Incandescent light bulb

An incandescent light bulb, also known as an incandescent lamp or incandescent light globe, is an electric light that makes light by Joule heating a filament until it glows. The filament sits inside a glass bulb that is either evacuated or filled with inert gas, which protects the filament from oxidation. Electric current reaches the filament through wires built into the glass. A bulb socket holds the bulb in place and connects it to electricity.

Incandescent bulbs come in many sizes, brightness levels, and voltage ratings. They do not need any extra regulating equipment, and they are cheap to make. They work on either alternating current or direct current. Because of this, the bulb was widely used in homes and businesses. It was used for table lamps, car headlamps, and flashlights, and for decorative and advertising lights.

Incandescent bulbs are much less efficient than other kinds of electric lighting. Less than 5% of the energy they use becomes visible light. The rest is given off as heat. A typical incandescent bulb produces 16 lumens per watt (lm/W) for a 120 V bulb, or 13 lm/W for a 230 V bulb. A compact fluorescent bulb produces 60 lm/W, and typical white LED lamps produce 100 lm/W. Because of this, incandescent bulbs have lost many customers to LED bulbs. Government rules aimed at saving energy have helped this change happen. In some places, these rules have been debated.

History

Historians Robert Friedel and Paul Israel say that Thomas Edison of General Electric's Edison's version was the first practical incandescent lamp. However, Joseph Swan made a working bulb that was used every day in London in 1881. Historian Thomas Hughes said Edison's success came from building a complete generator-based lighting system, with the lamp as one part of it.

Early inventors spent many years working with resistance and heat. In 1761, Ebenezer Kinnersley showed that a wire could be heated until it glowed, but these wires quickly burned up in air. In 1802, Humphry Davy used a large battery at the Royal Institution to heat a thin platinum strip. It was not bright or long-lasting enough to be useful. For about 40 years, researchers tried to make the carbon arc lamp work in homes. It was too bright and too dangerous for small spaces.

Other early experimenters tried different wires, carbon rods, and vacuum enclosures. They included James Bowman Lindsay in Dundee, Scotland, Marcellin Jobard, Warren De la Rue, John W. Starr, Jean Eugène Robert-Houdin, Moses G. Farmer, and Alexander Lodygin. Henry Woodward and Mathew Evans later patented a lamp with carbon rods. They sold it to Edison in 1879.

Edison began serious work on a practical lamp in 1878. He returned to a carbon filament. It lasted over 1,200 hours when made from bamboo from Japanese sources. Swan worked in Newcastle upon Tyne and improved vacuums and filaments. His house in Underhill, Low Fell, Gateshead was the first in the world to be lit by a light bulb. In 1881, the Savoy Theatre in the City of Westminster became the first theatre lit entirely by electricity.

Alessandro Cruto and Lewis Latimer improved carbon filaments. Heinrich Göbel later claimed to have made a bulb in 1854, but later research said this story was not true. Over time, Siemens tried tantalum. Carl Auer von Welsbach used osmium. Walther Nernst developed the Nernst lamp.

The tungsten filament became common. Sándor Just and Franjo Hanaman first made it, and Tungsram sold it. Filling bulbs with argon or nitrogen helped them last longer, as Irving Langmuir found. William D. Coolidge of General Electric Company developed a way to make tungsten wire. Burnie Lee Benbow patented the coiled coil filament. Hakunetsusha, a predecessor of Toshiba, found a way to make these filaments in large amounts. Marvin Pipkin patented a way to frost bulbs. Imre Bródy used krypton in lamps made at Ajka, Hungary, in a factory designed with Egon Orowan.

By 1964, the cost of a given amount of light had dropped about thirty times compared with Edison's first system. In the United States, sales grew from about 300,000 lamps in 1885 to 795 million in 1945.

Efficacy and efficiency

Further information: Luminous efficacy

Light bulbs are rated by their luminous efficacy. This tells us how much visible light a bulb gives off (luminous flux) compared to the electricity it uses. It is measured in lumens per watt (lm/W). Less than 5% of the power used by a typical tungsten incandescent bulb becomes visible light. Most of the rest is invisible infrared radiation, which we feel as heat. That heat is useful in some things, such as heat lamps in incubators, lava lamps, Edison effect bulbs, the Easy-Bake Oven toy, and infrared heaters used in industry. For the same amount of light, an incandescent bulb uses more power and gives off more heat than most other electric lights. This can make air conditioning systems work harder. The light it makes is similar to that of a black body radiator at the same temperature.

The luminous efficiency of a light source compares its efficacy with the highest possible efficacy, which is 683 lm/W. An ideal white light source could make about 250 lumens per watt, which is a luminous efficiency of 37%. Lights such as fluorescent lamps, high-intensity discharge lamps, and LED lamps are more efficient because they make light by luminescence. Their light can be adjusted to look like incandescent light or other color temperatures of white light. However, colors can sometimes look different under them. Incandescent bulbs cost much more to run than CFLs and LED lamps, so many governments have set efficiency standards that incandescent bulbs cannot meet. The European Union, the United States, Russia, Brazil, Argentina, Canada, and Australia are among those that have banned them. Some people worry about the health effects of fluorescent lamps, and others do not like that alternatives cost more to buy at first.

See also: Architectural lighting design

Some research has tried to make incandescent lamps more efficient. In 2007, General Electric announced a high efficiency incandescent project, but it ended in 2008 because progress was slow. Sandia National Laboratories studied a photonic lattice filament, but later work showed that the early promising results were not correct. Philips has sold hybrid bulbs that use reflective capsules and interference filters or photonic crystals. These send wasted infrared heat back to the filament.

Construction

An incandescent light bulb has a sealed glass bulb, called the envelope, with a tungsten filament inside. An electric current runs through the filament. Wires and a base with two or more conductors connect the bulb to electricity. A stem at the base lets the wires pass through the glass without air or gas leaking in or out. The current heats the filament to about 2,000 to 3,300 K, which is still below tungsten's melting point. The hot filament gives off light that is close to a continuous spectrum. Only part of that energy is visible light. Most of it becomes heat in the near-infrared range.

Bulbs have either clear or coated glass. Coated bulbs have kaolin clay on the inside, which softens the light. Makers may add pigments to change its color. Colored bulbs, such as "party bulbs" and Christmas tree lights, are made by coloring the glass with a dopant. This is often a metal like cobalt (blue) or chromium (green). Most modern bulbs are filled with an inert gas at a low pressure. The gas slows the filament's evaporation and prevents its oxidation. Common gases include argon mixed with nitrogen, krypton, xenon, hydrogen in special flashing lamps, and halogen in halogen lamps. Small lamps often use a vacuum, which gives good thermal insulation but does not stop evaporation. Gases with high atomic or molecular weight work best. Gas can still carry heat away through heat conduction and heat convection, and thermal conductivity also matters. If the envelope leaks, the hot filament reacts with air. This forms tungsten nitride, tungsten dioxide, tungsten pentoxide, and tungsten trioxide, which settle on nearby surfaces. Early carbon-filament bulbs used carbon monoxide, nitrogen, or mercury vapor, but these helped very little.

Outline of Glass bulb Low pressure inert gas (argon, nitrogen, krypton, xenon) Tungsten filament Contact wire (goes into stem) Contact wire (goes out of stem) Support wires (one end embedded in stem; conduct no current) Stem (glass mount) Contact wire (goes out of stem) Cap (sleeve) Insulation (vitrite) Electrical contact

Manufacturing

At first, light bulbs were made by hand, which took a lot of work. Later, machines were invented that could make bulbs faster and for less money. Until 1910, a team of three workers usually blew each bulb by hand into a mold. In the 1880s, workers at Corning Glass Works could make about 150 bulbs in one hour. The Westlake machine was made by Libbey Glass. Corning Glass Works soon began building its own machines, and the first one used in factories was the E-Machine.

In 1926, Corning put a new machine called the Ribbon Machine into its factory in Wellsboro, Pennsylvania. This machine was used to make incandescent bulbs into the 21st century. It works by moving a long strip of glass along a conveyor belt, heating it in a furnace, and blowing it into molds. A typical machine can make many thousands of bulbs in one hour. By the 1970s, only 15 ribbon machines in factories around the world made all the incandescent bulbs being sold. In 2016, the Osram-Sylvania factory in Wellsboro, Pennsylvania closed, and one of the last ribbon machines in the United States was shut down.

Filament

Carbon has the highest melting point of any element. In carbon arc lamps, it was shown to glow nearly as brightly as sunlight. However, carbon can sublimate before it melts, which made vacuum bulbs turn black quickly. The first filaments that worked well were made from carbonized paper or bamboo. Metal filaments were tried in 1897 and slowly replaced carbon around 1904. Tungsten can handle very high heat, but it was brittle and hard to shape. By 1910, William D. Coolidge at General Electric found a way to make tungsten bendable enough to draw into wire. Today, tungsten filaments usually fail because of grain boundary sliding, which makes the filament sag until it breaks. Filaments are often coiled, which slows evaporation and helps the bulb last longer. Electrical filaments are also used in hot cathodes of fluorescent lamps and vacuum tubes to supply electrons.

Tungsten slowly evaporates while a bulb is on, and hotter filaments evaporate faster. This means bulbs must balance good light output with how long they last. Irving Langmuir found that filling a bulb with an inert gas, instead of a vacuum, slows evaporation. Bulbs above about 25 watts are now usually filled with mostly argon and some nitrogen, or sometimes krypton. Xenon works well but is expensive, so it is used mainly in smaller lamps. Small differences in resistivity can create hot spots that wear through the filament, and a positive feedback loop makes these spots worse. Lamps that run on direct current can also wear out faster, and some rhenium alloys can help. A broken filament in a gas-filled bulb can form an electric arc, so thin lead-in wires or protective fuses are often built in. Evaporated tungsten also darkens the glass. Studying this problem led to the discovery of thermionic emission, the invention of the vacuum tube, and evaporation deposition, which is used to make mirrors and other optical coatings. Even a tiny amount of water vapor can make darkening worse, because it dissociates at the hot filament. To prevent this, small amounts of zirconium are placed inside the lamp as a getter to absorb oxygen.

Main article: Halogen lamp

A halogen lamp fills the bulb with a halogen gas at low pressure, along with an inert gas. This reduces uneven evaporation and stops the glass from darkening. The halogen cycle returns tungsten from the glass back to the filament, which helps the bulb last longer. Halogen bulbs are much smaller than ordinary bulbs and are useful where bright light is needed in a small space, such as fiber-optic lamps for optical microscopy or microscopes. Earlier, an ionization arc was used to heat a bead electrode instead of a wire, but these xenon arc lamps later took over.

Electrical characteristics

Incandescent lamps act almost like simple resistors. Their power factor is 1. This means the power they use is equal to the apparent power in the circuit. Incandescent light bulbs are usually sold by the electrical power they use. This depends mainly on the resistance of the filament when the bulb is on. For two bulbs with the same voltage and type, the bulb with higher power gives more light. The table shows the approximate typical amount of light, in lumens, for standard 120 volt bulbs at different powers. Bulbs made for 230 volts usually give a little less light, although the table shows the opposite at 25 watts. Their thinner filament has to run at a slightly lower temperature to last as long, which lowers energy efficiency. "Soft white" bulbs usually give a little less light than clear bulbs with the same power.

The resistance of the filament changes with temperature. When cold, a tungsten filament has about 1/15 of the resistance it has when it is lit. For example, a 100-watt, 120-volt bulb has a resistance of 144 ohms when lit, but only about 9.5 ohms when cold. Because these bulbs act like simple resistors, basic TRIAC dimmers can be used to control their brightness. Some electrical contacts carry a "T" symbol, showing they are made for the high rush of current that tungsten lamps need. For a 100-watt, 120-volt general-service lamp, the current settles in about 0.10 seconds, and the lamp reaches 90% of its full brightness after about 0.13 seconds.

Physical characteristics

Main article: Lightbulb socket

The filament in a tungsten bulb is hard to break when it is cold. When it is hot, it is more fragile. A bump on the outside may break the filament. It can also cause a rush of electric current that melts or burns away part of it. In most modern bulbs, part of the wire works like a fuse. If a broken filament causes an electrical short, that part melts and stops the current. This protects the wires that bring power to the bulb. A hot glass bulb may crack when it touches cold objects. When the glass breaks, the bulb implodes. Air then reaches the filament, which usually destroys it through oxidation.

Names for bulb shapes and sizes are set by national standards. Some names use a letter for the shape and a number for the size, such as A55 or PAR38.

Standards such as ANSI C79.1-2002, IS 14897:2000 and JIS C 7710:1988 list common names for bulb shapes.

Reflector bulbs use a parabolic aluminized reflector (PAR) to aim light more precisely. Multifaceted reflector bulbs are usually smaller and run on a lower voltage. GE calls its infrared-coated lamps "HIR," and Osram calls a similar coating "IRC." Large lamps may have a screw base or a bayonet base. Bayonet bases are often used in automotive lamps because they resist loosening from vibration. Small lamps may have a wedge base. A bi-pin base is often used for halogen or reflector lamps. In the late 1800s, manufacturers made many lamp bases that did not fit each other. General Electric's "Mazda" standard sizes were soon adopted across the US. Contacts in the lightbulb socket let electric current pass through the base to the filament.

Light output and lifetime

See also: Lamp rerating

Incandescent bulbs are very sensitive to changes in their power supply. A 5% drop in voltage doubles the life of the bulb, but it also cuts its light by about 16%. Long-life bulbs use this trade-off in places like traffic signal lamps. Early bulbs could last up to 2500 hours. In 1924, the Phoebus cartel agreed to limit bulb life to 1000 hours. When this was made public in 1953, General Electric and other big American makers were banned from limiting how long bulbs lasted.

The "Centennial Light" is recognized by the Guinness Book of World Records as having burned almost non-stop at a fire station in Livermore, California since 1901. It gives off only as much light as a four watt bulb. A similar 40-watt bulb in Texas has been lit since 21 September 1908. It was once kept in an opera house. Photoflood lamps used for photography focus on bright light instead of a long life, and some last only two hours. Lamps made for rough use may have extra support wires to make them stronger.