How does a fluorescent light work?

How does a fluorescent light work?

A fluorescent lamp consists of a gas-filled glass tube with an electrode at each end. This lamp emits light when a current of electrons passes through it from one electrode to the other and excites mercury atoms in the tube’s vapor. The electrons are able to leave the electrodes because those electrodes are heated to high temperatures and an electric field, powered by the electric company, propels them through the tube. However, the light that the mercury atoms emit is actually in the ultraviolet, where it can’t be seen. To convert this ultraviolet light to visible light, the inside surface of the glass tube is coated with a fluorescent powder. When this fluorescent powder is exposed to ultraviolet light, it absorbs the light energy and reemits some of it as visible light, a process called “fluorescence.” The missing light energy is converted to thermal energy, making the tube slightly hot. By carefully selecting the fluorescent powders (called “phosphors”), the manufacturer of the light can tailor the light’s coloration. The most common phosphor mixtures these days are warm white, cool white, deluxe warm white, and deluxe cool white.

The only other significant component of the fluorescent lamp is its ballast. This device is needed to control the current flow through the tube. Gas discharges such as the one that occurs inside the lamp are notoriously unstable—they’re hard to start and, once they do start, tend to become too intense. To regulate the discharge, the ballast controls the amount of current flowing through the tube. In most older lamps, this control is done by an electromagnetic device called an inductor. An inductor opposes current changes and keeps a relatively constant current flowing through the tube (although that current does stop and reverse directions each time the power line current reverses directions — 120 times a second or 60 full cycles, over and back, in the United States). Some modern fluorescent lamps use electronic ballasts—sophisticated electronic controls that regulate current with the help of transistor-like components.

Where does the extra energy go after ultraviolet light goes through the phosphor…

Where does the extra energy go after ultraviolet light goes through the phosphor coating? Is it lost as heat?

Yes. The extra energy is converted into heat by the phosphors. Their electrons absorb the light energy, convert some of that energy into heat, and then reemit the light. Since the new light contains less energy per particle (per photon) than the old light, it appears as visible rather than ultraviolet light.

How does an ultraviolet (“black light”) fluorescent tube work?

How does an ultraviolet (“black light”) fluorescent tube work?

Some ultraviolet fluorescent tubes are simply the mercury discharge tubes (as in a normal fluorescent tube) but without any phosphor coating on the inside of the tube and with a quartz glass tube that transmits 254 nanometer light. In such a bulb, the 254-nanometer light emitted by mercury vapor in a discharge is emitted directly from the tube without being converted into visible light. A filter somewhere in the system absorbs the small amount of visible light emitted by a low-pressure mercury discharge. For the longer wavelength black light used in most applications, other gases that emit lots of 300-400 nanometer light are used. Again, these tubes have no phosphor coatings to convert the ultraviolet light into visible light. One other way to make longer wavelength black light is to use a mercury discharge but to coat the inside of the tube with a phosphor that fluoresces ultraviolet light between 300 and 400 nanometer.

Why do fluorescent emissions of light not produce more heat?

Why do fluorescent emissions of light not produce more heat?

When an atom is excited by a collision and then emits energy as light, it converts most of the collision energy into light. Thus the gas in a fluorescent lamp experiences many collisions but emits most of the collision energy as light. The gas becomes slightly hot, but not nearly as hot as the filament of an incandescent bulb. The electrical energy arrives at the fluorescent bulb as a current of charged particles and most of this energy leaves the bulb as light, without ever becoming heat. However the electrical energy arriving at an incandescent bulb becomes heat first and then becomes light. The conversion of electrical energy to heat dramatically reduces the bulb’s ability to emit visible light efficiently.

How does radiation trapping work?

How does radiation trapping work?

Each atom has certain wavelengths of light that it is particularly capable of absorbing and emitting. For mercury, that special wavelength is about 254 nanometer (ultraviolet). For sodium, it is about 590 nanometer (orange-yellow). If you send a photon of the right 590 nanometer light at a sodium atom, there is a good chance that that atom will absorb it, hold it for a few billionths of a second, and then reemit it. The newly reemitted light will probably not be traveling in the same direction as before. Now if you have a dense gas of sodium vapor and send in your special photon of light, that photon will find itself bouncing from one sodium atom to another, like the metal ball in a huge pinball game. The photon will eventually emerge from the gas, but not before it has traveled a very long distance and spent a long time in the gas. It was “trapped” in the sodium vapor. This radiation trapping makes it hard for high-pressure gas discharges to emit their special wavelengths because those wavelengths of light become trapped in the gas.

Why do fluorescent tubes explode if broken (is it the compression of the gas)?

Why do fluorescent tubes explode if broken (is it the compression of the gas)?

Fluorescent tubes operate at very low pressure; roughly 1/1000th of an atmosphere. They do not explode when broken; they implode. The atmospheric pressure surrounding the tube crushes it as soon as it begins to crack. The tube shape of a typical fluorescent tube is chosen because it can withstand the enormous compressive forces of the atmosphere better than most other shapes.

Are flood lights incandescent or fluorescent? Why are they so bright?

Are flood lights incandescent or fluorescent? Why are they so bright?

Most modern commercial and industrial floodlights are fluorescent lamps. Fluorescent lamps are so much more energy efficient than incandescent lamps that they quickly pay for their higher cost by saving electricity. Fluorescent lamps also last much longer than incandescent lamps, particularly if they are left on for long periods of time. Fluorescent lamps age most during their start-up cycles. Even around the house, fluorescent floodlights are becoming popular. Fluorescent lamps using about 150 W of power are as bright as incandescent lamps using 500 W. Both are bright, but one is much more energy efficient.

Is a neon light actually a mercury/phosphor tube?

Is a neon light actually a mercury/phosphor tube?

Most “neon” lamps are mercury lamps with a colored phosphor coating on the inside. However the true neon lamp (that special red glow) is really neon gas glowing directly. Take a close look at an advertising lamp that contains a variety of colors. The mercury/phosphor ones will seem to emit light from their frosted glass walls. You are seeing the phosphors glowing. But the real neon lamp will emit light from its inside. The glass will be clear and you will see the glow originate in the gas itself.