Right
The
incandescent light bulb,
incandescent
lamp or
incandescent light globe is a
source of electric
light that works by
incandescence (a general term for
heat-driven light emissions which includes the simple case of
black body radiation). An
electric current passes through a thin
filament, heating it until it
produces light. The enclosing glass bulb prevents the
oxygen in air from reaching the hot filament, which
otherwise would be destroyed rapidly by
oxidation. Incandescent bulbs are also sometimes
called
electric lamps, a term also applied to the original
arc lamps.
Incandescent bulbs are made in a wide range of sizes and
voltages, from 1.5 volts to about 300 volts. They
require no external regulating equipment and have a low
manufacturing cost, and work well on either alternating current or
direct current. As a result the incandescent lamp is widely used in
household and commercial lighting, for portable lighting, such as
table lamps, car
headlamps,
flashlights, and for decorative and advertising
lighting.
Some applications of the incandescent bulb make use of the heat
generated, such as incubators, brooding boxes for
poultry, heat lights for
reptile tanks ,
infrared heating for industrial heating and
drying processes, and the
Easy-Bake
Oven toy. In cold weather the heat shed by incandescent lamps
contributes to building heating, but in hot climates lamp losses
increase the energy used by
air
conditioning systems.
Incandescent light bulbs are gradually being replaced in many
applications by other types of
electric
light such as (
compact)
fluorescent lamps,
high-intensity discharge
lamps,
light-emitting
diodes (LEDs), and other devices. These newer technologies give
more visible light and less heat for the same amount of electrical
energy input. Some jurisdictions, such as the
European Union are in the process of
phasing-out the use of
incandescent light bulbs in favor of more energy-efficient
lighting.
History of the light bulb
In addressing the question "Who invented the incandescent lamp?"
historians Robert Friedel and Paul Israel list 22 inventors of
incandescent lamps prior to
Joseph
Wilson Swan and
Thomas Edison.
They conclude that Edison's version was able to outstrip the others
because of a combination of three factors: an effective
incandescent material, a higher
vacuum than others were able to achieve (by use of
the
Sprengel pump) and a high
resistance lamp that made power
distribution from a centralized source economically viable.
Another historian, Thomas Hughes, has attributed Edison's success
to the fact that he invented an entire, integrated system of
electric lighting. "The lamp was a small component in his system of
electric lighting, and no more critical to its effective
functioning than the Edison Jumbo
generator, the Edison main and feeder,
and the parallel-distribution system. Other inventors with
generators and incandescent lamps, and with comparable ingenuity
and excellence, have long been forgotten because their creators did
not preside over their introduction in a system of lighting."
| Early evolution of the light bulb |
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Early pre-commercial research
In 1802,
Humphry Davy had what was then the most
powerful electrical battery in
the world at the Royal Institution
of Great Britain. In that year, he created
the first incandescent light by passing the current through a thin
strip of
platinum, chosen because the metal
had an extremely high
melting point.
It was not bright enough nor did it last long enough to be
practical, but it was the precedent behind the efforts of scores of
experimenters over the next 75 years. In 1809, Davy also created
the first
arc lamp by making a small but
blinding electrical connection between two
carbon charcoal rods
connected to a 2000-cell battery; it was demonstrated to the Royal
Institution in 1810.
Over the first three-quarters of the 19th century many
experimenters worked with various combinations of platinum or
iridium wires, carbon rods, and evacuated or semi-evacuated
enclosures. Many of these devices were demonstrated and some were
patented.
In 1835,
James Bowman Lindsay
demonstrated a constant electric light at a public meeting in
Dundee, Scotland. He stated that he could "read a book at a
distance of one and a half feet". However, having perfected the
device to his own satisfaction, he turned to the problem of
wireless telegraphy and did not
develop the electric light any further. His claims are not well
documented.
In 1840, British scientist
Warren de la
Rue enclosed a
coiled platinum
filament in a
vacuum tube and
passed an
electric current through it. The
design was based on the concept that the high
melting point of platinum would allow it to
operate at high temperatures and that the evacuated chamber would
contain fewer gas molecules to react with the platinum, improving
its longevity. Although an efficient design, the cost of the
platinum made it impractical for commercial use.
In 1841, Frederick de Moleyns of England was granted the first
patent for an incandescent lamp, with a
design using platinum wires contained within a vacuum bulb.
In 1845, American John W. Starr acquired a patent for his
incandescent light bulb involving the use of carbon filaments. He
died shortly after obtaining the patent. Aside from the information
contained in the patent itself, little else is known about
him.
In 1851,
Jean Eugène
Robert-Houdin publicly demonstrated incandescent light bulbs on
his estate in Blois, France. His light bulbs are on permanent
display in the museum of the Chateau of Blois.
In 1872
A. N. Lodygin invented an
incandescent light bulb. In 1874 he obtained a patent for his
invention.
In a suit filed by rivals seeking to get around Edison's lightbulb
patent, German-American inventor
Heinrich Göbel claimed he developed the
first light bulb in 1854: a carbonized
bamboo
filament, in a vacuum bottle to prevent oxidation, and that in the
following five years he developed what many call the first
practical light bulb. Despite a successful recreation of his lamp
in 1882,
Lewis Latimer demonstrated
that the bulbs which Göbel had purportedly built in the 1850s, had
actually been built much later, and found the glassblower who had
constructed the fraudulent exhibits. In a patent interference suit
in 1893, the judge ruled Göbel's claim "extremely
improbable".
In North America, parallel developments were taking place.
On July
24, 1874 a Canadian
patent was filed by a Toronto
medical
electrician named Henry
Woodward and a colleague Mathew
Evans. They built their lamps with different sizes and
shapes of
carbon rods held between
electrodes in glass cylinders filled with
nitrogen. Woodward and Evans attempted to
commercialize their lamp, but were unsuccessful. They ended up
selling their patent ( ) to Thomas Edison in 1879 .
Commercialization
Joseph Wilson Swan (1828–1914)
was a British physicist and chemist. In 1850, he began working with
carbonized paper filaments in an evacuated glass bulb. By 1860 he
was able to demonstrate a working device but the lack of a good
vacuum and an adequate supply of electricity resulted in a short
lifetime for the bulb and an inefficient source of light. By the
mid-1870s better pumps became available, and Swan returned to his
experiments.
With the help of Charles Stearn, an expert on vacuum pumps, in 1878
Swan developed a method of processing that avoided the early bulb
blackening. This received a British Patent No 8 in 1880. On 18
December 1878 a lamp using a slender carbon rod was shown at a
meeting of the Newcastle Chemical Society, and Swan gave a working
demonstration at their meeting on 17 January 1879. It was also
shown to 700 who attended a meeting of the Literary and
Philosophical Society of Newcastle on 3 February 1879. These lamps
used a carbon rod from an arc lamp rather than a slender filament.
Thus they had low resistance and required very large conductors to
supply the necessary current, so they were not commercially
practical, although they did furnish a demonstration of the
possibilities of incandescent lighting with relatively high vacuum,
a carbon conductor, and platinum lead-in wires. Besides requiring
too much current for a central station electric system to be
practical, they had a very short lifetime. Swan turned his
attention to producing a better carbon filament and the means of
attaching its ends. He devised a method of treating cotton to
produce 'parchmentised thread' and obtained British Patent 4933 in
1880. From this year he began installing light bulbs in homes and
landmarks in England. His house was the first in the world to be
lit by a lightbulb and so the first house in the world to be lit by
Hydro Electric power. In the early 1880s he had started his
company.
Thomas Edison began serious research
into developing a practical incandescent lamp in 1878. Edison filed
his first patent application for "Improvement In Electric Lights"
on October 14, 1878 ( ). After many experiments with platinum and
other metal filaments, Edison returned to a
carbon filament. The first successful test was on
October 22, 1879, and lasted 13.5 hours. Edison continued to
improve this design and by Nov 4, 1879, filed for a U.S. patent for
an electric lamp using "a carbon filament or strip coiled and
connected ... to platina contact wires." Although the patent
described several ways of creating the carbon filament including
using "cotton and linen thread, wood splints, papers coiled in
various ways," it was not until several months after the patent was
granted that Edison and his team discovered that a carbonized
bamboo filament could last over 1200 hours.
Hiram S. Maxim started a lightbulb company in 1878 to
exploit his patents and those of William Sawyer. His United States
Electric Lighting Company was the second company, after Edison, to
sell practical incandescent electric lamps.
They made their first
commercial installation of incandescent lamps at the Mercantile
Safe Deposit Company in New York City
in the fall of 1880, about six months after the
Edison incandescent lamps had been installed on the steamer
Columbia. In October 1880, Maxim patented a method of
coating carbon filaments with
hydrocarbons to extend their life.
Lewis Latimer, his employee at the time,
developed an improved method of heat-treating them which reduced
breakage and allowed them to be molded into novel shapes, such as
the characteristic "M" shape of Maxim filaments. On January 17,
1882, Latimer received a patent for the "Process of Manufacturing
Carbons," an improved method for the production of light bulb
filaments which was purchased by the United States Electric Light
Company. Latimer patented other improvements such as a better way
of attaching filaments to their wire supports.
In
Britain
, the Edison
and Swan companies merged into the Edison and Swan United Electric
Company (later known as Ediswan,
which was ultimately incorporated into Thorn Lighting Ltd). Edison was
initially against this combination, but after Swan
sued him and won, Edison was eventually forced to
cooperate, and the merger was made. Eventually, Edison acquired all
of Swan's interest in the company. Swan sold his United States
patent rights to the
Brush
Electric Company in June 1882.
Swan later wrote that Edison had a greater
claim to the light than he did, in order to protect Edison's
patents from claims against them in the United States
. In 1881, the Savoy Theatre
became the first public building in the world to be
lit entirely by electric lights.
In 1882, the first recorded set of
miniature incandescent lamps for lighting a
Christmas tree was installed. These
did not become common in homes for many years.
The
United
States Patent Office gave a ruling October 8, 1883, that
Edison's patents were based on the prior art of
William Sawyer and were invalid.
Litigation continued for a number of years. Eventually on October
6, 1889, a judge ruled that Edison's electric light improvement
claim for "a filament of carbon of high resistance" was
valid.
In the 1890s, the Austrian inventor
Carl Auer von Welsbach worked on
metal-filament mantles, first with
platinum
wire, and then
osmium, and produced an
operating version in 1898. In 1898 he patented the osmium lamp and
started marketing it in 1902, the first commercial metal filament
incandescent lamp.
In 1897, German physicist and chemist
Walther Nernst developed the
Nernst lamp, a form of incandescent lamp that
used a ceramic
globar and did not require
enclosure in a vacuum or inert gas. Twice as efficient as carbon
filament lamps, Nernst lamps were briefly popular until overtaken
by lamps using metal filaments.
In 1903, Willis Whitnew invented a metal-coated carbon filament
that would not blacken the inside of a light bulb.
On
December 13, 1904, Hungarian
Sándor Just and
Croatian Franjo
Hanaman were granted a Hungarian patent (No. 34541) for
a
tungsten filament lamp, which lasted
longer and gave a brighter light than the carbon filament.
Tungsten
filament lamps were first marketed by the Hungarian company Tungsram in 1905,
so this type is often called Tungsram-bulbs in many European
countries.
In 1906, the
General Electric
Company patented a method of making tungsten filaments for use
in incandescent light bulbs.
Sintered tungsten
filaments were costly, but by 1910
William David Coolidge (1873–1975)
had invented an improved method of making tungsten filaments. The
tungsten filament outlasted all other types of filaments and
Coolidge made the costs practical.
In 1913
Irving Langmuir found that
filling a lamp with
inert gas instead of a
vacuum resulted in twice the luminous efficacy and reduction of
bulb blackening. In 1924,
Marvin
Pipkin, an American chemist, patented a process for
frost the inside of lamp bulbs
without weakening them, and in 1947 he patented a process for
coating the inside of lamps with
silica.
In 1930,
Hungarian Imre Bródy filled
lamps with krypton gas in lieu of argon. He used krypton
and/or xenon filling of bulbs. Since the new gas was expensive, he
developed a process with his colleagues to obtain krypton from air.
Production of krypton filled lamps based on
his invention started at Ajka
in 1937, in
a factory co-designed by Polányi and Hungarian-born physicist
Egon Orowan.
By 1964, improvements in efficiency and production of incandescent
lamps had reduced the cost of providing a given quantity of light
by a factor of thirty, compared with the cost at introduction of
Edison's lighting system
Consumption of incandescent light bulbs grew rapidly in the United
States. In 1885 an estimated 300,000 general lighting service lamps
were sold, all with carbon filaments. When tungsten filament were
introduced, there were about 50 million lamp sockets in the United
States. In 1914 88.5 million lamps were used, (only 15% with carbon
filaments), and by 1945 annual sales of lamps were 795 million
(more than 5 lamps per person per year).
Cartels
Between 1924 and 1939 the international market for incandescent
light bulbs was controlled by the Phoebus cartel, which dictated
wholesale prices and whose members controlled most of the world
market for lamps.
Efficiency comparisons
Approximately 90% of the power consumed by an incandescent light
bulb is emitted as
heat, rather than as visible
light.
The effectiveness of an electric lighting source is determined by
two factors - the relative visibility of electromagnetic radiation,
and the rate at which the source converts electric power into
electromagnetic radiation.
Luminous efficacy of a light
source is a ratio of the visible light energy emitted ( the
luminous flux) to the total power input to the lamp.
Visible light is measured in
lumens, a
unit which is defined in part by the differing sensitivity of the
human eye to different wavelengths of light. Not all wavelengths of
visible electromagnetic energy are equally effective at stimulating
the human eye; the luminous efficacy of radiant energy is a measure
of how well the distribution of energy matches the perception of
the eye. The maximum efficacy possible is 683 lm/W for
monochromatic green light at 555 nanometres wavelength, the peak
sensitivity of the human eye. For white light, the maximum luminous
efficacy is around 240 lumens/watt, but the exact value is not
unique because the human eye can perceive many different mixtures
of visible light as "white".
The chart below lists values of overall luminous efficacy and
efficiency for several types of general service, 120 volt,
1000-hour lifespan incandescent bulb, and several idealized light
sources. A similar chart in the article on
luminous
efficacy compares a broader array of light sources to one
another.
| Type |
Overall luminous efficiency |
Overall luminous efficacy (lm/W) |
| 40 W tungsten incandescent |
1.9% |
12.6 |
| 60 W tungsten incandescent |
2.1% |
14.5 |
| 100 W tungsten incandescent |
2.6% |
17.5 |
| glass halogen |
2.3% |
16 |
| quartz halogen |
3.5% |
24 |
| high-temperature incandescent |
5.1% |
35 |
| ideal black-body radiator at
4000 K |
7.0% |
47.5 |
| ideal black-body radiator at 7000 K |
14% |
95 |
| ideal monochromatic 555 nm (green) source |
100% |
683 |
Unfortunately, the spectrum emitted by a
blackbody radiator does not match the sensitivity
characteristics of the human eye. Tungsten filaments radiate mostly
infrared radiation at temperatures where they remain solid (below
3683
kelvins / 3410°C / 6,170°F). Donald L.
Klipstein explains it this way: "An ideal thermal radiator produces
visible light most efficiently at temperatures around 6300 °C
(6600 K or 11,500 °F). 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." No known
material can be used as a filament at this ideal temperature, which
is hotter than the sun's surface. An upper limit for incandescent
lamp luminous efficacy is around 52 lumens per watt, the
theoretical value emitted by tungsten at its melting point.
For a given quantity of light, an incandescent light bulb produces
more heat (and consumes more power) than a
fluorescent lamp. Incandescent lamps' heat
output increases load on
air
conditioning in the summer, but the heat from lighting can
contribute to building heating in cold weather.
High-quality
halogen incandescent lamps
have higher efficacy, which will allow a 60 W bulb to provide
nearly as much light as a non-halogen 100 W. Also, a lower-wattage
halogen lamp can be designed to produce the same amount of light as
a 60 W non-halogen lamp, but with much longer life.
Many light sources, such as the
fluorescent lamp,
high-intensity discharge lamps
and
LED lamps offer higher efficiency, and
some have been designed to be
retrofitted in existing fixtures. These devices
produce light by
luminescence, instead
of heating a filament to incandescence. These mechanisms produce
discrete
spectral lines and so don't
have the broad "tail" of wasted invisible infrared emissions
produced by incandescent emitters. By careful selection of which
electron energy level transitions are used, the spectrum emitted
can be tuned to either mimic the appearance of incandescent sources
or else produce different
color
temperatures of white for visible light.
Cost of lighting
The desired product of any electric lighting system is light
(lumens), not power (watts). To compare incandescent lamp operating
cost with other light sources, the calculation must also consider
the lumens produced by each lamp. For commercial and industrial
lighting systems the comparison must also include the required
illumination level, the capital cost of the lamp, the labor cost to
replace lamps, the various depreciation factors for light output as
the lamp ages, effect of lamp operation on heating and air
conditioning systems, as well as the energy consumption. The
initial cost of an incandescent bulb is small compared to the cost
of the energy it will use.
Overall cost of lighting must also take into account light lost
within the lamp holder fixture; internal reflectors and updated
design of lighting fixtures can improve the amount of usable light
delivered. Since human vision adapts to a wide range of light
levels, a 10% or 20% decrease in lumens still may provide
acceptable illumination, especially if the changeover is
accompanied by cleaning of lighting equipment or improvements in
fixtures.
Measures to phase out use
Due to the higher energy usage of incandescent light bulbs in
comparison to more energy efficient alternatives, such as
compact fluorescent lamps and
LED lamps, many governments have introduced
measures to phase-out their use, by setting minimum efficacy
standards higher than can be achieved by general service
lamps.
Efforts to improve efficiency
Due to the measures noted above, there have been recent efforts to
improve the efficiency of incandescents. For example the consumer
lighting division of
General
Electric announced that they are working on a "high efficiency
incandescent" (HEI) lamp, which they claim could ultimately be as
much as four times more efficient than current incandescents,
although their initial production goal is to be approximately two
times more efficient.
U.S.
Department of Energy research at Sandia
National Laboratories
initially indicated the potential for dramatically
improved efficiency from a photonic lattice filament.
However, later work indicated that initially promising results were
in error.
Prompted by U.S. legislation mandating increased bulb efficiency by
2012, new "hybrid" incandescent bulbs have been introduced by
Philips. The "Halogena Energy Saver"
incandescent is 30 percent more efficient than traditional designs,
using a special chamber to reflect formerly-wasted heat back to the
filament to provide additional lighting power.
Construction
Incandescent light bulbs consist of a
glass
enclosure (the envelope, or bulb) with a
filament of
tungsten wire inside the bulb, through which an
electric current is passed. Contact
wires and a base with two (or more) conductors provide electrical
connections to the filament. Incandescent light bulbs usually
contain a stem or glass mount anchored to the bulb's base which
allows the electrical contacts to run through the envelope without
gas/air leaks. Small wires embedded in the stem in turn support the
filament and/or its lead wires. The bulb is filled with an
inert gas such as
argon to
reduce
evaporation of the
filament.
An electrical current heats the filament to typically 2000 K to
3300 K (about 3100–5400°F), well below tungsten's melting point of
3695 K (6192°F). Filament temperatures depend on the filament type,
shape, size, and amount of current drawn. The heated filament emits
light that approximates a
continuous
spectrum. The useful part of the emitted energy is
visible light, but most energy is given off as
heat in the near-
infrared wavelengths.
Three-way light bulbs have two filaments and three conducting
contacts in their bases. The filaments share a common ground, and
can be lit separately or together. Common wattages include
30–70–100, 50–100–150, and 100–200–300, with the first two numbers
referring to the individual filaments, and the third giving the
combined wattage.
While most light bulbs have clear or frosted glass, other kinds are
also produced, including the various colors used for
Christmas tree lights and other decorative
lighting.
Neodymium-containing glass is
sometimes used to provide a more natural-appearing light.
 |
- Outline of Glass bulb
- Low pressure inert gas (argon, neon, nitrogen)
- Tungsten filament
- Contact wire (goes out of stem)
- Contact wire (goes into stem)
- Support wires
- Stem (glass mount)
- Contact wire (goes out of stem)
- Cap (sleeve)
- Insulation (vitrite)
- Electrical contact
|
Many arrangements of electrical contacts are used. Large lamps may
have a screw base (one or more contacts at the tip, one at the
shell) or a bayonet base (one or more contacts on the base, shell
used as a contact or used only as a mechanical support). Some
tubular lamps have an electrical contact at either end. Miniature
lamps may have a wedge base and wire contacts, and some automotive
and special purpose lamps have screw terminals for connection to
wires. Contacts in the lamp socket allow the electric current to
pass through the base to the filament. Power ratings for
incandescent light bulbs range from about 0.1
watt to about 10,000 watts.
The glass bulb of a general service lamp can reach temperatures
between 200 and 260 degrees Celsius (400 to 550 degrees
Fahrenheit). Lamps intended for high power operation or used for
heating purposes will have envelopes made of hard glass or fused
quartz.
Filament
The first successful light bulb
filaments
were made of
carbon (from carbonized paper or
bamboo). Early carbon filaments had a
negative
temperature coefficient of resistance - as they got hotter,
their electrical resistance decreased. This made the lamp sensitive
to fluctuations in the power supply, since a small increase of
voltage would cause the filament to heat up, reducing its
resistance and causing it to draw even more power and heat even
further. In the "flashing" process, carbon filaments were heated by
current passing through them, while in an evacuated vessel
containing hydrocarbon (gasoline) vapor. The carbon deposited by
this treatment improved the uniformity and strength of filaments,
and their efficiency. A metallized or graphitized filament was
first heated in a high-temperature oven before flashing and lamp
assembly; this transformed the carbon into graphite, which further
strengthened and smoothed the filament, and as a byproduct had the
advantage of changing the lamp to a positive temperature
coefficient like a metallic conductor. This helped stabilize power
consumption, temperature and light output against minor variations
in supply voltage.
In 1902 the
Siemens company developed a
tantalum lamp filament. These lamps were
more efficient than even graphitized carbon filaments and could
operate at higher temperatures. Since the metal had a lower
resistivity than carbon, the tantalum lamp filament was quite long
and required multiple internal supports. The metal filament had the
property of gradually shortening in use; the filaments were
installed with large loops which tightened in use. This made lamps
in use for several hundred hours quite fragile. Metal filaments had
the property of breaking and re-welding, though this would usually
decrease resistance and shorten the life of the filament. General
Electric bought the rights to use tanalum filaments and produced
them in the United States until 1913.
From 1898 to around 1905
osmium was also used
as a lamp filament in Europe, but the metal was so expensive that
used broken lamps could be returned for part credit. It could not
be made for 110 V or 220 V so several lamps were wired in series
for use on standard voltage circuits.
In 1906 the
tungsten filament was
introduced, which is still used. Tungsten metal was initially not
available in a form that allowed it to be drawn into fine wires. By
1910, a process was developed by W. D. Coolidge at
General Electric for production of a
ductile form of tungsten. The process required pressing chemically
produced tungsten powder into bars, then several steps of
sintering, swaging, and then wire drawing. It was found that very
pure tungsten formed filaments that sagged in use, and that a very
small "doping" treatment with potassium, silicon, and aluminum
oxides at the level of a few hundred parts per million, greatly
improved the life and durability of the tungsten filaments.
To improve the efficiency of the lamp, the filament usually
consists of coils of coiled fine wire, also known as a 'coiled
coil.' For a 60-watt 120-volt lamp, the uncoiled length of the
tungsten filament is usually 22.8 inches or 580 mm , and
the filament diameter is 0.0018 inches (0.045 mm). The
advantage of the coiled coil is that evaporation of the tungsten
filament is at the rate of a tungsten cylinder having a diameter
equal to that of the coiled coil. Due to the coils creating gaps ,
such a filament has a lower surface area than the perceived surface
area of the filament, and so evaporation is reduced. If the
filament is then run hotter to bring back evaporation to the same
rate, the resulting filament is a more efficient light
source.
There are several different shapes of filament used in lamps, with
differing characteristics. Manufacturers designate the types with
codes such as C-6, CC-6, C-2V, CC-2V, C-8, CC-88, C-2F, CC-2F,
C-Bar, C-Bar-6, C-8I, C-2R, CC-2R, and Axial.
Electrical filaments are also used in
hot
cathodes of
fluorescent lamps
and
vacuum tubes as a source of
electrons or in vacuum tubes to heat an
electron-emitting electrode.
Reducing filament evaporation
One of the problems of the standard electric light bulb is
evaporation of the filament. Small variations in
resistivity along the filament cause
"hot spots" to form at points of higher resistivity ; a variation
of diameter of only 1% will cause a 25% reduction in service life.
The hot spots evaporate faster than the rest of the filament,
increasing resistance at that point—a
positive feedback which ends in the
familiar tiny gap in an otherwise healthy-looking filament.
Irving Langmuir found that an inert
gas, instead of vacuum, would retard evaporation. General service
incandescent light bulbs over about 25 watts in rating are now
filled with a mixture of mostly
argon and some
nitrogen, or sometimes
krypton.
Xenon gas, much more
expensive, is used occasionally in small bulbs, such as those for
flashlights. Since a filament breaking in a gas-filled bulb can
form an
electric arc which may spread
between the terminals and draw very heavy current, intentionally
thin lead-in wires or more elaborate protection devices are
therefore often used as
fuses
built into the light bulb.More nitrogen is used in higher-voltage
lamps to reduce the possibility of arcing.
During ordinary operation, the tungsten of the filament evaporates;
hotter, more-efficient filaments evaporate faster. Because of this,
the lifetime of a filament lamp is a trade-off between efficiency
and longevity. The trade-off is typically set to provide a lifetime
of several hundred to 2,000 hours for lamps used for general
illumination. Theatrical, photographic, and projection lamps may
have a useful life of only a few hours, trading life expectancy for
high output in a compact form. Long-life general service lamps have
lower efficiency but are used where the cost of changing the lamp
is high compared to the value of energy used.
Filament notching describes another phenomenon that limits
the life of lamps. Lamps operated on direct current develop random
stair-step irregularities on the filament surface, reducing the
cross section and further increasing heat and evaporation of
tungsten at these points. In small lamps operated on direct
current, lifespan may be cut in half compared to AC operation.
Different alloys of tungsten and
rhenium can
be used to counteract the effect.
If a light bulb envelope leaks, the hot tungsten filament reacts
with air, yielding an aerosol of brown
tungsten nitride, brown
tungsten dioxide, violet-blue
tungsten pentoxide, and yellow
tungsten trioxide which then deposits on
the nearby surfaces or the bulb interior.
Bulb blackening
In a conventional lamp, the evaporated tungsten eventually
condenses on the inner surface of the glass envelope, darkening it.
For bulbs that contain a vacuum, the darkening is uniform across
the entire surface of the envelope. When a filling of inert gas is
used, the evaporated tungsten is carried in the thermal convection
currents of the gas, depositing preferentially on the uppermost
part of the envelope and blackening just that portion of the
envelope. An incandescent lamp which gives 93% or less of its
initial light output at 75% of its rated life is regarded as
unsatisfactory, when tested according to IEC Publication 60064.
Light loss is due to filament evaporation and bulb blackening.
Study of the problem of bulb blackening lead to the discovery of
the
Edison effect,
thermionic emission and invention of the
vacuum tube.
A very small amount of water vapor inside a light bulb can
significantly affect lamp darkening. Water vapor dissociates into
hydrogen and oxygen at the hot filament. The oxygen attacks the
tungsten metal, and the resulting tungsten oxide particles travel
to cooler parts of the lamp. Hydrogen from water vapor reduces the
oxide, reforming water vapor and continuing this
water
cycle. The equivalent of a drop of water distributed over
500,000 lamps will significantly increase darkening. Small amounts
of substances such as
zirconium are placed
within the lamp as a
getter to react with any
oxygen that may bake out of the lamp components during
operation.
Some old, high-powered lamps used in theater, projection,
searchlight, and lighthouse service with heavy, sturdy filaments
contained loose tungsten powder within the envelope. From time to
time, the operator would remove the bulb and shake it, allowing the
tungsten powder to scrub off most of the tungsten that had
condensed on the interior of the envelope, removing the blackening
and brightening the lamp again.
Halogen lamps
The
halogen lamp reduces uneven
evaporation of the filament and darkening of the envelope by
filling the lamp with a
halogen gas at low
pressure, rather than an inert gas. The
halogen cycle increases the lifetime of the
bulb and prevents its darkening by redepositing tungsten from the
inside of the bulb back onto the filament. The halogen lamp can
operate its filament at a higher temperature than a standard gas
filled lamp of similar power without loss of operating life.
Incandescent arc lamps
A variation of the incandescent lamp did not use a hot wire
filament, but instead used an arc struck on a spherical bead
electrode to produce heat. The electrode then became incandescent,
with the arc contributing little to the light produced. Such lamps
were used for projection or illumination for scientific instruments
such as
microscopes. These arc lamps ran
on relatively low voltages and incorporated tungsten filaments to
start ionization within the envelope. They provided the intense
concentrated light of an
arc lamp but were
easier to operate. Developed around 1915, these lamps were
displaced by mercury and
xenon arc
lamps.
Electrical characteristics
Incandescent lamps are nearly pure resistive loads with a
power factor of 1. This means the actual power
consumed (in
watts) and the apparent power (in
volt-amperes) are equal. The actual
resistance of the filament is temperature-dependent. The cold
resistance of tungsten-filament lamps is about 1/15 the
hot-filament resistance when the lamp is operating. For example, a
100-watt, 120-volt lamp has a resistance of 144
ohms when lit, but the cold resistance is much lower
(about 9.5 ohms) . Since incandescent lamps are resistive loads,
simple
triac dimmers can be used to
control brightness. Electrical contacts may carry a "T" rating
symbol indicating that they are designed to control circuits with
the high inrush current characteristic of tungsten lamps. For a
100-watt, 120 volt general-service lamp, the current stabilizes in
about 0.10 seconds, and the lamp reaches 90% of its full brightness
after about 0.13 seconds.
Power
Comparison of efficacy by power (120 Volt lamps)
| Power (W) |
Output (lm) |
Efficacy (lm/W) |
| 5 |
25 |
5 |
| 15 |
110 |
7.3 |
| 25 |
200 |
8.0 |
| 35 |
350 |
10.0 |
| 40 |
500 |
12.5 |
| 50 |
700 |
14.0 |
| 55 |
800 |
14.5 |
| 60 |
850 |
14.2 |
| 65 |
1,000 |
15.4 |
| 70 |
1,100 |
15.7 |
| 75 |
1,200 |
16.0 |
| 90 |
1,450 |
16.1 |
| 95 |
1,600 |
16.8 |
| 100 |
1,700 |
17.0 |
| 135 |
2,350 |
17.4 |
| 150 |
2,850 |
19.0 |
| 200 |
3,900 |
19.5 |
| 300 |
6,200 |
20.7 |
Incandescent light bulbs are usually
marketed according to the
electrical power consumed. This is measured
in
watts and depends mainly on the
resistance of the filament, which in
turn depends mainly on the filament's length, thickness, and
material. For two bulbs of the same voltage, type, color, and
clarity, the higher-powered bulb gives more light.
The table shows the approximate typical output, in
lumens, of standard incandescent light bulbs at
various powers. Note that the lumen values for "soft white" bulbs
will generally be slightly lower than for standard bulbs at the
same power, while clear bulbs will usually emit a slightly brighter
light than correspondingly powered standard bulbs.
Physical characteristics
Bulb shapes, sizes, and terms
Incandescent light bulbs come in a range of shapes and sizes.The
names of the shapes may be slightly different in some regions.Many
of these shapes have a designation consisting of one or more
letters followed by one or more numbers, e.g. A55 or PAR38. The
letters represent the shape of the bulb. The numbers represent the
maximum diameter, either in eighths of an inch, or in millimetres,
depending on the shape and the region. For example, 63 mm
reflectors are designated R63, but in the U.S. they are known as
R20 (2.5 inches). However, in both regions, a PAR38 reflector
is known as PAR38.
Common shapes:
- General Service
- Light emitted in (nearly) all directions. Available either
clear or frosted.
- Types: General (A), Mushroom
- High Wattage General Service
- Lamps greater than 200 watts.
- Types: Pear-shaped (PS)
- Decorative
- lamps used in chandeliers, etc.
- Types: Candle (B), Twisted Candle, Bent-tip Candle (CA &
BA), Flame (F), Fancy Round (P), Globe (G)
- Reflector (R): Reflective coating inside the bulb directs light
forward. Flood types (FL) spread light. Spot types (SP) concentrate
the light. Reflector (R) bulbs put approximately double the amount
of light (foot-candles) on the front central area as General
Service (A) of same wattage.
- Types: Standard Reflector (R), Elliptical Reflector (ER), Crown
Silvered
- Parabolic Aluminized Reflector (PAR): Parabolic Aluminized
Reflector (PAR) bulbs control light more precisely. They produce
about four times the concentrated light intensity of General
Service (A), and are used in recessed and track lighting.
Weatherproof casings are available for outdoor spot and flood
fixtures.
- 120V Sizes:PAR 16, 20, 30, 38, 56 and 64
- 230V Sizes:Par 38, 56 and 64
- Available in numerous spot and flood beam spreads. Like all
light bulbs, the number represents the diameter of the bulb in 1/8s
of an inch. Therefore, a PAR 16 is 2" in diameter, a PAR 20 is 2.5"
in diameter, PAR 30 is 3.75" and a PAR 38 is 4.75" in
diameter.
- Multifaceted Reflector (MR)
- HIR: "HIR" is a GE designation for a lamp
with an infrared reflective coating. Since less heat escapes, the
filament burns hotter and more efficiently. The Osram designation for a similar coating is "IRC".
Lamp bases
40 watt light bulbs with standard E10, E14 and E27 Edison screw
base
Very small lamps may have the filament support wires extended
through the base of the lamp, and can be directly soldered to a
printed circuit board for connections. Some reflector-type lamps
include screw terminals for connection of wires. Most lamps have
metal bases that fit in a socket to support the lamp and conduct
current to the filament wires. In the late 19th century
manufacturers introduced a multitude of incompatible lamp bases.
General Electric introduced
standard base sizes for tungsten incandescent lamps under the
Mazda trademark in 1909. This
standard was soon adopted across the United States, and the Mazda
name was used by many manufacturers under license through 1945.
Today most incandescent lmaps for general lighting service use an
Edison screw or double contact
bayonet base. Bayonet base lamps are
frequently used in
automotive
lamps to resist loosening due to vibration. A
bipin base is often used for halogen or reflector
lamps.
Lamp bases may be secured to the bulb with a cement, or by
mechanical crimping to indentations molded into the glass bulb.
The double-contact bayonet cap on an incandescent bulb
Miniature lamps used for some automotive lamps or decorative lamps
have
wedge-bases which have a partial plastic
or even completely glass base. In this case, the wires wrap around
to the outside of the bulb, where they press against the contacts
in the socket. Miniature Christmas bulbs use a plastic wedge base
as well.
Lamps intended for use in optical systems (such as film
projectors, microscope illuminators, or
stage lighting instruments have
bases with alignment features so that the filament is positioned
accurately within the optical system. A screw-base lamp may have a
random orientation of the filament when the lamp is installed in
the socket.
Voltage, light output, and lifetime
Incandescent lamps are very sensitive to changes in the supply
voltage. These characteristics are of great practical and economic
importance.
For a supply voltage
V near the rated voltage of the lamp:
- Light output is approximately proportional to
V 3.4
- Power consumption is approximately proportional to
V 1.6
- Lifetime is approximately proportional to V
−16
- Color temperature is approximately proportional to
V 0.42
This means that a 5% reduction in operating voltage will more than
double the life of the bulb, at the expense of reducing its light
output by about 20%. This may be a very acceptable trade off for a
light bulb that is in a difficult-to-access location (for example,
traffic lights or fixtures hung from high ceilings). "Long-life"
bulbs take advantage of this tradeoff. Since the value of the
electric power they consume is much more than the value of the
lamp, general service lamps emphasize efficiency over long
operating life. The objective is to minimize the cost of light, not
the cost of lamps.
The relationships above are valid for only a few percent change of
voltage around rated conditions, but they do indicate that a lamp
operated at much lower than rated voltage could last for hundreds
of times longer than at rated conditions, albeit with greatly
reduced light output.
The Centennial Light
is a light bulb which is accepted by the
Guinness Book of
World Records as having been burning almost continuously
at a fire station in Livermore,
California
, since 1901. However, the bulb is powered by
only 4 watts.
A similar story can be told of a 40-watt
bulb in Texas
which has
been illuminated since September 21, 1908. It once resided
in an
opera house where notable
celebrities stopped to take in its glow, but is now in an area
museum.
In flood lamps used for
photographic
lighting, the tradeoff is made in the other direction. Compared to
general-service bulbs, for the same power, these bulbs produce far
more light, and (more importantly) light at a higher color
temperature, at the expense of greatly reduced life (which may be
as short as 2 hours for a type P1 lamp). The upper limit to the
temperature at which metal incandescent bulbs can operate is the
melting point of the metal. Tungsten
is the metal with the highest melting point, 3695 K (6192°F). A
50-hour-life projection bulb, for instance, is designed to operate
only 50
°C (90
°F)
below that melting point. Such a lamp may achieve up to 22
lumens/watt, compared with 17.5 for a 750-hour general service
lamp.
Lamps designed for different voltages have different luminous
efficacy. For example, a 100-watt, 120-volt lamp will produce about
17.1 lumens per watt. A lamp with the same rated lifetime but
designed for 230 V would produce only around 12.8 lumens/watt, and
a similar lamp designed for 30 volts (train lighting) would produce
as much as 19.8 lumens/watt. Lower voltage lamps have a thicker
filament, for the same power rating. They can run hotter for the
same lifetime before the filament evaporates.
The wires used to support the filament make it mechanically
stronger, but remove heat, creating another tradeoff between
efficiency and long life. Many general-service 120-volt lamps use
no additional support wires, but lamps designed for "rough service"
or "vibration service" may have as many as five. Low-voltage lamps
have filaments made of heavier wire and do not require additional
support wires.
Very low voltages are inefficient since the lead wires would
conduct too much heat away from the filament, so the practical
lower limit for incandescent lamps is 1.5 volts. Very long
filaments for high voltages are fragile, and lamp bases become more
difficult to insulate, so lamps for illumination are not made with
rated voltages over 300 V. Some infrared heating elements are made
for higher voltages, but these use tubular bulbs with widely
separated terminals.
See also
References
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