Science Fair Project Encyclopedia
A fluorescent lamp is a type of lamp that uses electricity to excite mercury vapor in argon or neon gas, producing short-wave ultraviolet light. This light then causes a phosphor to fluoresce, producing visible light.
Fluorescent lamps are more efficient than incandescent light bulbs of an equivalent brightness. This is because more of the energy input is converted to usable light and less is converted to heat (allowing fluorescent lamps to run cooler). They also have a longer lamp life.
However, unlike incandescent lamps, fluorescent lamps always require auxilliary equipment (a ballast).
The earliest ancestor of the fluorescent lamp is probably the device by Heinrich Geissler who obtained in 1856 a bluish glow from a gas sealed in a tube, excited with an induction coil. Though he is remembered as a physicist, it is interesting to note that Geissler was educated as a glassblower, which was certainly of some value for this earliest realization.
In 1894, D. McFarlane Moore created the Moore lamp, a commercial gas discharge lamp meant to compete with the incandescent light bulb of his former boss Thomas Edison. The gases used were nitrogen and carbon dioxide emitting respectively pink and white light, and had moderate success.
In 1901, Peter Cooper Hewitt demonstrated the mercury-vapor lamp, which was emitting in the blue-green spectrum and thus was unfit for most practical purposes. It was, however, very close to the modern design, and had some applications in photography where color was not yet an issue, thanks to its much higher efficiency than incandescent lamps.
It remained to Edmund Germer and coworkers to propose in 1926 to coat the tube with fluorescent powder which converts ultraviolet light emitted by a rare gas into better spectrally distributed light (also bringing up high pressure of the gas at the same time). Germer is today recognized as the inventor of fluorescent lamp.
A fluorescent light bulb is filled with a gas containing argon and mercury vapor, sometimes referred to as plasma when electrified. The inner surface of the bulb is coated with a fluorescent paint made of varying blends of metallic and rare-earth phosphor salts. The bulb's cathode emits electrons which ignite the plasma under the influence of the voltage applied to the light bulb. Then plasma electrons bombard the mercury vapor causing it to emit ultraviolet (UV) light at a wavelength of 254nm. The UV light is absorbed by the bulb's fluorescent coating, which re-radiates the energy at lower frequencies (longer wavelengths) to emit visible light. The blend of phosphors controls the color of the light, and along with the bulb's glass prevents the harmful UV light from escaping.
Fluorescent lamps are negative resistance devices: as more current flows through them and more gas is ionized, the resistance of the fluorescent lamp drops and this would allow even more current to flow through them! Connected directly to a constant-voltage mains power line, a fluorescent lamp would rapidly self-destruct due to the unlimited current flow. Because of this, fluorescent lamps are always used with some sort of auxiliary electronics that regulates the current flow in the tube. This auxiliary device is commonly called a ballast.
While the ballast could be (and occasionally is) as simple as a resistor, substantial power is wasted in a resistive ballast so ballasts usually use a reactance (inductor or capacitor) instead. For operation from mains voltage, the use of simple inductor (a so-called "magnetic ballast") is common. In countries that use 120 vac mains, the mains voltage is insufficient to light large fluorescent lamps so the ballast for these larger fluorescent lamps is often a step-up autotransformer with substantial leakage inductance (so as to limit the current flow). Either form of inductive ballast may also include a capacitor for power factor correction. More sophisticated ballasts may employ transistors or other semiconductor components to convert mains voltage into high-frequency ac while also regulating the current flow in the lamp. These are referred to as "electronic ballasts".
The mercury atoms in the fluorescent tube must be ionized before the arc can "strike" within the tube. For small lamps, it does not take much voltage to strike the arc and starting the lamp presents no problem, but larger tubes require a substantial voltage (in the range of a thousand volts). In some cases, that is exactly how it is done: "instant start" fluorescent tubes simply use a high enough voltage to break down the gas and mercury column and thereby start arc conduction. These tubes can be identified by the facts that
- they have a single pin at each end of the tube and
- the lampholders that they fit into have a "disconnect" socket at the low-voltage end to assure that the mains current is automatically removed so that a person replacing the lamp can not receive a high-voltage electric shock.
In other cases, a separate starting aid must be provided. Old fluorescent designs used a combination filament/cathode at each end of the lamp combined with a mechanical or automatic switch that would initially connect the filaments in series and thereby "preheat" the filaments prior to striking the arc. Because of thermionic emission, the filaments would readily emit electrons into the gas column creating a glow discharge near the filaments. Then, when the starting switch opened up, the inductive ballast would create a voltage surge which would (usually) strike the arc. If so, the impinging arc then kept the filament/cathode warm. If not, the starting sequence was repeated. If the starting aid was automatic, this often led to the situation where an old fluorescent lamp would flash time and time again as the starter repeatedly tried to start the worn-out lamp. More advanced starters would "trip out" in this situation and not attempt another start until manually reset.
Newer lamp and ballast designs (known as "rapid start" lamps) provide true filament windings within the ballast; these rapidly and continuously warm the filaments/cathodes using low-voltage ac. Unfortunately, there is no inductive voltage surge produced so the lamps must usually be mounted near a grounded (earthed) reflector to allow the glow discharge to propagate through the tube and initiate the arc discharge. Electronic ballasts often revert to a style in-between the preheat and rapid-start styles: a capacitor or other electronic circuit may join the two filaments, providing a conduction path that preheats the filaments but which is subsequently shorted out by the arc discharge. Generally this capacitor also forms, together with the inductor that provides current limiting in normal operation, a resonant circuit increasing the voltage across the lamp so that it can easily start. Some electronic ballasts use soft start, the output AC frequency is started above the resonance frequency of the output circuit of the ballast, and after the filaments are heated, the frequency is rapidly decreased. If the frequency approaches the resonant frequency of the ballast, the output voltage will increase so much that the lamp will ignite. If the lamp does not ignite an electronic circuit stops the operation of the ballast.
Fluorescent light bulbs come in many shapes and sizes. An increasingly popular one is the compact fluorescent light bulb (CF). Many compact fluorescent lamps integrate the auxiliary electronics into the base of the lamp allowing them to then screw into a regular light bulb socket.
Unfortunately, many people find the color spectrum produced by some fluorescent lighting to be harsh and displeasing. It is common for a healthy person to appear with a sickly bluish skin tone under fluorescent lighting, and many pigments have a slightly different color when viewed under fluorescent light versus incandescent. This is mainly the case with fluorescent lamps containing the older halophosphate type phosphors (chemical formula Ca5(PO4)3(F,Cl):Sb3+,Mn2+), usually labeled as "cool white". The bad color reproduction is due to the fact that this phosphor mainly emits yellow and blue light, and relatively little green and red. To the eye, this mixture looks white, but light reflected from surfaces has a distorted color. More expensive fluorescent lamps use a triphosphor mixture, based on europium and terbium ions, that have emission bands that are more evenly distributed over the spectrum of visible light and hence lead to more natural color reproduction.
In the US, Residential use of fluorescent lighting remains low (generally limited to kitchens, basements, hallways and other areas), but schools and businesses find the cost savings of fluorescents to be significant and only rarely use incandescent lights. Typical lighting arrangements may include fluorescent tubes sending different tints of white, in order to provide good color reproduction. In other countries, Residential use of fluorescent lighting varies depending on the price of energy and the enviromental concerns of the local population as well as the acceptability of the light output.
Because they contain mercury, a toxic material, in quantities of a few milligrams per unit, in many areas throughout the world government regulations require that fluorescent bulbs must be properly disposed of. This generally applies only to large commercial buildings which produce many waste bulbs, though restrictions vary widely.
Fluorescent Bulb Identification
Bulbs are typically identified by a code such as F##T##, where F is for fluorescent, the first number indicates the power in watts (or strangely, length in inches in very long bulbs), the T indicates that the shape of the lamp is tubular, and the last number is diameter in eighths of an inch. Typical diameters are T12 (1½" or 38mm) for residential bulbs with old magnetic ballasts, T8 (1" or 25mm) for commercial energy-saving bulbs with electronic ballasts, and T5 (5/8" or 16mm) for very small bulbs which may even operate from a battery-powered device. High-output bulbs are brighter and draw more electrical current, have different ends on the pins so they cannot be used in the wrong fixture or with the wrong bulb, and are labeled F##T12HO, or F##T12VHO for very high output.
Blacklights, sun lamps, and germicidal lamps
Blacklights are a subset of fluorescent lamps that are used to provide long-wave ultraviolet light (at about 360nm wavelength). They are built in the same fashion as conventional fluorescent lamps but the glass tube is coated with a phosphor that converts the short-wave UV within the tube to long-wave UV rather than to visible light.
Most blacklights (so-called "BLB" or "BlackLight-Blue" lamps) are also made from more-expensive deep blue glass rather than clear glass. The deep blue glass filters out most of the visible colors of light directly emitted by the mercury vapor discharge, producing proportionally more UV light and less visible light so your blacklight posters look better. The blacklight lamps used in bug zappers does not require this refinement so it is usually omitted in the interest of low cost.
Finally, germicidal lamps contain no phosphor at all and their tubes are made of fused quartz that is transparent to the short-wave UV directly emitted by the mercury discharge. The UV emitted by these tubes will kill germs, ionize oxygen to ozone, and cause eye and skin damage. Besides their uses to kill germs and create ozone, they are sometimes used by geologists to identify certain species of minerals by the color of their fluorescence. When used in this fashion, they are fitted with filters in the same way as Blacklight-Blue lamps are; the filter passes the short-wave UV and blocks the visible light produced by the mercury discharge.
If you live in a dry cold climate with lots of static electricity, try this: Put on your best static gathering socks and take hold of a short flourescent tube. Then shuffle about on the carpet to gather a robust static charge. Now discharge by gently touching the lamp electrodes to anything electrically grounded. Instead of the usual little spark the entire tube will flash as the electrons course (painlessly) out of your body. This also applies with Van de Graaf generators ; simply touch the light to the sphere or touch the sphere while holding the light. Warning: this may produce a rather "jolty" shock.
Alternatively, if you happen to have a Tesla coil handy, you can fully illuminate the fluorescent lamp at quite a distance from the Tesla coil simply by holding the detached lamp in your hand and possibly touching one of its terminals.
If you live near high voltage power lines you might try standing underneath them at night while holding a fluorescent tube. The strong electric field created by power lines will cause a very small (harmless) current flow through the tube and it should give off at least a feeble glow. Obviously you should never do this during stormy weather and no attempt should ever be made to get closer to the lines using, for instance, a ladder.
A new type of fluorescent lamp, called induction lamp, is available since 1990. The most common form has the shape of an incandescent lamp. There is no electrical connection going through the glass of the bulb, as with an incandescent lamp, or a conventional fluorescent lamp. From the bottom side of the glass bulb, a tube is protruding to the inside of the bulb. An antenna, mostly called power coupler is placed in this tube. This antenna consists of a tubular ferrite core on which a coil is wound. The coil receives electric power from a special electronic ballast, that generates a relative high frequency between 2.5 and 3.0 MHz. A special resonance circuit in the ballast produces an initial high voltage on the antenna to start the gas discharge, thereafter the voltage is reduced to the normal running level. The system can be seen as a transformer, the power coupler forming the primary coil, and the gas discharge arc in the bulb forming the one turn secondary coil and the load. In all other conventional lamps the electrodes are the life limiting parts. Since the induction lamp has no electrodes, it can have a very long service life. For induction lamp systems with separate ballast, the service life can be as long as 100 000 hours (11.4 years continuous operation), for induction lamps with integrated ballast this is 15 000 to 30 000 hours. High quality electronic circuits are needed for the ballast to attain such a long service life. The lamps have special application areas: situations where replacement costs are high.
- NASA: The Fluorescent Lamp: A plasma you can use
- How Stuff Works: Are fluorescent bulbs really more efficient than normal light bulbs?
- How Stuff Works: How Fluorescent Lamps Work
- The Lighting Design Lab: Should I Turn Off Fluorescent Lighting When Leaving A Room?
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