Incandescent including halogen light bulbs
Copyright (C) 1996 Donald L. Klipstein (Jr) (don@misty.com)
It is fairly well known that Thomas Alvin Edison invented the first
reasonably practical incandescent lamp, using a carbon filament
in a bulb containing a vacuum. Since that time, the incandescent
lamp has been improved by using tantalum and later tungsten filaments,
which evaporate more slowly than carbon.
Nowadays, incandescent lamps are still made with tungsten filaments.
The filament of an incandescent lamp is simply a resistor. If electrical
power is applied, it is converted to heat in the filament. The filament's
temperature rises until it gets rid of heat at the same rate that
heat is being generated in the filament. Ideally, the filament gets
rid of heat only by radiating it away, although a small amount of
heat energy is also removed from the filament by thermal conduction.
The filament's temperature is very high, generally over 2000 degrees
Celsius, or generally over 3600 degrees Fahrenheit. In a "standard"
75 or 100 watt 120 volt bulb, the filament temperature is roughly
2550 degrees Celsius, or roughly 4600 degrees Fahrenheit. At high
temperatures like this, the thermal radiation from the filament
includes a significant amount of visible light.
In a 120 volt, 100 watt "standard" bulb with a rated
light output of 1750 lumens, the efficiency is 17.5 lumens per watt.
This compares poorly to an "ideal" of 242.5 lumens per
watt for one idealized type of white light, or 681 lumens per watt
ideally for the yellowish-green wavelength of light that the human
eye is most sensitive to.
Other types of incandescent light bulbs have different efficiencies,
but all generally have efficiencies near or below 35 lumens per
watt. Most household incandescent bulbs have efficiencies from 8
to 21 lumens per watt. Higher efficiencies near 35 lumens per watt
are only achieved with photographic and projection lamps with very
high filament temperatures and short lifetimes of a few hours to
around 40 hours.
The reason for this poor efficiency is the fact that tungsten filaments
radiate mostly infrared radiation at any temperature that they can
withstand. An ideal thermal radiator produces visible light most
efficiently at temperatures around 6300 Celsius (6600 Kelvin or
11,500 degrees Fahrenheit). Even at this high temperature, a lot
of the radiation is either infrared or ultraviolet, and the theoretical
luminous efficiency is 95 lumens per watt.
Of course, nothing known to any humans is solid and usable as a
light bulb filament at temperatures anywhere close to this. The
surface of the sun is not quite that hot.
There are other ways to efficiently radiate thermal radiation using
higher temperatures and/or substances that radiate better at visible
wavelengths than invisible ones. This is covered by Part II of the
Great Internet Light Bulb Book, Discharge Lamps. The efficiency
of an incandescent bulb can be increased by increasing the filament
temperature, which makes it burn out more quickly.
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.
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.
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.
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".
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.
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.
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).
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.
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.
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.
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.
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, the
filaments are designed to run 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.
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.
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 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.
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.)
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 GamColor no. 1510 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 GamColor 1510 is available at some theatrical supply shops.
Written by Don Klipstein (Jr).