Incandescent light is given off when an object is heated until it glows. To emit white light, an object must be heated to at least 1,341°F (727°C); at lower temperatures, redder colors are emitted. White-hot iron in a forge is incandescent, as is red lava flowing down a volcano, as are the red burners on an electric stove. The most common example of incandescence is the white-hot filament in the light bulb of an incandescent lamp.
In 1802, Sir Humphry Davy (1778-1829) showed that electricity running through thin strips of metal could heat them to temperatures high enough that they would give off light; this is the basic principle by which all incandescent lamps work. In 1820, De La Rue demonstrated a lamp made of a coiled platinum wire in a glass tube with brass endcaps. When the current was switched on, electricity ran through the endcaps and through the wire (the filament). The wire was heated by its resistance to the current until it glowed white-hot, producing light. Between this time and the 1870s, the delicate lamps were unreliable, short-lived, and expensive to operate. The lifetime was short because the filament would burn up in air. To combat the short lifetime, early developers used thick low-resistance filaments, but heating them to incandescence required large currents—and generating large currents was costly.
Thomas Edison (1847–1931) is well-known as “the inventor of the light bulb,” but he was, in fact, only one of several researchers that created early electrical incandescent lamps in the 1870s. These researchers include Joseph Swan, Frederick DeMoyleyns, and St. George Lane-Fox in England, as well as Moses Farmer, Hiram Maxim, and William Sawyer in the United States.
Edison’s contribution was an understanding of the necessary electrical properties for lamps. He knew that a system for delivering electricity was needed to make lamps practical; that it should be designed so that the lamps are run in parallel, rather than in series; and that the lamp filament should have high, rather than low, resistance. Because voltage in a circuit equals the current times the resistance, one can reduce the amount of current by increasing the resistance of the load. Increasing the resistance also reduces the amount of energy required to heat the filament to incandescence.
Edison replaced low-resistance carbon or platinum filaments with a high-resistance carbon filament. This lamp had electrical contacts connected to a cotton thread that had been burned to char (carbonized) and placed in a glass container with all the air pumped out. The vacuum, produced by a pump developed only a decade earlier by Herman Sprengel, dramatically increased the lifetime of the filament. The first practical version of the electric light bulb was lit on October 19, 1879, which burned for 40 hours, and produced 1.4 lumens per watt of electricity.
An incandescent non-electric lamp still in use is the Welsbach burner, commonly seen in camping lanterns. This burner, invented in 1886 by Karl Auer, Baron von Welsbach, consists of a mantle made of knit cotton soaked in oxides (originally nitrates were used) that is burned to ash the first time it is lit. The ash holds its shape and becomes incandescent when placed over a flame—and is much brighter than the flame itself.
Incandescent lamps come in a huge variety of shapes and sizes, but all share the same basic elements as De La Rue’s original incandescent lamp. Each is contained by a glass or quartz envelope. Current enters the lamp through a conductor in an airtight joint or joints. Wires carry current to the filament, which is held up and away from the bulb by support wires. Changes in the specifics of incandescent lamps have been made to increase efficiency, lifetime, and ease of manufacture.
Although the first common electric lamps were incandescent, many lamps used today are not: Fluorescent lamps, neon signs, and glow-discharge lamps, for example, are not incandescent. Fluorescent lamps are more energy-efficient than incandescent lamps, but may not offer a desired color output. Energy-efficient compact fluorescent lamps today offer good color performance, rapid-on characteristics, and although more costly to install, they save buyers money by using less energy over a longer bulb lifetime. A compact fluorescent bulb that produces as much white light as a 60-watt (W) incandescent bulb uses only about 14 W.
Today, filaments are made of coiled tungsten, a high-resistance material that can be drawn into a wire and has both a high melting point of 6,120°F (3,382°C) and a low vapor pressure, which keeps it from melting or evaporating too quickly. It also has the useful characteristic of having a higher resistance when hot than when cold. If tungsten is heated to melting, it emits 53 lumens per watt. (Lamp filaments are not heated as high to keep the lamp lifetime reasonable, but this gives the upper limit of light available from such a filament.) The filament shape and length are also important to the efficiency of the lamp. Most filaments are coiled, and some are double and triple coiled. This allows the filament to lose less heat to the surrounding gas as well as indirectly heating other portions of the filament.
Most lamps have one screw-type base, through which both wires travel to the filament. The base may be sealed by a flange seal (for lamps 0.8 in [20 mm] or larger) or a low-cost butt seal for lamps smaller than 0.8 (20 mm) in diameter with smaller wires that carry 1 amp or less. The bases are cemented to the bulbs. In applications that require precise positioning of the filament, two-post or bayonet-type bases are preferred.
The bulb may be made from either a regular lead or lime glass or a borosilicate glass that can withstand higher temperatures. Even higher temperatures require the use of quartz, high-silica, or aluminosilicate glasses. Most bulbs are chemically etched inside to diffuse light from the filament. Another method of diffusing the light uses an inner coating of powered white silica.
Lower wattage lights have all the atmosphere pumped out, leaving a vacuum. Lights rated at 40 W or more use an inert fill gas that reduces the evaporation of the tungsten filament. Most use argon, with a small percentage of nitrogen to prevent arcing between the lead-in wires. Krypton is also occasionally used because it increases the efficiency of the lamp, but it is also more expensive. Hydrogen is used for lamps in which quick flashing is necessary.
As the bulb ages, the tungsten evaporates, making the filament thinner and increasing its resistance. This reduces the wattage, the current, the lumens, and the luminous efficacy from the lamp. Some of the evaporated tungsten also condenses on the bulb, darkening it and resulting in more absorption at the bulb. (You can tell whether a bulb has a fill gas or is a vacuum bulb by observing the blackening of an old bulb: vacuum bulbs are evenly coated, whereas gas-filled bulbs show blackening concentrated at the uppermost part of the bulb.) Tungsten-halogen lamps are filled with a halogen (bromine, chlorine, fluorine, or iodine) gas and degrade much less over their lifetimes. When tungsten evaporates from the filament, instead of being deposited on the bulb walls, it forms a gaseous compound with the halogen gas. When this compound is heated (near the filament), it breaks down, redepositing tungsten onto the filament. The compactness and lifelong performance of such lamps is better than regular lamps. The temperature is higher (above 5,121°F [2,827°C]) in these lamps than in regular lamps, thus providing a higher percentage of visible and ultraviolet output. Linear tungsten-halogen bulbs may be coated with filters that reflect infrared energy back at the filament, thus raising the efficiency dramatically without reducing the lifetime.
The efficiency of the light is determined by the amount of visible light it sheds for a given amount of energy consumed. Engineering the filament material increases efficiency. Losses come from heat lost by the filament to the gas around it, loss from the filament to the lead-in wires and supports, and loss to the base and bulb.
Most of the output of the lamp is in the infrared region of the spectrum, which is fine if you want a heat lamp, but not ideal for a visible light source. Only about 10% of the output of a typical incandescent lamp is visible, and much of this is in the red and yellow parts of the spectrum (which are closer to the infrared region than green, blue, or violet). One way of providing a color balance more like daylight is to use a glass bulb with a blue tinge that absorbs some of the red and yellow. This increases the color temperature, but reduces the total light output.
Tradeoffs in design
Temperature is one of several tradeoffs in the design of each lamp. A high filament temperature is necessary, but if it is too high then the filament will evaporate quickly, leading to a short lifetime. Too low a temperature and little of the radiation will be visible. For tungsten-halogen lamps, the temperature must be at least 500° F (260° C) to insure operation of the regenerative cycle. Also, although the filament must be hot, the bulb and base have temperature limits, as does the cement that binds them. Many bulbs have a heat button that acts as a heat shield between the filament and the base. The position of the bulb (base-down for a table lamp, but base-up for a hanging ceiling lamp) also changes the amount of heat to which the base is exposed, which alters the lifetime of the bulb.
If the voltage at which the bulb is operating changes, this changes the filament resistance, temperature, current, power consumption, light output, efficacy (and thus color temperature), and lifetime of the bulb. In general, if the voltage increases, all the other characteristics increase—except for lifetime, which decreases. (None of these relationships are linear.)
With so many different parameters to be balanced in each lamp, it is no wonder that thousands of different lamps are available for a myriad of purposes. Large lamps (including general purpose lamps), miniature lamps (such as Christmas tree lights), and photographic lamps (such as those for shooting movies) cover the three major classes of lamp.
General service lamps are made in ranges from 10 W to 1500 W. The higher-wattage lamps tend to be more efficient at producing light, so it is more energy-efficient to operate one 100-W bulb than two 50-W bulbs. On the other hand, long-life bulbs (which provide longer lifetimes by reducing the filament temperature) are less efficient than regular bulbs but may be worth using in situations where changing the bulb is a bother or may a hazard.
Spotlights and floodlights generally require accurately positioned, compact filaments. Reflectorized bulbs, such as those used for car headlights (these are
Chromaticity —The color quality of light that depends on its hue and saturation. Brightness is not an aspect of chromaticity.
Color temperature —The absolute temperature of a blackbody radiator having a chromaticity equal to that of the light source. Usually used as a way of describing the color characteristics of a light source.
Filament —Part of a lamp that is heated until incandescent; the light source.
Lumen —Luminous flux through a solid angle. One lumen is the amount of light emitted into one steradian from a light source that emits one candela (the intensity of light from one standardized candle).
tungsten-halogen bulbs) or overhead downlights (such as those used in track lighting) are made with reflectors built into the bulb: The bulb’s shape along one side is designed so that a reflective coating on that inner surface shapes the light into a beam.
Lamps used for color photography have to provide a good color balance, keep the same balance throughout their lives, and interact well with the film’s sensitivity. These lamps tend to be classified according to their color temperatures, which range from 5,301°F (2,927°C) for photography, 5,571°F (3,077°C) for professional movies, to 8,541°F (4,727°C) for “daylight blue” lamps, and even some “photographic blue” lamps that approximate sunshine and have a color temperature of 9,441°F (5,227°C).
British Broadcasting Corporation (BBC). “Light Bulbs: Not Such a Bright Idea.” February 3, 2006. <http://news.bbc.co.uk/2/hi/science/nature/4667354.stm> (accessed November 2, 2006).