The Great Internet Light Bulb Book, Part I
Incandescent including halogen light bulbs
Copyright (C) 1996, 2000, 2005, 2006 Donald L. Klipstein (Jr) (don@misty.com) Freely distributing copies of this entire document or un-HTML-ized text versions thereof is permitted and encouraged.
History of Incandescent Bulbs
It is widely regarded that Thomas Alva Edison invented the first reasonably practical incandescent lamp, using a carbon filament in a bulb containing a vacuum. Edison's first successful test occurred in 1879.
There were earlier incandescent lamps, such as one by Heinrich Goebel made with a carbon filament in 1854. This incandescent lamp had a carbonized bamboo filament and was mentioned as lasting up to 400 hours. At least some sources regard Goebel as the inventor of the incandescent lamp.
Joseph Wilson Swan began trying to make carbon-based incandescent lamps in 1850 and made one in 1860 that was workable except for excessively short life due to poor vacuum. He made more successful inccandescent lamps after better vacuum pumps became available in the mid 1870's.
Since that time, the incandescent lamp has been improved by using tantalum and later tungsten filaments, which evaporate more slowly than carbon. Nowadays, incandescent lamps are still made with tungsten filaments.
Basic Principles
The filament of an incandescent lamp is simply a resistor. If electrical power is applied, it is converted to heat in the filament. The filament's temperature rises until it gets rid of heat at the same rate that heat is being generated in the filament. Ideally, the filament gets rid of heat only by radiating it away, although a small amount of heat energy is also removed from the filament by thermal conduction.
The filament's temperature is very high, generally over 2000 degrees Celsius, or generally over 3600 degrees Fahrenheit. In a "standard" 75 or 100 watt 120 volt bulb, the filament temperature is roughly 2550 degrees Celsius, or roughly 4600 degrees Fahrenheit. At high temperatures like this, the thermal radiation from the filament includes a significant amount of visible light.
Luminous Efficiency
In a 120 volt, 100 watt "standard" bulb with a rated light output of 1750 lumens, the efficiency is 17.5 lumens per watt. This compares poorly to an "ideal" of 242.5 lumens per watt for one idealized type of white light, or 683 lumens per watt ideally for the yellowish-green wavelength of light that the human eye is most sensitive to. Other types of incandescent light bulbs have different efficiencies, but all generally have efficiencies near or below 35 lumens per watt. Most household incandescent bulbs have efficiencies from 8 to 21 lumens per watt. Higher efficiencies near 35 lumens per watt are only achieved with photographic and projection lamps with very high filament temperatures and short lifetimes of a few hours to around 40 hours. The reason for this poor efficiency is the fact that tungsten filaments radiate mostly infrared radiation at any temperature that they can withstand. An ideal thermal radiator produces visible light most efficiently at temperatures around 6300 Celsius (6600 Kelvin or 11,500 degrees Fahrenheit). Even at this high temperature, a lot of the radiation is either infrared or ultraviolet, and the theoretical luminous efficiency is 95 lumens per watt. Of course, nothing known to any humans is solid and usable as a light bulb filament at temperatures anywhere close to this. The surface of the sun is not quite that hot. There are other ways to efficiently radiate thermal radiation using higher temperatures and/or substances that radiate better at visible wavelengths than invisible ones. The efficiency of an incandescent bulb can be increased by increasing the filament temperature, which makes it burn out more quickly.
Vacuum vs. gas-filled bulbs
At first, incandescent bulbs were made with a vacuum inside them. Air oxidizes the filament at high temperatures. Later, it was discovered that filling the bulb with an inert gas such as argon or an argon-nitrogen mixture slows down evaporation of the filament. Tungsten atoms evaporating from the filament can be bounced back to the filament by gas atoms. The filament can be operated at a higher temperature with a fill gas than with a vacuum. This results in more efficient radiation of visible light. So why are some bulbs still made with a vacuum? The reason is that a fill gas conducts heat away from the filament. This conducted heat is energy that cannot be radiated by the filament and is lost, or wasted. This mechanism reduces the bulb's efficiency of producing radiation. If this is not offset by the advantage of operating the filament at a higher temperature, then the bulb is more efficient with a vacuum.
One property of thermal conduction from the filament to the gas is the strange fact that the amount of heat conducted is roughly proportional to the filament's length, but does not vary much with the filament's diameter. The reason this occurs is beyond the scope of this document. However, this means that bulbs with thin filaments and lower currents are more efficient with a vacuum, and higher current bulbs with thicker filaments are more efficient with a fill gas. The break-even point seems to be very roughly around 6-10 watts per centimeter of filament. (This can vary with filament temperature and other factors. The break-even point may be higher in larger bulbs where convection may increase heat removal from the filament by the gas.) Sometimes, premium fill gases such as krypton or xenon are used. These gases have larger atoms that are better at bouncing evaporated tungsten atoms back to the filament. These gases also conduct heat less than argon. Of these two gases, xenon is better, but more expensive. Either of these gases will significantly improve the life of the bulb, or result in some improvement in efficiency, or both. Often, the cost of these gases makes it uneconomical to use them.
How light bulbs burn out
Due to the high temperature that a tungsten filament is operated at, some of the tungsten evaporates during use. Furthermore, since no light bulb is perfect, the filament does not evaporate evenly. Some spots will suffer greater evaporation and become thinner than the rest of the filament. These thin spots cause problems. Their electrical resistance is greater than that of average parts of the filament. Since the current is equal in all parts of the filament, more heat is generated where the filament is thinner. The thin parts also have less surface area to radiate heat away with. This "double whammy" causes the thin spots to have a higher temperature. Now that the thin spots are hotter, they evaporate more quickly. It becomes apparent that as soon as a part of the filament becomes significantly thinner than the rest of it, this situation compounds itself at increasing speed until a thin part of the filament either melts or becomes weak and breaks.
Why bulbs often burn out when you turn them on
Many people wonder what goes on when you turn on a light. It is often annoying that a weak, aging light bulb will not burn out until the next time you turn it on. The answer here is with those thin spots in the filament. Since they have less mass than the less-evaporated parts of the filament, they heat up more quickly. Part of the problem is the fact that tungsten, like most metals, has less resistance when it is cool and more resistance when it is hot. This explains the current surge that light bulbs draw when they are first turned on. When the thin spots have reached the temperature that they would be running at, the thicker, heavier parts of the filament have not yet reached their final temperature. This means that the filament's resistance is still a bit low and excessive current is still flowing. This causes the thinner parts of the filament to get even hotter while the rest of the filament is still warming up. This means that the thin spots, which run too hot anyway, get even hotter when the thicker parts of the filament have not yet fully warmed up. This is why weak, aging bulbs can't survive being turned on.
Why burnout is sometimes so spectacular
When the filament breaks, an arc sometimes forms. Since the current flowing through the arc is also flowing through the filament at this time, there is a voltage gradient across the two pieces of the filament. This voltage gradient often causes this arc to expand until it is across the entire filament. Now, consider a slightly nasty characteristic of most electric arcs. If you increase the current going through an arc, it gets hotter, which makes it more conductive. Obviously, this could make things a bit unstable, since the more conductive arc would draw even more current. The arc easily becomes conductive enough that it draws a few hundred amps of current. At this point, the arc often melts the parts of the filament that the ends of the arc are on, and the arc glows with a very bright light blue flash. Most household light bulbs have a built-in fuse, consisting of a thin region in one of the internal wires. The extreme current drawn by a burnout arc often blows this built-in fuse. If not for this fuse, people would frequently suffer blown fuses or tripped circuit breakers from light bulbs burning out. Although the light bulb's internal fuse will generally protect household fuses and circuit breakers, it may fail to protect the more delicate electronics often found in light dimmers and electronic switching devices from the current surges drawn by "burnout arcs".
How bad a current surge bulbs draw when turned on
It is fairly well known that a cold light bulb filament has less resistance than a hot one. Therefore, a light bulb draws excessive current until the filament warms up. Since the filament can draw more than ten times as much current as usual when it is cold, some people are concerned about excessive energy consumption from turning on light bulbs. The degree of this phenomenon has become a matter of urban folklore. However, the filament warms up very rapidly. The amount of energy consumed to warm up a cold filament is less than it would consume in one second of normal operation.
Making bulbs last longer
Long-life bulbs
Many light bulbs are made to operate with a slightly lower filament temperature than usual. This makes the bulbs last much longer with a slight reduction of efficiency. Reduced Power Reducing the voltage applied to a light bulb will reduce the filament temperature, resulting in a dramatic increase in life expectancy. One device sold to do this is an ordinary silicon diode built into a cap that is made to stick to the base of a light bulb. A diode lets current through in only one direction, causing the bulb to get power only 50 percent of the time if it is operated on AC. This effectively reduces the applied voltage by about 30 percent. The life expectancy is increased very dramatically. However, the power consumption is reduced by about 40 percent (not 50 since the cooler filament has less resistance) and light output is reduced by reduced by about 70 percent (cooler filaments are less efficient at radiating visible light).