Vacuum
vs. gas-filled bulbs
At first, incandescent bulbs were
made with a vacuum inside them. Air oxidizes the filament at high temperatures.
Later, it was discovered that filling the bulb with an inert gas such as argon
or an argon-nitrogen mixture slows down evaporation of the filament. Tungsten
atoms evaporating from the filament can be bounced back to the filament by gas
atoms. The filament can be operated at a higher temperature with a fill gas than
with a vacuum. This results in more efficient radiation of visible light. So why
are some bulbs still made with a vacuum? The reason is that a fill gas conducts
heat away from the filament. This conducted heat is energy that cannot be
radiated by the filament and is lost, or wasted. This mechanism reduces the
bulb's efficiency of producing radiation. If this is not offset by the advantage
of operating the filament at a higher temperature, then the bulb is more
efficient with a vacuum.
One property of thermal conduction from the filament to the gas is the strange
fact that the amount of heat conducted is roughly proportional to the filament's
length, but does not vary much with the filament's diameter. The reason this
occurs is beyond the scope of this document.
However, this means that bulbs with thin filaments and lower currents are more
efficient with a vacuum, and higher current bulbs with thicker filaments are
more efficient with a fill gas. The break-even point seems to be very roughly
around 6-10 watts per centimeter of filament. (This can vary with filament
temperature and other factors. The break-even point may be higher in larger
bulbs where convection may increase heat removal from the filament by the gas.)
Sometimes, premium fill gases such as krypton or xenon are used. These gases
have larger atoms that are better at bouncing evaporated tungsten atoms back to
the filament. These gases also conduct heat less than argon. Of these two gases,
xenon is better, but more expensive. Either of these gases will significantly
improve the life of the bulb, or result in some improvement in efficiency, or
both. Often, the cost of these gases makes it uneconomical to use them.
How
light bulbs burn out
Due to the high temperature that a
tungsten filament is operated at, some of the tungsten evaporates during use.
Furthermore, since no light bulb is perfect, the filament does not evaporate
evenly. Some spots will suffer greater evaporation and become thinner than the
rest of the filament.
These thin spots cause problems. Their electrical resistance is greater than
that of average parts of the filament. Since the current is equal in all parts
of the filament, more heat is generated where the filament is thinner. The thin
parts also have less surface area to radiate heat away with. This "double
whammy" causes the thin spots to have a higher temperature. Now that the thin
spots are hotter, they evaporate more quickly.
It becomes apparent that as soon as a part of the filament becomes significantly
thinner than the rest of it, this situation compounds itself at increasing speed
until a thin part of the filament either melts or becomes weak and breaks.
Why
bulbs often burn out when you turn them on
Many people wonder what goes on when
you turn on a light. It is often annoying that a weak, aging light bulb will not
burn out until the next time you turn it on.
The answer here is with those thin spots in the filament. Since they have less
mass than the less-evaporated parts of the filament, they heat up more quickly.
Part of the problem is the fact that tungsten, like most metals, has less
resistance when it is cool and more resistance when it is hot. This explains the
current surge that light bulbs draw when they are first turned on.
When the thin spots have reached the temperature that they would be running at,
the thicker, heavier parts of the filament have not yet reached their final
temperature. This means that the filament's resistance is still a bit low and
excessive current is still flowing. This causes the thinner parts of the
filament to get even hotter while the rest of the filament is still warming up.
This means that the thin spots, which run too hot anyway, get even hotter when
the thicker parts of the filament have not yet fully warmed up. This is why
weak, aging bulbs can't survive being turned on.
Why
burnout is sometimes so spectacular
When the filament breaks, an arc
sometimes forms. Since the current flowing through the arc is also flowing
through the filament at this time, there is a voltage gradient across the two
pieces of the filament. This voltage gradient often causes this arc to expand
until it is across the entire filament.
Now, consider a slightly nasty characteristic of most electric arcs. If you
increase the current going through an arc, it gets hotter, which makes it more
conductive. Obviously, this could make things a bit unstable, since the more
conductive arc would draw even more current. The arc easily becomes conductive
enough that it draws a few hundred amps of current. At this point, the arc often
melts the parts of the filament that the ends of the arc are on, and the arc
glows with a very bright light blue flash. Most household light bulbs have a
built-in fuse, consisting of a thin region in one of the internal wires. The
extreme current drawn by a burnout arc often blows this built-in fuse. If not
for this fuse, people would frequently suffer blown fuses or tripped circuit
breakers from light bulbs burning out.
Although the light bulb's internal fuse will generally protect household fuses
and circuit breakers, it may fail to protect the more delicate electronics often
found in light dimmers and electronic switching devices from the current surges
drawn by "burnout arcs".
How bad
a current surge bulbs draw when turned on
It is fairly well known that a cold
light bulb filament has less resistance than a hot one. Therefore, a light bulb
draws excessive current until the filament warms up.
Since the filament can draw more than ten times as much current as usual when it
is cold, some people are concerned about excessive energy consumption from
turning on light bulbs.
The degree of this phenomenon has become a matter of urban folklore. However,
the filament warms up very rapidly. The amount of energy consumed to warm up a
cold filament is less than it would consume in one second of normal operation.
Making
bulbs last longer
Long-life bulbs
Many light bulbs are made to operate
with a slightly lower filament temperature than usual. This makes the bulbs last
much longer with a slight reduction of efficiency.
Reduced
Power
Reducing the voltage applied to a
light bulb will reduce the filament temperature, resulting in a dramatic
increase in life expectancy.
One device sold to do this is an ordinary silicon diode built into a cap that is
made to stick to the base of a light bulb. A diode lets current through in only
one direction, causing the bulb to get power only 50 percent of the time if it
is operated on AC. This effectively reduces the applied voltage by about 30
percent. (Reducing the voltage to its original value times the square root of .5
results in the same power consumption as applying full voltage half the time.)
The life expectancy is increased very dramatically. However, the power
consumption is reduced by about 40 percent (not 50 since the cooler filament has
less resistance) and light output is reduced by reduced by about 70 percent
(cooler filaments are less efficient at radiating visible light).
Soft-start devices
Since bulbs usually burn out during
the current surge that occurs when they are turned on, one would expect that
eliminating the surge would save light bulbs.
In fact, such devices are available. Like the diode-based ones, they are
available in a form that is built into caps that one could stick onto the tip of
the base of a light bulb. These devices are "negative temperature coefficient
thermistors", which are resistors having a resistance that decrease when they
heat up.
When the bulb is first started, the thermistor is cool and has a moderately high
resistance that limits current flowing through the bulb. The current flowing
through the thermistor's resistance generates heat, and the thermistor's
resistance decreases. This allows the current to increase in a fairly gradual
manner, and the filament warms up in a uniform manner.
However, this extends the life of the bulbs less than one might think. If the
filament has thin spots that cannot survive the current surge that occurs when
the bulb is turned on, then the filament is already in very bad shape. At this
time, the thin spots are significantly hotter than the thicker parts of the
filament and are evaporating rather rapidly. As described earlier, this process
is accelerating. If the thin spots are protected from surges, the life of the
bulb would be extended by only a few percent.
Additional life extension occurs only because the thermistor keeps enough
resistance to result in enough heat to keep it fairly conductive. This
resistance slightly reduces power to the bulb, extending its life somewhat and
making it slightly dimmer.
DC vs.
AC operation
As tungsten atoms evaporate from the
filament, a very small percentage of them are ionized by the small amounts of
short-wave ultraviolet light being radiated by the filament, the electric field
around the filament, or by free electrons that escape from the filament by
thermionic emission. These tungsten ions are positively charged, and tend to
leave the positive end of the filament and are attracted to the negative end of
the filament. The result is that light bulbs operated on DC have this specific
mechanism that would cause uneven filament evaporation.
This mechanism is generally not significant, although it has been reported that
light bulbs sometimes have a slight, measurable decrease in lifetime from DC
operation as opposed to AC operation.
In a few cases, AC operation may shorten the life of the bulb, but this is rare.
In rare cases, AC may cause the filament to vibrate enough to significantly
shorten its life. In a few other rare cases involving very thin filaments, the
filament temperature varies significantly throughout each AC cycle, and the peak
filament temperature is significantly higher than the average filament
temperature.
Ordinarily, one should expect a light bulb's life expectancy to be roughly equal
for DC and AC.
Why
making bulbs last longer often does not pay
You may have heard that the life
expectancy of a light bulb is roughly inversely proportional to the 12th or 13th
power of the applied voltage. And that power consumption is roughly proportional
to voltage to the 1.4 to 1.55 power, and that light output is roughly
proportional to the 3.1 to 3.4 power of applied voltage. This would make the
luminous efficiency roughly proportional to applied voltage to the 1.55 to 2nd
power of applied voltage.
Now, if a slight reduction in applied voltage results in a slight to moderate
loss of efficiency and a major increase in lifetime, how could this cost you
more?
The answer is in the fact that the electricity consumed by a typical household
bulb during its life usually costs many times more than the bulb does. Bulbs are
so cheap compared to the electricity consumed by them during their lifetime that
it pays to make them more efficient by having the filaments run hot enough to
burn out after only several hundred to about a thousand hours or so.
Here is an example
with actual numbers (using U.S. dollars, in 1996):
Suppose you have 10 "standard" 100
watt 120 volt bulbs with a rated lifetime of 750 hours. Such bulbs typically
cost around 75 cents in the U.S. The electricity used by all ten of these bulbs
is 1 kilowatt, which would typically cost about 9 cents per hour (approximate
U.S. average).
Over 750 hours, this would cost (on an average) $67.50 for the electricity plus
$7.50 for 10 bulbs, or $75.
Now, suppose you use these bulbs with 110 volts instead of 120.
These bulbs would consume about 87.8 watts instead of 100. However, they would
only produce 76 percent of their normal light output (and this is a slightly
optimistic figure). To restore the original light output, you need 13 of these
bulbs. (And this will fall very slightly short.) Using 13 bulbs that consume
87.8 watts apiece results in a power consumption of 1141 watts. Over 750 hours
at 9 cents per KWH, this would cost $77. This is more than the $75 cost of
running 10 bulbs at full voltage even if the bulbs never burn out at 110 volts.
At 110 volts instead of 120, the life expectancy of the bulbs may be tripled.
One third of 13 times 75 cents is about $3.25, which adds to the $77 cost of
electricity to result in an average total cost of $80.25 for 750 hours.
This example should explain why you often get the most light for the least money
using standard bulbs rather than longer-lasting ones.
How to
minimize lighting costs
Higher wattage bulbs tend to be more
efficient than lower wattage ones. One reason for this is the fact that thicker
filaments can be operated at a higher temperature, which is better for radiating
visible light.
Another reason is that since higher wattage bulbs would lead you to use fewer
bulbs, you buy fewer bulbs and the cost of bulbs becomes less important. To
optimize cost effectiveness in this case of higher wattage light bulbs, the
filaments are designed to run even hotter to improve energy efficiency to reduce
your electricity costs.
Smaller bulbs use less electricity apiece, making the cost of the bulb more
important. This is why lower wattage bulbs are often designed to last 1500 to a
few thousand hours instead of 750 to 1000 hours. Designing the bulbs to last
longer reduces their light output and energy efficiency.
To minimize your cost of both electricity and bulbs, you should use as few bulbs
as possible, using higher wattage bulbs. To get the same amount of light with
lower wattage bulbs, you need both more electricity and more bulbs.
An even better way to reduce your lighting costs is to use fluorescent, compact
fluorescent, or HID (mercury, metal halide, or sodium) lamps since these are 3
to 5 times as efficient as incandescent lamps.
Halogen
Bulbs
The
halogen cycle, What are halogen bulbs?
A halogen bulb is an ordinary
incandescent bulb, with a few modifications. The fill gas includes traces of a
halogen, often but not necessarily iodine. The purpose of this halogen is to
return evaporated tungsten to the filament.
As tungsten evaporates from the filament, it usually condenses on the inner
surface of the bulb. The halogen is chemically reactive, and combines with this
tungsten deposit on the glass to produce tungsten halides, which evaporate
fairly easily. When the tungsten halide reaches the filament, the intense heat
of the filament causes the halide to break down, releasing tungsten back to the
filament.
This process, known as the halogen cycle, extends the life of the filament
somewhat. Problems with uneven filament evaporation and uneven deposition of
tungsten onto the filament by the halogen cycle do occur, which limits the
ability of the halogen cycle to prolong the life of the bulb. However, the
halogen cycle keeps the inner surface of the bulb clean. This lets halogen bulbs
stay close to full brightness as they age.
In order for the halogen cycle to work, the bulb surface must be very hot,
generally over 250 degrees Celsius (482 degrees Fahrenheit). The halogen may not
adequately vaporize or fail to adequately react with condensed tungsten if the
bulb is too cool. This means that the bulb must be small and made of either
quartz or a high-strength, heat-resistant grade of glass known as "hard glass".
Since the bulb is small and usually fairly strong, the bulb can be filled with
gas to a higher pressure than usual. This slows down the evaporation of the
filament. In addition, the small size of the bulb sometimes makes it economical
to use premium fill gases such as krypton or xenon instead of the cheaper argon.
The higher pressure and better fill gases can extend the life of the bulb and/or
permit a higher filament temperature that results in higher efficiency. Any use
of premium fill gases also results in less heat being conducted from the
filament by the fill gas, meaning more energy leaves the filament by radiation,
meaning a slight improvement in efficiency.
Lifetime and efficiency of halogen bulbs
A halogen bulb is often 10 to 20
percent more efficient than an ordinary incandescent bulb of similar voltage,
wattage, and life expectancy. Halogen bulbs may also have two to three times as
long a lifetime as ordinary bulbs, sometimes also with an improvement in
efficiency of up to 10 percent. How much the lifetime and efficiency are
improved depends largely on whether a premium fill gas (usually krypton,
sometimes xenon) or argon is used.
Halogen
Bulb Failure Modes
Halogen bulbs usually fail the same way that ordinary incandescent bulbs do,
usually from melting or breakage of a thin spot in an aging filament.
Thin spots can develop in the filaments of halogen bulbs, since the filaments
can evaporate unevenly and the halogen cycle does not redeposit evaporated
tungsten in a perfect, even manner nor always in the parts of the filament that
have evaporated the most.
However, there are additional failure modes.
One failure mode is filament notching or necking. Since the ends of the filament
are somewhat cool where the filament is attached to the lead wires, the halogen
attacks the filament at these points. The thin spots get hotter, which stops the
erosion at these points. However, parts of the filament even closer to the
endpoints remain cool and suffer continued erosion. This is not so bad during
continuous operation, since the thin spots do not overheat. If this process
continues long enough, the thin spots can become weak enough to break from the
weight of the filament.
One major problem with the "necked" ends of the filament is the fact that they
heat up more rapidly than the rest of the filament when the bulb is turned on.
The "necks" can overheat and melt or break during the current surge that occurs
when the bulb is turned on. Using a "soft-start" device prevents overheating of
the "necks", improving the bulb's ability to survive "necking". Soft-start
devices will not greatly extend the life of any halogen bulbs that fail due to
more normal filament "thin spots" that run excessively hot.
Some halogen bulbs may usually burn out due to filament end necking, and some
others may usually burn out from thin, hot spots forming in the filament due to
uneven filament evaporation/recovery. Therefore, some models may have a
significantly extended life from "soft-starting" and some other models may not.
It is generally not a good idea to touch halogen bulbs, especially the more
compact, hotter-running quartz ones. Organic matter and salts are not good for
hot quartz. Organic matter such as grease can carbonize, leaving a dark spot
that absorbs radiation from the filament and becomes excessively hot. Salts and
alkaline materials (such as ash) can sometimes "leach" into hot quartz, which
typically weakens the quartz, since alkali and alkaline earth metal ions are
slightly mobile in hot glasses and hot quartz. Contaminants may also cause hot
quartz to crystallize, weakening it. Any of these mechanisms can cause the bulb
to crack or even violently shatter. If a quartz halogen bulb is touched, it
should be cleaned with alcohol to remove any traces of grease. Traces of salt
will also be removed if the alcohol has some water in it.
Since the hotter-running quartz halogen bulbs could possibly violently shatter,
they should only be operated in suitable fully enclosed fixtures.
Use of
Halogen Bulbs with Dimmers
Dimming a halogen bulb, like dimming
any other incandescent lamp, greatly slows down the formation of thin spots in
the filament due to uneven filament evaporation. However, "necking" or
"notching" of the ends of the filament remains a problem. If you dim halogen
lamps, you may need "soft-start" devices in order to achieve a major increase in
bulb life.
Another problem with dimming of halogen lamps is the fact that the halogen cycle
works best with the bulb and filament at or near specific optimum temperatures.
If the bulb is dimmed, the halogen may fail to "clean" the inner surface of the
bulb. Or, tungsten halide that results may fail to return tungsten to the
filament. Halogen bulbs have sometimes been known to do strange and scary things
when greatly dimmed.
Halogen bulbs should work normally at voltages as low as 90 percent of what they
were designed for. If the bulb is in an enclosure that conserves heat and a
"soft-start" device is used, it will probably work well at even lower voltages,
such as 80 percent or possibly 70 percent of its rated voltage. However, do not
expect a major life extension unless soft-starting is used. Even with
soft-starting, do not expect to more than double or possibly triple the life of
any halogen bulb already rated to last 2,000 hours or more. Even with soft
starting, the life of these bulbs will probably not continue to improve much as
voltage is reduced to less than about 90 percent of the bulb's voltage rating.
Dimmers can be used as soft-start devices to extend the life of any particular
halogen bulbs that usually fail from "necking" of the ends of the filament. The
bulb can be warmed up over a period of a couple of seconds to avoid overheating
of the "necked" parts of the filament due to the current surge that occurs if
full voltage is applied to a cold filament. Once the bulb survives starting, it
is operated at full power or whatever power level optimizes the halogen cycle
(usually near full power)
The dimmer may be both "soft-starting" the bulb and operating it at slightly
reduced power, a combination that often improves the life of halogen bulbs. Many
dimmers cause some reduction in power to the bulb even when they are set to
maximum.
(A suggestion from someone who starts expensive medical lamps by
turning up a dimmer and reports major success in extending the life of expensive
special bulbs from doing this.)
Ultraviolet from Halogen Bulbs
There is some common concern about
the ultraviolet output of halogen bulbs, since they operate at high filament
temperatures and the bulbs are made of quartz instead of glass. However, the
filament temperature of halogen bulbs rated to last 2,000 hours or more is only
slightly greater than that of standard incandescent lamps, and the UV output is
only slightly higher. Halogen fixtures typically have a glass or plastic shield
to confine any possible bulb explosions, and these shields absorb the small
traces of shortwave and mediumwave UV that gets through the quartz bulb.
Higher temperature photographic and projection bulbs are different. The much
higher filament temperature of shorter life bulbs results in possibly
significant hazardous UV. For maximum safety, use these bulbs in fixtures or
equipment designed to take these bulbs, and in a manner consistent with the
fixture or equipment instructions.
For those who want to take special precautions against UV, a UV blocking clear
filter gel such as the GAM no. 1510 or Rosco "UV Filter" (03114) may be a
practical solution. This filter gel withstands use moderately close to halogen
lamps and withstands heat to maybe 100 to 150 Celsius or so. This filter gel can
be placed immediately outside the glass shield of most fixtures, although the
tubular shield in many popular 300 watt torchiere lamps gets too hot for the
filter gel.
The GAM 1510 and Rosco "UV Filter" is available at some theatrical supply shops.
Written by Don Klipstein (Jr).
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