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A Fluorescent lighting system consists of two or three main components: (1) The fluorescent lamp, (2) The Ballast, and (3) the Starter system. Depending on the particular fluorescent lighting system, the starter may be a replaceable component, a starter may not be required, or the starter function may be integrated into the ballast. The starting function may also rely on the physical design of the fixture. Each of these components is discussed in detail in Section 2.
The basic concept behind a fluorescent lamp is that a flow of electrical current occurs between two metal conductors placed in a glass tube, a process also known as arcing. That current flow passes through the gases in the tube (argon and a small amount of mercury in a gaseous phase) and excites the atoms of gas. The excited atoms emit photons, some of which are vibrating at a frequency known as ultraviolet light. The ultraviolet light strikes a phosphor coating on the inside of the glass. The phosphor responds to the ultraviolet light by producing a bright visible light.
That all sounds simple enough, but by itself, a fluorescent lamp won't do anything. For a fluorescent lamp to start working, the potential of the electricity provided to the electrical conductors (called cathodes*) inside the lamp must be greater than the initial electrical resistance of the gas in the lamp so that the electricity may begin arcing through the gas. There are two ways to overcome this initial electrical resistance: (1) Lower the electrical resistance of the gas in the lamp, or (2) temporarily raise the electrical potential supplied to the lamp to a level greater than the resistance of the gas, so that arcing may begin. The Starter (or if absent, the Ballast) creates either or both of these conditions to start the lamp. In fact, up to half of the wiring in some fluorescent fixtures is used only while starting the lamps.
There are three different systems used in starting a traditional fluorescent lamp. They are: (1) Pre-Heat, (2) Rapid Start, and (3) Instant Start. Each starting method is described in later sections.
* A cathode is an electrical conductor, usually a wire or plate inside each end of the glass tube that an electrical current can be applied to, similar to the cathode found in a radio vacuum tube (called "valves" in some countries).
Technically, all gas discharge lamps have a cathode and an anode. Electrons always depart the cathode and impact the anode after traveling through the gas of the lamp, completing the electrical circuit. However, when alternating current is used, the cathode and anode switch roles many times each second (60 times in most western hemisphere countries, 50 times per second in most other places). When alternating current is used, the electrical conductor at both ends of the tube are both called cathodes.
As it turns out, all fluorescent lamps utilize a glass tube (even the compact screw-in fluorescent lamps have a small tube or tubes in there somewhere (it may be covered by a plastic housing) and the tube is actually what makes these devices be fluorescent lamps and not some other form of lighting, such as a near relative known as mercury vapor lighting. When the electrical connectors or other fittings are attached to the tube to make the complete assembly, that finished assembly is now called a lamp. This is also true in incandescent lighting, where a bulb (or globe) and screw base are combined to make a lamp.
Despite all that, the general public is so used to calling incandescent lamps by the simpler name "light bulbs", some manufacturers have given up on trying to enforce the distinction, and some will refer to fluorescent lamps as fluorescent bulbs in their publications and packaging.
The term "Bottle" is even older, and typically is not used to describe any fluorescent lamp today. The term "bottle" now seems to be reserved for use in devices made of glass containing gas where light output is not the functional goal of the device. For example, the glow-bulb used in starters is sometimes referred to as the "starter bottle".
The term "Globe" is sometimes to describe the glass envelope used in incandescent, mercury vapor and the various types of sodium lamps. Use of the "Globe" term dates back to Thomas Edisons original carbon filament incandescent lamps, but in modern times, it typically only refers to the outer glass of a lamp with two glass envelopes, one inside the other.
For example, a incandescent halogen lamp has an inner tube where the filament is, and surrounding it is a protective outer globe. As with fluorescent lamps, once all the parts are assembled into a unit, it is called a lamp. "Globe" is not used in any situation when referring to a fluorescent lamp.
A complete assembly of fluorescent lamps and all components of fluorescent fixtures is known collectively as a Fluorescent Lighting System.
In addition to Neon, Argon (another Noble gas) is the most commonly-used gas used in gas discharge lighting. Argon is used because it is a source of ultraviolet light, as well as producing visible light with a deep-blue color. In fluorescent lamps and some "neon" lighting, mercury is also added to argon gas, and the combination generates large amounts of ultraviolet light. That ultraviolet light can in turn be used to excite phosphors coating the inside of the tube, and the phosphors emit visible light. A wide variety of visible light colors can be produced by using different phosphors, sometimes in conjunction with colored glass tubing. Although the glass of the tube only blocks some ultraviolet light, most is unable to pass through the layers of phosphor in the inside of the tube.
Frequently, a small amount of argon gas is added to neon lamps. This combination has a lower break-down voltage than pure neon. The break-down voltage is the voltage level needed to overcome the electrical resistance of the gas in the lamp and start the current flow through the gas. Adding this small amount of argon (as little as .01%) does not significantly change the color of the light produced by the lamp.
Gas discharge lighting (including those lamps using neon, argon, krypton and other gases or combinations) are called "cold cathode" devices. This is because the cathodes, metal conductors on the inside of each end of the tube, are not heated to encourage electrons to travel through the gas. Instead, a voltage greater than the total resistance of the gas of all the tubes in the circuit is typically used. By comparison an electron tube (such as a television picture tube or old radio tubes) uses a heated filament as the cathode. When heated, a cathode gives up electrons more easily and more consistently.
Gas discharge lighting systems use thousands of volts to create the arc through the gas in the glass tubing. The more twists and turns in the tubing, the higher the voltage must be to travel the same distance if the tubing was in a straight line. A typical eight foot long argon-mercury vapor tube with a phosphor that produces green light will require between 4,000 and 5,000 volts to start and operate, but consume only use a few milliamperes of current while operating. Because of the low current used, the light output of "neon" lighting is minimal compared to fluorescent lighting.
Fluorescent lighting systems use far more current to obtain more light. It also operates at lower (and safer) voltages. For example, an eight-foot fluorescent lamp initially lights when around 600 volts is supplied to the cathodes, but then the voltage immediately goes down. While the lamp is operating, the voltage drop across the lamp is around 125 volts, and the lamps consume about 75 watts each, sometimes as much as 25 times the amount of power used by a gas discharge lamp of the same length.
A very close relative of the fluorescent lamp is the mercury vapor lamp. The mercury vapor lamp also uses a small tube or globe containing argon and mercury, but it is only an inch or two in length. Most mercury vapor lamps use an initial high voltage to start the electrical arc through the argon and mercury, heating the mercury until it boils and becomes a gas. Some lamp models use an incandescent filament to speed the heating process. In such lamps, a temperature-sensing mechanism (usually a measurement of current flow or elapsed time) built into the lamp decides when to end the warm-up process.
The standard low-end mercury vapor lamp has no phosphor coating at all. Normally, that would mean that both the visible light and the harmful ultraviolet light produced would escape from the lamp. However, a second glass globe surrounds the inner one. The inner one is where the argon and mercury gas reside, while the outer globe is filled with another gas, usually nitrogen. This gas absorbs most of the harmful ultraviolet radiation coming from the inner bulb (much like the earths upper atmosphere absorbs ultraviolet light from the sun), so that only the visible light produced by the lamp can escape.
Because the visible light from a standard mercury vapor lamp is a harsh bright blue light and suitable to only a few applications (street, security and other outdoor lighting are the bulk of the uses), some special mercury vapor lamp designs place a phosphor coating on the inside of the outer bulb of glass, and fill the outer globe with gasses that don't absorb ultraviolet light. As with a fluorescent lamp, the ultraviolet light is able to strike the phosphor and this creates whatever color light the phosphor produces, most commonly a purpleish white. Such lamps are very similar to fluorescent lighting, employing many of the same techniques. Phosphor-coated mercury vapor lamps are commonly used in warehouses, sports stadiums, some large stores and other areas.
Like fluorescent lamps, mercury vapor lamps also use a ballast to prevent the lamp from consuming more and more current as it gets hotter and hotter. The ballast is usually located in the fixture, although for safety reasons, highway lighting systems sometimes place the ballast in the lamp pole base so that if the pole is struck, the heavy ballast won't fall the forty feet or more and strike anybody.
One annoying artifact of mercury vapor lighting is the fact that it takes several minutes for the lamps to start. In fact, typically the lamps must be cool before they will begin to start, so if already-operating lamps are turned off or there is a brief electrical outage, it may take five to ten minutes for the lamps to restart. A common example of this is when football games have special lighting for the half-time show and turn off the main stadium lighting, it may take ten minutes for the stadium lamps to warm-up and become bright enough to resume the game. Broadcasts of professional games usually run a lot of commercials following half-time shows while waiting for the field to become bright enough for the television cameras to operate properly.
Another outdoor lighting system is known as sodium or sodium-halogen. These are hybrid gas discharge systems, but are more distant relatives of fluorescent lighting than mercury vapor lighting is. Sodium lighting falls into two classes: low pressure and high pressure. Low pressure sodium lamps produce a yellow-green light and are less-common today, while high-pressure sodium-halogen produces an orange light.
Both types of sodium lighting uses the element sodium, which is heated by a current flow until it becomes luminescent. In some ways, this is similar to the Lime lighting system used in theaters in the last half of the 19th century. Lime lighting heated a chunk of limestone by passing three types of burning gasses over it until the limestone glowed bright yellow. The light produced was then focused through lenses and mirrors and directed onto the stage. The phrase "in the limelight" comes from the use of this system. Not only was the limelight system complicated, it also tended to burn-down theaters.
It should be explained that in sodium-halogen or incandescent-halogen lamps, the halogen gas does not produce any of the light. Instead, halogen gas is used to increase the life expectancy of the lamp by helping to keep gas atoms from entering the lamp and shortening the lamps life. (Helium and Hydrogen atoms can pass through glass.) The word "halogen" is frequently used in a generic way to describe a variety of gasses that are used to displace other gases, including the Halon gases used for fire-supression. Lamps containing halogen invariably contain two globes or bulbs, one inside the other, and the contents of each bulb is sealed from the other.
The use of halogen allows incandescent lamps to be operated at a higher filament temperature, which means more of the energy leaving the filament will be visible light rather than being below the visible spectrum, energy that is just perceived as heat, without significantly reducing the operating life of the lamp.
By modern standards, both types of sodium lighting are unsuitable for almost all indoor lighting, although sodium halogen lighting has been used in warehouses and briefly as a classroom lighting system in a few pilot programs in schools in the 1970s and 1980s. Most of these school systems have since been replaced with fixtures and lamps that produce a better quality light.
| Color Name |
Neon |
Sodium Halogen |
Sodium (Low Pressure) |
Argon |
Argon plus Mercury Vapor |
Argon plus Mercury Vapor With Phosphor |
| Dominant visible-light Wavelengths (in Nanometers) | ~640nm | ~620nm | ~550nm | ~460nm | ~420nm | Mix, depends on phosphors selected |
Section 2: The Fluorescent Lamp, Ballast and Starter (HTML) [NEXT]
Section 3: Fluorescent Lamp Colors, Shapes and Sizes (HTML)
Section 8: Fluorescent Lighting Energy Savings and Product Comparisons (HTML)
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