Incandescent Light

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). 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.

Basic structure

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.

Color temperature

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.)