Fluorescent Lamps

Lets start with this not new but mysterious thing.
 We find fluorescent lamps everywhere around ourselves - in homes, offices , streets . But still its a mystery for most of us that what's going on inside these glowing beauties and why are they better than those traditional incandescent bulbs.

Background :
 As we know,Light is a form of energy that can be released by an atom. It is made up of many small particle-like packets that have energy and momentum but no mass. These particles, called light photons, are the most basic units of light. Atoms release light photons when their electrons become excited. When an atom gains or loses energy, the change is expressed by the movement of electrons. When something passes energy on to an atom -- heat, for example -- an electron may be temporarily boosted to a higher orbital (farther away from the nucleus).The electron only holds this position for a tiny fraction of a second; almost immediately, it is drawn back toward the nucleus, to its original orbital. As it returns to its original orbital,

the electron releases the extra energy in the form of a photon, in some cases a light photon.

The wavelength of the emitted light depends on how much energy is released, which depends on the particular position of the electron. Consequently, different sorts of atoms will release different sorts of light photons. In other words, the color of the light is determined by what kind of atom is excited.

This is the basic mechanism at work in nearly all light sources. The main difference between these sources is the process of exciting the atoms. In an incandescent light source, such as an ordinary light bulb or gas lamp, atoms are excited by heat; in a light stick, atoms are excited by a chemical reaction. Fluorescent lamps have one of the most elaborate systems for exciting atoms.

Construction and Working :

The central element in a fluorescent lamp is a sealed glass tube. The tube contains a small bit of mercury and an inert gas, typically argon, kept under very low pressure. The tube also contains a phosphor powder, coated along the inside of the glass. The tube has two electrodes, one at each end, which are wired to an electrical circuit. The electrical circuit, which we'll examine later, is hooked up to an alternating current (AC)

supply.

When you turn the lamp on, the current flows through the electrical circuit to the electrodes. There is a considerable voltage across the electrodes, so electrons will migrate through the gas from one end of the tube to the other. This energy changes some of the mercury in the tube from a liquid to a gas. As electrons and charged atoms move through the tube, some of them will collide with the gaseous mercury atoms.These collisions excite the atoms, bumping electrons up to higher energy levels. When the electrons return to their original energy level, they release light photons.

As we saw in the last section, the wavelength of a photon is determined by the particular electron arrangement in the atom. The electrons in mercury atoms are arranged in such a way that they mostly release light photons in the ultraviolet wavelength range. Our eyes don't register ultraviolet photons, so this sort of light needs to be converted into visible light to illuminate the lamp.This is where the tube's phosphor powder coating comes in. Phosphors are substances that give off light when they are exposed to light. When a photon hits a phosphor atom, one of the phosphor's electrons jumps to a higher energy level and the atom heats up.

When the electron falls back to its normal level, it releases energy in the form of another photon. This photon has less energy than the original photon, because some energy was lost as heat. In a fluorescent lamp, the emitted light is in the visible spectrum -- the phosphor gives off white light we can see. Manufacturers can vary the color of the light by using different combinations of phosphors.

When the lamp first turns on, the path of least resistance is through the bypass circuit,and across the starter switch. In this circuit, the current passes through the electrodes on both ends of the tube. These electrodes are simple filaments, like you would find in an incandescent light bulb. When the current runs through the bypass circuit, electricity heats up the filaments. This boils off electrons from the metal surface, sending them into the gas tube, ionizing the gas.

At the same time, the electrical current sets off an interesting sequence of events in the starter switch. The conventional starter switch is a small discharge bulb, containing neon or some other gas. The bulb has two electrodes positioned right next to each other. When electricity is initially passed through the bypass circuit, an electrical arc (essentially, a flow of charged particles) jumps between these electrodes to make a

connection. This arc lights the bulb in the same way a larger arc lights a fluorescent bulb.One of the electrodes is a bimetallic strip that bends when it is heated. The small amount of heat from the lit bulb bends the bimetallic strip so it makes contact with the other electrode. With the two electrodes touching each other, the current doesn't need to jump as an arc anymore. Consequently, there are no charged particles flowing through the gas, and the light goes out. Without the heat from the light, the bimetallic strip cools,

bending away from the other electrode. This opens the circuit. Inside the casing of a conventional fluorescent starter there is a small gas discharge lamp.

By the time this happens, the filaments have already ionized the gas in the fluorescent tube, creating an electrically conductive medium. The tube just needs a voltage kick across the electrodes to establish an electrical arc. This kick is provided by the lamp's ballast, a special sort of transformer wired into the circuit.

When the current flows through the bypass circuit, it establishes a magnetic field in part of the ballast. This magnetic field is maintained by the flowing current. When the starter switch is opened, the current is briefly cut off from the ballast. The magnetic field collapses, which creates a sudden jump in current -- the ballast releases its stored energy.

The ballast, starter switch and fluorescent bulb are all wired together in a simple circuit.

This surge in current helps build the initial voltage needed to establish the electrical arc through the gas. Instead of flowing through the bypass circuit and jumping across the gap in the starter switch, the electrical current flows through the tube. The free electrons collide with the atoms, knocking loose other electrons, which creates ions. The result is a plasma, a gas composed largely of ions and free electrons, all moving freely. This creates a path for an electrical current.The impact of flying electrons keeps the two filaments warm, so they continue to emit new electrons into the plasma. As long as there is AC current, and the filaments aren't worn out, current will continue to flow through the tube.

The problem with this sort of lamp is it takes a few seconds for it to light up. These days, most fluorescent lamps are designed to light up almost instantly.

Comparison with conventional incandescent lamp :

Conventional incandescent light bulbs also emit a good bit of ultraviolet light, but they do not convert any of it to visible light. Consequently, a lot of the energy used to power an incandescent lamp is wasted. A fluorescent lamp puts this invisible light to work, and so is more efficient. Incandescent lamps also lose more energy through heat emission than do fluorescent lamps. Overall, a typical fluorescent lamp is four to six times more efficient than an incandescent lamp. People generally use incandescent lights in the home,however, since they emit a "warmer" light -- a light with more red and less blue.