Vapor Lamps

Key Terms

ballast
Bohr model
energy levels
fluorescent lights
phosphorescence
photon
spontaneous emission

Introduction

Vapor lamps produce light when electricity (moving electrons) passes through a gas under low pressure.  It does not rely on "heat" to make light, ... it uses the atomic properties of the individual atoms to produce light.  Here is some science you need to understand:

The nature of light and atoms

You have been taught from a young age that an atom consists of a nucleus (made up of protons and neutrons) and the electrons orbit this nucleus.  This is true, however, the rules of quantum mechanics make the story a bit more complicated.  Here are some additional things you may not have already learned:

• Electrons circling the nucleus of an atom only orbit at certain "energy levels" or states.
   
  • Normally the electrons orbit at the lowest possible level called the "ground state".  This places the electrons as close to the nucleus as they can get.  When this happens the atom emits no radiation.
     
  • If an atom absorbs energy from its surroundings, one or more of the outer electrons will move to a higher orbit.
    • Heat is an easy way to "excite" an atom.
    • Electricity passing through the atoms can also excite them.
    • An atom can even absorb energy in light, which moves the electrons to higher orbits.
       
  • The atom loses energy when the electron moves back toward the ground state.  This is called spontaneous emission because it happens on its own.
    • The released energy is in the form of light (or other invisible electromagnetic radiation).
    • The atom releases a "particle of light" called a photon.
    • The bigger the jump (transition), the more energy the atom releases.
    • The color of the light emitted from the atom depends on the size of the transition.  The bigger the jump, the "bluer" the light.  A photon of blue light has more energy than a photon of red light.  At the same token, radio waves have the lowest photon energy, and gamma rays top the energy chart.
    • The exact equation is: where h is known as Planck's constant and c is the speed of light.  Notice that as wavelength decreases, photon energy increases.

The Bohr Model - the hydrogen atom

Niels Bohr solved this mystery of light and the atom.   We will use his hydrogen model to illustrate the idea since hydrogen is the simplest atom (one proton and one electron).  Below is an animation which depicts a crude representation of the energy levels the single electron is allowed to orbit in (black circles).  If left undisturbed, the electron orbits in the ground state ... the closest it can get to the nucleus.  If the atom absorbs energy from its environment, the electron moves to a higher orbit.  When it drops back down, the atom emits a photon of radiation.

The Hydrogen model (animation)

The energy levels are not uniform, like steps on a staircase.  Instead, they become progressively closer the farther they appear from the nucleus.  The transition from orbit 2 down to orbit 1 (ground state) is such a large energy transition that the photon released lies in the ultraviolet section of the electromagnetic spectrum.   There are only 3 downward transitions in the hydrogen atom that produce light visible to our eyes.  These are depicted in the diagram below.

To see this you would need to fill a tube with low pressure hydrogen gas and zap it with a source of high voltage electricity.  The electrons moving through the gas transfer some of their energy to the hydrogen atoms.  The atoms would then release this energy.  Since this is happening to millions of atoms at the same time, the net effect is an eerie orange looking color.  The next step is to pass this light through a prism to reveal the separate red, blue-green, and violet light coming from the gas.

If interested, click link 5.5.a to see a Java applet ... notice that the "bluer" the light ... the bigger the electron jump.

Vapor Lamps

Mercury vapor lamps are also known as fluorescent lights.  Although there are several different types of fluorescent light designs, we will just cover the typical one you may have in your home or office - the hot cathode type.

The Basics

Inside each tube is an inert gas (typically argon) and a tiny bit of mercury.  At each end of the tube are two electrical connections which (on the inside) look very much like the filament found in an incandescent light bulb.  However, the function of these filaments (called tungsten electrodes) is not to give off light, but rather, electrons ... which boil off the surface when heated.  Driven by voltages established at each end of the tube, these free electrons are compelled to move across the tube which interacts with the mercury vapor .... exciting those atoms as electrons are driven to higher orbits.  The mercury atoms then spontaneously release that energy in the form of photons in the UV portion of the spectrum.   These UV photons must pass through a phosphor coating on the inside of the bulb, which absorbs this energy and releases it in the form of many photons all within the visible portion of the spectrum.  The combined effect is white light.

It's not that simple ...

First, the free electrons that boil off the tungsten electrodes and start moving will interact with the argon gas and ionize it.  This, in turn, create the conditions found in lightning ... a discharge arc.  This is necessary because the mercury within the tube is in the liquid state and the discharge generates energy to turn the mercury into a vapor.  There are several ways to kick start this process and most involve a much higher start-up voltage.  Once an arc is established the voltage can drop to a lower level.  Since this current is driven by AC voltages, the current is constantly flipping direction.

.... but we don't want too much current.  Enter the ballast.

The need for ballast

Electrons moving through a vapor behave a bit different from electrons moving through a conducting solid.  In solids, the electrical resistance remains fairly constant.  However, in a vapor lamp things are more complicated.  To establish a conducting path when the fluorescent light is first turned on, heating elements emit electrons which quickly ionize some of the vapor.  As stated earlier, this is needed to establish a conducting path.  However, once established, the electrons start moving across the vapor, and they tend to ionize even more gas atoms.  As a result, the overall electrical resistance drops and, left unchecked, the amperage would jump to unsafe levels (as in a bolt of lightning).  You need a way to limit the current and that is what the ballast does.

One type of ballast (called a magnetic ballast) is simply a wire wrapped around a metal bar.  AC currents run through this device before running through the vapor lamp.  This particular type of ballast is no longer used because someone invented a better ballast in the form of an electronic ballast.  However, we can still discuss the magnetic ballast since it is fairly straight forward.  

A magnetic ballast is just the addition of an inductor in the circuit.  If you recall from an earlier section (unit 3 - induction), an inductor is really nothing more than a wire wrapped around a metal bar.  In this particular application, as AC current moves through the system, the "eddy currents" produced in the bar generate their own changing magnetic field ... which then induce a "back emf" or back current in the coil (remember the metal detector?).  The direction of this "back current" in the coil is in opposition to the current that first started all the action.  This tends to act like a "current brake" in the system and prevents the vapor lamp from going into this "run away" condition we first mentioned.  It makes sense that old timers call the ballast a "choke" ... because it chokes the current.  All vapor lamps require a ballast to run safely.

The magnetic ballast

Actually the ballast does more than just limit the current.  It also provides a boost voltage when the lamp is first turned on (to generate the discharge arc) as well as preheat the tungsten electrodes.  No ballast ... no vapor lamp.

Click link 5.5.b for a video

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