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Basic Physics of the Incandescent Lamp (Lightbulb) Dan Macisaac, Gary Kanner,Andgraydon Anderson

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Basic Physics of the Incandescent Lamp (Lightbulb) Dan Macisaac, Gary Kanner,Andgraydon Anderson Basic Physics of the Incandescent Lamp (Lightbulb) Dan MacIsaac, Gary Kanner,andGraydon Anderson ntil a little over a century ago, artifi- transferred to electronic excitations within the Ucial lighting was based on the emis- solid. The excited states are relieved by pho- sion of radiation brought about by burning tonic emission. When enough of the radiation fossil fuels—vegetable and animal oils, emitted is in the visible spectrum so that we waxes, and fats, with a wick to control the rate can see an object by its own visible light, we of burning. Light from coal gas and natural say it is incandescing. In a solid, there is a gas was a major development, along with the near-continuum of electron energy levels, realization that the higher the temperature of resulting in a continuous non-discrete spec- the material being burned, the whiter the color trum of radiation. and the greater the light output. But the inven- To emit visible light, a solid must be heat- tion of the incandescent electric lamp in the ed red hot to over 850 K. Compare this with Dan MacIsaac is an 1870s was quite unlike anything that had hap- the 6600 K average temperature of the Sun’s Assistant Professor of pened before. Modern lighting comes almost photosphere, which defines the color mixture Physics and Astronomy at entirely from electric light sources. In the of sunlight and the visible spectrum for our Northern Arizona University. United States, about a quarter of electrical eyes. It is currently impossible to match the He received B.Sc. (physics) consumption is attributed to lighting, half of color mix of sunlight with any filament and B.Ed. (math and sci- that in incandescent lamps. So an understand- because we have no substance that can be ence education) degrees ing of the basic electrical and optical charac- heated to this temperature and remain solid. from Mount Allison Univ- ersity, an M.A. (science edu- teristics of the lightbulb is an appropriate sub- Of all solid filament materials, tungsten has cation) from the University ject for study in the introductory physics class. the highest known melting temperature (3680 of British Columbia, and an The first incandescent lamps consisted of a K) and the lowest rate of evaporation (vapor M.S. (physics) and Ph.D. filament of carbon wire in an evacuated glass pressure) of the pure metals. Carbon can with- (science education) from bulb, two ends of the wire being brought out stand higher temperatures without melting, Purdue University. His inter- through a sealed cap and then to the electric but evaporates too rapidly. Compounds and ests are in physics educa- supply. A major improvement was the devel- alloys (usually metal carbides and nitrides) tion research and high- opment of metallic filaments, particularly with higher melting temperatures and lower school teacher preparation, those made of tungsten, now used almost evaporation rates exist, but these are brittle and his hobbies include exclusively in lightbulbs. and tend to disassociate at these very high classical trumpet and motor- Incandescent lighting is very economical temperatures. cycle riding. and nonhazardous to manufacture, although Tungsten’s high melting point and low Department of Physics less energy efficient than other lighting tech- vaporization pressure make it the metal of and Astronomy 1 Northern Arizona nologies. The mercury vapor fluorescent choice, but the vaporization pressure limits University tube is more efficient; however, many states maximum useful filament temperature to Flagstaff, AZ 86011-6010 are passing restrictive regulations regarding about 3000 K. It is extremely difficult to [email protected] disposal of such tubes. Currently, the most maintain an average temperature higher than efficient commercial light source (in terms of about 2900 K in standard incandescent bulbs, visible light output to input energy) is the resulting in radiation distributions like those high-pressure sodium arc lamp. shown in Sidebar 1. At these temperatures, only a small fraction of the radiated energy Incandescent Lighting Technology occurs in the visible wavelengths—less than Incandescence occurs when electrical 10%, with most remaining energy radiated resistive heating creates thermally excited away at infrared (IR) wavelengths. Hence, atoms. Some of the thermal kinetic energy is incandescent filaments are quite inefficient 520 THEPHYSICSTEACHER Vol. 37, Dec.1999 Basic Physics of the Incandescent Lamp (Lightbulb) Bulb envelope Fill gas for visible light production, and their light is quite reddish-yellow. Low-temperature fila- ments (2500 to 2700 K) are particularly rich in red spectral energy and tend to bring out red in skin complexions, making people appear healthier. Higher temperature fila- ments (2800 to 2900 K) are relatively richer in blue wavelengths, and are paradoxically Filament called “cooler” lights in the color sense. Lead-in While far from ideal for visible light emis- Wires sion, these filament temperatures are still enormous (about 3000 K, or 5000 8F), likely the hottest phenomenon we will closely encounter in our lifetimes, unless we are Glass rod welding. Achieving and sustaining these tem- peratures requires many technical innova- tions (Fig. 1). Lighting manufacturers describe the effi- Fuse Exhaust ciency measurement orefficacyof incandes- Tube cent lamps as the amount of visible light pro- duced in lumens per watt (LPW) of electrical Base power consumed. A bar of tungsten heated to its melting point has a theoretical maximum efficacy of 52 LPW. Practical studio flood Gary S. Kanneris a visiting Assistant Professor at the Eyelet lamps achieve 33 LPW, standard 60-W household lamps with a rated lifetime of 1000 Department of Physics and Astronomy at Northern Fig. 1. Components of a standard household lightbulb hours achieve 14.5 LPW, or 870 lumens total. Arizona University while on of 60 W, 120 VAC. Looking at the Lightbulb leave of absence from Los • Bulb envelopeis made of soft soda-lime glass; Alamos National Laboratory, top operating temperature ~ 400 8C. A lightbulb’s glass envelope is designed to where he has been a techni- keep water vapor and oxygen away from the • Fill gas—usually argon to retard filament evapo- cal staff member in a metal- ration with some nitrogen to eliminate arcing— filament, which would otherwise oxidize the lurgy group, after having circulates by convection. metal within seconds. Household bulbs are served as postdoc there for • Exhaust tube extends through bulb base and is usually made of soda-lime glass. Silica and several years. He received used to evacuate, flush, and fill bulb before being Pyrex™ (borosilicate) can be used for higher a B.A. in physics from sealed off. temperatures, to improve durability, or for Brandeis University, an M.S. in physics from Brown • Base, made of aluminum and brass, is cement- ultraviolet transparency. The intricate coiled- ed to the bulb; cement failure is first sign of bulb’s coil mechanical design of the filament is University, and a Ph.D. in overheating. physics from the University designed to retain as much thermal energy as of Utah. His research inter- • Eyelet is contact point to which electrical hot wire possible while increasing surface area and ests are in optical properties is soldered. Electrical return wire is soldered to aspect ratio (Fig. 2). The lead-in molybdenum threaded side portion of base. of semiconductors, and he wires that support the filament and conduct likes to compete in • Fuseprotects household circuit by melting if fila- current to it retain their strength at high tem- ment arcs. triathlons. peratures. The National Electric Code speci- Department of Physics • Glass rodwith button that supports wires placed fies that the lead-in wire soldered to the eye- in it. and Astronomy let (the bottom brass button) must incorporate Northern Arizona • Lead-in wiresmade ofthree welded metal sec- a short fuse element and then be connected to University tions carry current to and from filament, passing Flagstaff, AZ 86011-6010 through glass seals called the stem press. Wires the “hot” ac connection. Return is through the are designed to match expansion coefficient of threaded aluminum side of the base. All mod- the glass. ern household bulbs over 25 W are internally • Coiled-coil tungsten filamentis designed to fused. When properly installed, it is the hot maintain temperature and glows yellow/orange side that is fused (hence the current use of hot at 3000 K (5000 8F). Filament is supported by mechanical clamps to lead-in and tie wires. polarized plugs for lamps). An electrostatically applied inside coat of fine silica powder called standard coat Basic Physics of the Incandescent Lamp (Lightbulb) Vol. 37, Dec. 1999 THEPHYSICSTEACHER 521 Fig. 2. Scanning electron micrograph (SEM) of a coiled-coil filament from a 60-W lamp (500X magnifi- cation). Filament winding is an incredi- ble technical feat; readers are encour- aged to break a bulb and examine the fila- ment closely with a hand lens. spreads the brilliant filament light diffusely cacy for melting tungsten will be surpassed. by Mie scattering.2 In high-temperature bulbs, an additional reflector may be used to Filament Coils keep the base cool by preventing radiative The automated mass manufacturing of Graydon Anderson com- warming and restricting the convection flow incandescent filaments intended for operation pleted his Ph.D. in physical of the “fill gas” throughout the base. at over 2500 K is a significant technical chemistry at Cornell Most modern bulbs use argon as the inert achievement. Just drawing brittle tungsten University in 1975. For most fill gas, with a small amount of nitrogen to into wire required the development of a spe- of the time since then he impede arcing. Argon’s high molecular cial process involving doping with potassium, has worked as an experi- mentalist at the Los Alamos weight and low thermal conductivity retards pressing and sintering tungsten ingots, then National Laboratory on a the tungsten’s evaporation and insulates the swaging, lubricating, and finally drawing the very diverse set of projects, filament, thereby allowing for high tempera- wire.
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