Category: incandescent light bulbs

How does a “touch lamp” work? — LAM, Enosburg Falls, VT

A touch lamp detects your touch by looking for changes in the electric properties of the lamp’s surfaces. It monitors these properties by putting a fluctuating electric charge on them. As electric current flows toward the bulb through the lamp’s wires, it passes through an electronic device that places a high frequency (about 60 kHz) alternating current onto those wires. This added current causes the lamp’s surfaces to take on a small fluctuating electric charge—first positive, then negative, then positive, over and over again. This surface charging involves electrostatic forces, which extend long distances between charged objects, and occurs even though the lamp’s surfaces aren’t directly connected to the lamp’s wires. The more surface the lamp has, the more easily it can hold that electric charge—the greater the lamp’s electric capacitance.

When you plug the lamp in, the electronic device uses its fluctuating charge to determine how easy it is to add or subtract charge from the lamp’s surfaces. In other words, it measures the lamp’s capacitance. It then begins to look for changes in that capacitance. When you touch the lamp, or even come close to its surfaces, your body effectively adds to the lamp’s surface and its capacitance increases significantly. The electronic device detects this increase in capacitance and switches the lamp’s state from on to off or from off to on. The fact that you don’t have to touch the lamp to affect its capacitance means that a touch lamp can have insulating paint on its metal surfaces yet still detect your touch. You can also buy touch lamp modules that plug into the wall and turn the lamp that’s connected to them into a touch lamp. These modules are so sensitive to capacitance changes in the lamp that you can trigger them just by touching the lamp cord.

What makes a three-way touch lamp work? What makes a three-way light bulb work? – CY

A three-way touch lamp is much like a simple touch lamp—it detects your touch by applying a high frequency alternating charge to the lamp’s surfaces and uses this fluctuating charge to measure the lamp’s electric capacitance—the ease with which charge can moved on or off the lamp’s surfaces. When you touch the lamp, the lamp’s capacitance changes and the lamp’s electronics detect this change.

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.

How is infrared light produced?

There are many ways of producing infrared light. First, any warm surface emits infrared light. For example, a heat lamp or an electric space heater emits enormous amounts of it. That’s because the thermal radiation of a warm object lies mostly in the invisible infrared portion of the electromagnetic spectrum.

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.

How does a halogen lamp get so hot?

Like all incandescent bulbs, a halogen lamp creates its light as visible thermal radiation from an extremely hot tungsten wire. In fact, the wire in a halogen lamp is allowed to get even hotter than the one in a normal bulb. But while the glass envelope of a normal bulb gets only moderately hot during use, the glass envelope of a halogen bulb gets extremely hot. That’s because the halogen bulb is using a chemical trick to keep tungsten atoms from getting away from the filament. Each time one of those tungsten atoms tries to leave, it’s picked up by halogen molecules inside the glass envelope and returned to the filament. These halogen molecules can even pick the tungsten atoms up off the glass envelope and return them to the filament, but only if the glass envelope is allowed to get extremely hot. That’s why the glass envelope of the halogen bulb is allowed to run so hot—if it weren’t, it would accumulate the tungsten atoms permanently and it would darken. And since the tungsten atoms wouldn’t be returned the filament, the filament wouldn’t last as long.

How hot is a match when it is ignited? Is the initial point of combustion hotter than when it is just burning? — TB, Excelsior, MN

You can usually judge the temperature of a hot object by its color—the brighter and whiter the light, the hotter the object. A candle flame has a temperature of roughly 1700° C while an incandescent light bulb has a temperature of about 2500° C. To my eye, a struck match briefly becomes brighter and whiter than a candle flame, so I would guess that its peak temperature is somewhere in the mid 2000° C range. Once the chemicals in the head have been used up, the flame temperature drops to about 1700° C.

How can one be fire safe while dealing with incandescent and fluorescent light bulbs? — TJ, Woodbridge, VA

Fluorescent tubes produce relatively little heat, so they’re relatively fire safe already. However, incandescent light bulbs become very hot and you have to be careful with them to avoid fires. First, make sure that the bulb can get rid of its waste heat. That means that you shouldn’t wrap the bulb in insulation because it needs to transfer its waste heat to the air. Second, keep flammable materials away from the bulb, particularly above the bulb since hot air from the bulb rises upward.

What types of gas are used in light bulbs and how do their effects differ? — SF, Westfield, NJ

The glass envelope of an incandescent bulb can’t contain air because tungsten is flammable when hot and would burn up if there were oxygen present around it. One of Thomas Edison’s main contributions to the development of such bulbs was learning how to extract all the air from the bulb. But a bulb that contains no gas won’t work well because tungsten sublimes at high temperatures—its atoms evaporate directly from solid to gas. If there were no gas in the bulb, every tungsten atom that left the filament would fly unimpeded all the way to the glass wall of the bulb and then stick there forever. While there are some incandescent bulbs that operate with a vacuum inside, most common incandescent lamps contain a small amount of argon and nitrogen gases.

Argon and nitrogen are chemically inert, so that the tungsten filament can’t burn in the argon and nitrogen, and each argon atom or nitrogen molecule is massive enough that when a tungsten atom that’s trying to leave the filament hits it, that tungsten atom may rebound back onto the filament. The argon and nitrogen gases thus prolong the life of the filament. Unfortunately, these gases also convey heat away from the filament via convection. You can see evidence of this convection as a dark spot of tungsten atoms that accumulate at the top of the bulb. That black smudge consists of tungsten atoms that didn’t return to the filament and were swept upward as the hot argon and nitrogen gases rose.

However, some premium light bulbs contain krypton gas rather than argon gas. Like argon, krypton is chemically inert. But a krypton atom is more massive than an argon atom, making it more effective at bouncing tungsten atoms back toward the filament after they sublime. Krypton gas is also a poorer conductor of heat than argon gas, so that it allows the filament to convert its power more efficiently into visible light. Unfortunately, krypton is a rare constituent of our atmosphere and very expensive. That’s why it’s only used in premium light bulbs, together with some nitrogen gas.

Incidentally, the filament in many incandescent bulbs is treated with a small amount of a phosphorus-based “getter” that reacts with any residual oxygen that may be in the bulb the first time the filament becomes hot. That’s how the manufacturer ensures that there will be no oxygen in the bulb for the tungsten filament to react with.

How does a halogen cooktop unit heat up food? — BS, Logan, UT

A halogen cooktop unit uses thermal radiation to transfer heat to a pot or pan. All objects emit thermal radiation, but that radiation isn’t visible until an object’s temperature is at least 500° C. At higher temperatures, a significant fraction of an object’s thermal radiation is visible light. In a halogen cooktop unit, an electrically heated tungsten filament is heated to the point where it emits a large amount of thermal radiation. Since the filament is small, it takes only a second or two for the filament to reach full temperature and begin emitting its intense thermal radiation. Any dark object above the unit will absorb this thermal radiation and experience a rise in temperature. When you turn off the unit, the filament cools rapidly and stops emitting its thermal radiation. The filament itself is protected from oxygen in the air by a heat-resistant glass envelope that’s filled with halogen gas. This gas helps to keep the filament intact and prevents it from depositing tungsten atoms on the insides of the glass envelope.

Why is an incandescent light bulb hotter than a fluorescent light? — TJ, Woodbridge, VA

An incandescent light bulb produces light by heating a small filament of tungsten to about 2500° C. At that temperature, the thermal radiation that the filament emits includes a substantial amount of visible light. But the filament also emits a great deal of infrared light (heat light) and it also transfers heat via conduction and convection to the glass bulb around it. When you put your hand near the bulb, you feel both the infrared light and the heat that has worked its way to the surface of the bulb. The bulb feels hot.

In contrast, a fluorescent lamp tries to produce light without heat. It collides electrons with mercury atoms to produce an atomic emission of ultraviolet light. This ultraviolet light is then converted to visible light by the layer of white phosphor powders on the inside of the lamp’s glass envelope. In principle, this whole activity can be performed without creating any thermal energy. However, many unavoidable imperfections cause the lamp to convert some of the electric energy it consumes into thermal energy. Nonetheless, the lamp only becomes warm rather than hot.

Which gives off more heat energy, an incandescent light bulb or a fluorescent lamp? Which is more efficient to use in the summer or winter? — TJ, Woodbridge, VA

An incandescent lamp turns its electric power completely into heat. Even the visible light it gives off is actually thermal radiation. A fluorescent lamp tries not to produce heat—the light it produces is non-thermal (it doesn’t involve hot materials). While a fluorescent lamp is only partly successful at not producing heat, it’s still several times more energy efficient than an incandescent lamps—fluorescents produces several times as much illumination for the same amount of electric power. This statement is true both in summer and winter, although fluorescent bulbs lose some of their energy efficiencies in very cold or very hot weather. Fluorescent lamps work best at temperatures between about 15° C and 40° C.