Fluorescent lamp
A fluorescent lamp is a type of electric lamp that excites argon and mercury vapor to create luminescence. Fluorescent lights are more efficient than conventional incandescent lamps because less of the energy is converted to heat. Instead, more is converted to usable light.
History
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 to be 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 1857, French physicist Henri Becquerel had the idea of a tube encapsulating fluorescent gas while leading investigations on fluorescence, phosphorescence and radioactivity.
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 gas 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 his 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 high pressure of the gas at the same time). Germer is today recognized as the inventor of fluorescent lamp.
General Electric later bought Germer's patent and under the impulsion of George Inman brought the fluorescent lamp to wide commercial use in 1938.
Principle
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 bombards the vapor with electrons 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 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 auxilliary electronics that regulates the current flow in the tube. This auxilliary 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".
Usage
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 auxilliary 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.
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.
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.
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 doesn't require this refinement so it is usually omitted in the interest of low cost.
Sun Lamps contain a different phosphor that emits more strongly in medium-wave UV, provoking a tanning response in human skin.
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.
Fluorescent Fun
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.
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.
External links
- 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