Incandescent Lamp Filaments
Nearly every source of light depends, at the fundamental level, on the release of energy from atoms that have been excited in some manner. Standard incandescent lamps, derived directly from the early models of the 1800s, now commonly utilize a tungsten filament in an inert gas atmosphere, and produce light through the resistive effect that occurs when the filament temperature increases as electrical current is passed through. This interactive tutorial demonstrates the sub-atomic activity within a conducting incandescent lamp filament that results in resistance to current flow, and ultimately leads to the emission of infrared and visible light photons.
Upon initialization, the tutorial window displays a coiled filament similar to those utilized in the common incandescent lamp, or light bulb. Below the coiled filament is presented a greatly enlarged longitudinal section of the filament wire illustrating idealized details of the atoms that make up the conducting material. In order to function as the light source in a lamp, the ends of the coiled filament would be connected to the opposite sides (positive and negative) of an electrical circuit for the supply of power. With sufficient voltage applied to the circuit, current will flow in the form of mass movement of free electrons from the negative to the positive side of the circuit. The tutorial allows the visitor to produce this effect by moving the Voltage slider from its initial left-hand position (zero volts) to the right, which will cause light to be emitted from the filament. The emission increases as the slider is moved farther to the right (up to a maximum of 12 volts), functioning in a manner similar to a household lamp dimmer switch. In an actual incandescent lamp, the filament becomes very hot from resistance to the movement of the electrons, and the light produced is accompanied by a significant amount of heat generation.
The atomic structure of the filament material is represented in the cross-sectional diagram by the bronze-colored balls, with the larger versions illustrating atomic nuclei of the conducting metal, and arranged in a regular geometric pattern typical of a structured solid atomic lattice. The rapidly moving smaller particles represent electrons of the outer atomic energy levels, which are freely mobile in conducting metallic materials. These free electrons are a characteristic of conductors, and normally move randomly from one atom to another. When an electrical potential (voltage) is applied to the system, a portion of the free electrons move from one pole of the filament toward the other (left to right in the tutorial). The mass movement of free electrons under an applied potential constitutes current flow, and is represented in the tutorial by the yellow electrons moving through the wire filament. Adjacent to the Voltage slider is a rectangular yellow box that displays the approximate color temperature of light being emitted by the filament.
Note that increasing the voltage by adjusting the slider corresponds to a greater number of electrons moving, or "flowing", through the conductor. In the course of moving through the filament, the flowing electrons collide with other bound electrons of the metal atoms, as well as with the nuclei in some cases. A proportion of the collisions result in excitation of the metallic electrons to higher energy levels, which may produce light emission upon returning to the lower stable energy level. Continuous collisions between electrons produce a resistance to the flow of the mobile electrons, and atoms of the filament are induced to vibrate by the interaction with the moving electrons. The vibrational energy results in the production of a significant amount of heat, and a characteristic of resistive filament lamps is that only about ten percent of their energy input is turned into light, most of the remainder being emitted as heat (infrared electromagnetic radiation). Electrons are able to flow more easily through a larger filament, and consequently resistance becomes higher as filament wire diameter is reduced, if the same energy is supplied.
The basic mechanism that leads to the release of light photons is similar for most light source types, and involves the excitation of electrons from a lower energy level to a higher level, followed by release of the excess energy when the excited electron returns to its original (ground-level) energy state. The primary difference in light sources is in the process by which the excitation is produced. For conventional incandescent lamps, the excitation relies upon the heating and vibrational motion that may excite bound electrons temporarily to higher energy levels. Although most of the energy released from a metal filament is in the form of heat and infrared light, if it is heated to sufficiently high temperature, visible light wavelengths are also produced. The energy of the light produced, and consequently its wavelength and color temperature, depends upon the specific atomic energy levels that are involved in the initial excitation and subsequent photon release. In order to produce sufficient visible light to be useful, a typical lamp filament must be heated to extremely high temperatures (over 2000 degrees Celsius), and only a few common materials can be utilized without melting or evaporating rapidly. Tungsten has nearly ideal properties for this application, and is the most commonly used metal for the production of lamp filaments.
Matthew J. Parry-Hill, Thomas J. Fellers and Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310.
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