gather light from wider angles. But past a
NA of about .60, equivalent to an acceptance angle of around 70 degrees, there
isn’t much useful light available. "Mode stripping" in the fiber
removes much of the light at the extremes of the acceptance angle. In
discussing or reading about optical fiber, the term "mode" often
comes up. "Single-mode" and "multimode" are used to
describe two basic classes of fiber optics. A mode is simply a path that a ray
of light can take through an optical conductor. Thus, "single-mode"
refers to an optical conductor that allows only one path for the light ray to
follow. In order for this to happen, the optical conductor must be very
small... generally on the order of 50 microns, or about 1/2000 of an inch. But
the advantages gained for applications like high speed data transmission are so
significant, that use of such tiny structures are routine and relatively simple
to use. For the job
of illumination however, much larger fibers are more efficient. These are
"multimode" fibers; countless modes in the case of the very large
fiber we manufacture. But the "order" of the modes, which (in a
serious over simplification) refers to the relative number of
"bounces" the light ray takes as it passes through the fiber, has
limits. On the low end, the lowest order mode possible would be a straight path
right down the fiber with no bounces. On the other end of the scale, the
highest order mode is the ray following a path right at the critical angle. So
long as it doesn’t exceed this angle, a light ray following this high-order
mode will travel from end to end. But like a
driver careening down a one-lane mountain road with no shoulder, these highest
order modes are prone to being lost "over the edge" if there
something goes wrong. Because they are so close to the critical angle, these
rays are the first to be lost or "stripped" if something causes them
to exceed the critical angle. There are many things that can cause this to
happen. Microscopic deviations from a smooth surface, scattering effects, too
tight of bend radii or clamps tightened too tight can all have this affect. The
point here is that the highest order modes is lost pretty quickly. There are no
perfectly smooth, flat, transparent materials and so some mode stripping is
always going to occur. This is why again, contrary to popular belief, output
angles are not the same as input angles. EVEN OUTPUT For side
illuminated applications, the evenness of the light spread along the length of
the fiber is as important as overall clarity. By taking advantage of controlled
molecular-scattering phenomena, we have tailored our side-illuminating fiber to
achieve a high degree of evenness along lengths up to 260-ft. (80m). Evenness
is also highly dependent upon the illuminator and "launch"
conditions. Projecting the light from the lamp into the fiber at narrower-than-
normal angles can improve the evenness of light over longer lengths. But there
is no agreed upon method for gauging "evenness" or even an agreement
on just what should be measured. Reputation and testing a sample on a
subjective level is often the only way to get an idea of what to expect. Manufacturing
quality is also an important factor affecting the evenness of the glow-effect.
Material purity and consistency, and the quality of the core/clad interface
will contribute significantly to the quality of the glow. The installation will
also affect glow quality. Tight bend radii, tight clamping or tracking,
excessive bending and kinking prior to installation will produce deleterious
affects. After the
optical considerations have been addressed, the mechanical aspects of
installing fiber need to be looked at. Some of these will affect what is
practically possible, particularly heat. An ideal
light source for fiber illumination would contain no invisible radiation- no
infrared and no ultraviolet. But aside from lasers (which are monochromatic or
quasi-chromatic at best) no such source exists. But even if there was such a
source, the contribution of visible light to the heat load on a fiber can be
considerable, with varying results depending on the fiber material. Why?
Because no medium is loss-less and virtually all fibers will have
"absorbency" losses. This is where atomic and molecular structures
resonate with photons of various wavelengths of light and convert them to heat
(seeing also the section on spectral attenuation, above). So as the visible
radiant energy applied to the end of a plastic optical fiber increases, so too
heat increases. Additionally, because it is impossible (or nearly so) to
produce enough visible light to be useful without producing infrared energy,
there is further heat load contribution to the system by the effects of the
infrared light. THERMAL RADIATION Heat will
cause a polymer fiber to burn if it is too high for too long. Far more common
however, are changes in the optical and mechanical properties of the fiber that
occur well before burning. The extra heat can further "polymerize"
the core materials and affect the way they transmit light and the stiffness of
the fiber. As it works out, the more flexible a fiber is, the poorer it’s
transmissive qualities. The plastisizers used to make the fiber flexible absorb
a certain amount of light. These along with several other factors, many which
are mutually exclusive, dictate a balancing act for the chemist formulating the
fiber polymer. A choice has to be made: either sacrifice clarity for
flexibility, or sacrifice flexibility for clarity. Fiber Optic Products fiber
is formulated to favor clarity over flexibility. The penalty is not severe, the
fiber is flexible when installed, but becomes stiffer with
photo/thermal-activity. Fiber clarity is improved and color shifting reduced
over time by the same activity. But in spite
of using some heat to advantage, service temperatures that are too high will
degrade the fiber over time. Illuminator design can become very complex for
this reason. Illuminator designers have opted for safer, lower wattage halogen
lamps, or using lamps such as the metal halide light variety which have
inherently less infrared energy. Always follow the light source manufacturers’
instructions for both using the illuminator and connecting the fibers to it. An
interesting note: we have had customers report the sensation of heat
accompanying the light from the end of some fibers. This isn’t infrared energy
being transmitted through the fiber, but rather the visible light being
converted to heat by the skin and underlying tissues and blood supply. "UV" EXPOSURE At the other
end of the visible spectrum is ultraviolet. When it comes to polymers,
"ultra-violence" is more like it! (A nod to Anthony Burgess, author
of "A Clockwork Orange") Ultraviolet light wreaks havoc on polymers
by breaking chemical bonds within the molecular structure of the material. The
result is varied depending on the polymer in question. Nearly all polymers used
in optical fiber will turn yellow, brown, or dark red when exposed to UV over
time. Even the very best fiber will eventually turn color after just a few
months of spring/summer exposure if exposed to the sun without protection (we
know, we make it and we test it). So outdoor installations simply have to be
protected from UV exposure, there is no way around it. No one, not even us will
warrantee their product for unprotected outdoor use. UV from daylight is not
the only source of these problematic wavelengths. Nearly every available light
source used for lighting fiber will produce some UV energy. This is even true
of quartz-halogen lamps, and especially true of metal-halide lamps. It is
crucial that the illuminator manufacturer provide means for reducing UV to
negligible levels before the light is launched into the fiber. And one more
point- UV light is scattered all over the sky and can still be a problem even
if the fiber is shielded from direct sunlight. UV light also is strongly
reflected by water, so pools and spas need more than a simple overhang to
effectively protect the fiber. BEND RADIUS LIMITATIONS Bending too tight (8 times the fiber diameter is the limit!), kinking, repeated tight radii flexing, stepping on, placing heavy (20lbs+) objects on, and localized heating of the fiber can destroy the core clad interface quality and so reduce transmission. Our fiber is durable, it will take a lot of punishment of certain kinds, but as with everything,, care and attention must be paid for proper results. Firm yet gentle fixturing and tracking is the rule, use sweeping elbows in conduit runs, don’t run the fiber over hot water pipes without insulation, and make sure that the uncoiling is done carefully. |
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