(reprinted with Breefcase's permission from works written in reefs.org)



Fluorescent lamps are a type of gas discharge tube. A pair of electrodes, one at each end of the tube, are sealed inside along with a drop of mercury and some inert gases (usually argon or metal halides), at very low pressure (close to a vacuum). The inside of the tube is coated with a phosphor which produces visible light at the desired color when excited with the ultra-violet (UV) radiation produced by the gas plasma.

When the lamp is off, the mercury/gas mixture is non-conductive. When power is first applied, a high voltage (several hundred volts) is needed to initiate the gas plasma discharge. However, once this takes place, a much lower voltage is needed to maintain it.

The electric current passing through the low pressure gases emits quite a bit of UV (but not much visible light). The gas discharge's radiation is almost entirely Mercury radiation, even though the gas mixture is
mostly inert gas and generally only around something like 1 percent Mercury vapor. The internal phosphor coating very efficiently converts most of the UV to visible light.

The mix of the phosphor(s) inside the tube is used to tailor the light spectrum to the intended application. Thus full-spectrum bulbs and Actinics really differ only in the phosphor mix used to coat the inside of the tube.

So actually, the metal halide gases inside a fluorescent lamp never "condense" into a liquid or solid - they are all gases at normal room temperature and pressure, even when sealed in a partial vacuum tubular lamp.
(Don't mistake that white powder that comes from inside some fluorescent tubes when they get broken for "condensed gas" -- that's the phosphor that coated the inside of the tube. Don't breathe the small amount of toxic Mercury vapor released, either.)

The gases inside the tube do IONIZE when exposed to a particular electrical potential, and it does take an electrical ballast to "kick start" the ionization process. The reason some lamps need to cool a bit in order to be re-lit is that the ballast is designed to ionize the gas inside a COLD lamp, not a hot one still partially ionized.

All "gas discharge" light sources that I'm aware of require a ballast to run. Just as lead ballast gives stability to a boat, (or a balloon, or a diver), an electrical ballast controls the stable operation of a fluorescent light. Gas discharge lamps are "zero resistance" or "negative resistance" elements. As the gases inside the tube ionize, the resistance of the ionized plasma created inside the tube decreases. This will cause the resistance to approach zero while the current draw approaches infinity (theoretically - obviously, your wall outlet can't really provide infinite current).

No known metal electrode can survive the extreme amount of current that ionized gas can draw, so without a ballast to limit the current supplied, the electrodes at the end of the fluorescent tube would burn up even though the ionized plasma would survive.

The traditional means by which gases are excited enough to ionize to a glowing plasma is through the "tar" or "core and coil" ballast. I doubt that many modern core and coil ballasts still actually have any tar in them. The coil (copper wire wound around a metal core) steps up the incoming 110 volt, 60 Hertz voltage that comes out of your wall outlet (in America at least) to a voltage high enough to excite the gases.

A more modern means by which metal halide gas discharge lamps can be ionized to a plasma is with an electronic ballast, which works by increasing the frequency of the incoming voltage, rather than stepping up the voltage itself. In this approach, the 60 Hz line voltage is raised to about 20 thousand Hertz (kHz) or even higher. The use of such "radio frequency" (RF) energy to excite gases requires a more complicated ballast, which is really just a type of miniature RF transmitter. (Ice caps are this type of ballast.)

Electronic ballasts have several advantages over tar ballasts. First, to ionize gases to a plasma, high frequency waves require less energy than lower frequency waves, so less power is consumed.

Second, the heat generated by the coil of a tar ballast is eliminated, making for cooler operation of the ballast.

Third, the range of spectrums, or colors, of the light produced by bulbs fired by electronic RF ballasts is wider - tests have shown that Actinic bulbs burn "bluer" with electronic ballasts.

A possible final advantage is longer bulb life, since current draw at start-up is less which lengthens electrode life, and voltage is better regulated throughout the "burn" as the bulb heats, which prevents the electro-chemical breakdown of the color phosphors inside the tube and prevents (or delays) color-shifting of the phosphor.



Reefscapes.net 2002

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