|MLA Citation:||Bloomfield, Louis A. "Incandescent Lightbulbs" How Everything Works 22 Jan 2018. Page 4 of 5. 22 Jan 2018 <http://www.howeverythingworks.org/prints.php?topic=incandescent_light_bulbs&page=4>.|
In a three-way touch lamp, the lamp's electronics control 4 different light levels alternately: dim, medium, bright, and off. How these light levels are obtained depends on the lamp. If the lamp uses a three-way light bulb, which contains two separate filaments, then it can obtain the 3 brightness levels by turning on one or both of the filaments. It uses just the small filament for dim, just the large filament for medium, and both filaments for bright. That's exactly what a normal three-way lamp does.
But if the lamp uses a normal bulb and obtains three light levels from it, then it uses the same technique as a dimmer switch. In this technique, an electronic switching device called a triac is used to limit the times during which electric current can flow through the bulb and deliver power to it. In the bright setting, the triac permits current to flow through the bulb at all times and the bulb appears as bright as possible. But in the dim or medium settings, the triac prevents current from flowing at certain times. The triac takes advantage of the fact that the power flowing through a household lamp is alternating current—current that reverses directions 120 times a second (in the United States) for a total of 60 full cycles of reversal, over and back, each second (60 Hz). At the beginning of each current reversal, the electronic devices that control the triac start a timer. This timer allows those devices to wait a certain amount of time before they trigger the triac and allow it to begin carrying current to the light bulb. Once triggered, the triac will allow current to flow through the bulb until the next reversal of current in the power line. Thus the amount of energy that reaches the bulb during each half-cycle of the power line depends on how long the electronic devices wait before triggering the triac. The longer they wait, the less energy will reach the bulb and the dimmer it will glow. In the bright setting, the triac is triggered immediately after each current reversal so that power always flows to the bulb and it glows brightly. But in the medium and dim settings, the triac is triggered well into the half-cycle that follows the reversal. A normal dimmer gives you complete control over this delay, but a three-way touch switch only provides three preset delays. The medium setting has a medium delay while the dim setting has a long delay.
Second, many light-emitting electronic devices emit infrared light. For example, the light emitting diodes in a television remote control unit emit infrared light. In this case, the infrared light is emitted by electrons that are shifting from one group of quantum levels in a semiconductor to another group—from conduction levels to valence levels. This emission isn't thermal radiation; it doesn't involve heat.
Lastly, some infrared light is produced by lasers. In this case, excited atoms or atomic-like systems amplify passing infrared light to produce enormous numbers of identical light particles—identical photons. Infrared industrial lasers are commonly used to machine everything from greeting cards to steel plates.
As energy becomes more and more precious, making the most of it becomes more and more important. I would suggest saving these 130-volt bulbs for fixtures that are so difficult to reach that you want to avoid changing bulbs at all costs. In more easily accessible fixtures, replacing bulbs is only a minor inconvenience associated with improved energy efficiency. Better still, switch to fluorescent lamps—which are much more energy efficient than even the best incandescent lamps.
However, there are several complications when using this technique to measure a person's temperature. First, anything that lies between the person and you, and that absorbs or emit thermal radiation, will affect your measurement. That's because some of the thermal radiation that appears to be coming from the person may be coming from those in between things. Fortunately, air is moderately transparent to thermal radiation but many other things aren't. In fact, to get an accurate reading of person's temperature, you'd have to cool the telescope and the light detector so that they don't add their own thermal radiation to what you observe. You'd also have to use a mirror telescope because glass optics absorb infrared light.
Second, the temperature that you observe will be that of the person's skin and not their inner core temperature. That's because the person's skin absorbs any infrared light from inside the person and it emits its own infrared light to the world around the person. You can't observe infrared light from inside the person because the person's skin blocks your view. All you see is their skin temperature.
The filament is formed by drawing tungsten metal into a very fine wire. This wire, typically only 42 microns (0.0017 inches) in diameter is first wound into a coil and then this coil is itself wound into a coil. The mandrels used in these two coiling processes are trapped in the coils and must be dissolved away with acids after the filament has been annealed.
The finished filament is clamped or welded to the power leads, which have already been embedded in a glass supporting structure. This glass support is inserted into a bulb and the two glass parts are fused together. A tube in the glass support allows the manufacturer to pump the air out of the bulb and then reintroduce various inert gases. When virtually all of the oxygen has been eliminated from the bulb, the tube is cut off and the opening is sealed. Once the base of the bulb has been attached, the bulb is ready for use.
Only the propane lamp needs a mantle to produce bright light. That's because the hot gas molecules that are produced by propane combustion aren't very good at radiating their thermal energy as visible light. The mantle extracts thermal energy from the passing gas molecules and becomes incandescent—it converts much of its thermal energy into thermal radiation, including visible light. Mantles are actually delicate ceramic structures consisting of metal oxides, including thorium oxide. Thorium is a naturally occurring radioactive element, similar to uranium, and lamp mantles are one of the few unregulated uses of thorium.
The light emitted by these oxide mantles is shorter in average wavelength than can be explained simply by the temperature of the burning gases, so it isn't just thermal radiation at the ambient temperature. The mantle's unexpected light emission is called candoluminescence and is thought to involve non-thermal light emitted as the result of chemical reactions and radiative transitions involving the burning gases and the mantle oxides.
In contrast, the acetylene miner's lamp works pretty well without a mantle. I think that's because the flame contains lots of tiny carbon particles that act as the mantle and emit an adequate spectrum of yellow thermal radiation. Many of these particles then go on to become soot. A candle flame emits yellow light in the same manner.
One last feature of a properly constructed miner's lamp, a safety lamp, is that it can't ignite gases around it even if those gases are present in explosive concentrations. That's because the lamp's flame is surrounded by a fine metal mesh. This mesh draws heat out of any gas within its holes and thus prevents the flame inside the mesh from igniting any gas outside the mesh.
This tungsten recycling process dramatically slows the filament's decay. Although the filament gradually develops thin spots that eventually cause it to fail, the filament can operate at a higher temperature and still last two or three times as long as the filament of a regular bulb. The hotter filament of a halogen bulb emits relatively more blue light and relatively less infrared light than a regular bulb, giving it a whiter appearance and making it more energy efficient.
But while both of these techniques work fine at short distances, they aren't so good at large distances in a dense forest. A better scheme is to look for his thermal radiation. All objects emit thermal radiation to some extent and the spectral character of this thermal radiation depends principally on the temperatures of the objects. If the person is hotter than his surroundings, as is almost always the case, he will emit a different spectrum of thermal radiation than his surrounds. Light sensors that operate in the deep infrared can detect a person's thermal radiation and distinguish it from that of his cooler surroundings. Still, viewing that thermal radiation requires a direct line-of-sight from the person to the infrared sensor, so if the forest is too dense, the person is untrackable.
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