How Do Fluorescent Bulbs Produce Light and How Many Different Types Are There?

Fluorescent lights were invented for one purpose: to confuse you. I’m glad to see that they’re doing their job.

When an ordinary incandescent lightbulb burns out you can just screw in a new one with the help of a certain number of friends, depending on your vocation or ethnicity. But when a fluorescent light burns out you look at the tube to find out what kind to replace it with and you see markings that look something like “F20CW-T12.” If you replace it with the “F15W-A10” that you saw in the store, will it explode when you turn it on?

Cheer up. ‘Tis better to light a candle and read this book than to curse the darkness.

First, let’s decipher those hieroglyphics on the tube. They’re a secret code that divulges everything about the bulb. Not to you, the poor consumer, of course, but to the people who make and sell them, who apparently have a need to appear smarter than you are.

I’m going to tell you how the secret code works. (I suppose that now they’ll have to kill me.)

Any given fluorescent tube is either straight, U-shaped or circular in shape; it has a certain wattage; it gives off a certain color of light; and it has a certain diameter. The letters and numbers on the tube give this information in that order: shape, wattage, color, diameter. The only trouble is that you have to know how this information is coded.

For shape, it’s a U or a C for U-shaped or circular, and no letter at all if it’s straight. Then comes the wattage: 4, 5, 8, 13, 15, 20, 30, 40 or whatever. (The wattage is generally lower than for comparable light-producing incandescents, because fluorescent lighting is from two to four times more efficient.) Then comes the color code: W for white, CW for cool white, WW for warm white, plus abbreviations for other exotic colors that we needn’t bother with. Last comes the tube’s diameter, but it is given, would you believe?, in eighths of an inch: T8 means a tube that is eight-eighths of an inch in diameter, which any sane human being would call one inch. A T12 tube is twelve-eighths or one-and-a-half inches in diameter, and so on.

Pop Quiz: Describe the properties of an F40CWT10 fluorescent bulb. (Answer at the end of this section.)

Oh, I forgot to tell you: The codes always begin with an F for “fluorescent,” presumably to keep you from trying to screw them into an ordinary lamp socket. (How many idiots does it take to screw a fluorescent tube into an incandescent lamp socket?)

As an alert consumer, you may have noticed that you can’t replace an 18-inch-long tube with one that is 24 inches long. The manufacturers graciously give you enough credit to make that decision on your own, so you won’t find a length code on the tubes.

Okay, now. How do the things work?

You know that ordinary incandescent lamps, including halogen lamps, make light by electrically heating a filament to white heat. The outside of the lamp bulb can get up to temperatures of several hundred degrees. Fluorescent lamps work on an entirely different principle.

The fluorescent tube is filled with a small amount of inert gas (usually argon) plus a drop’s worth of mercury. At each end is a small filament that is heated by the electric current so that it emits electrons. (You don’t know why a hot filament emits electrons? Go stand in the corner.

The electrons emitted from the filaments fly through the gas in the tube to get from one filament to the other, and in the process they collide with mercury atoms, which have been vaporized by the filaments’ heat. The mercury atoms absorb the collision energy and spit it out again as light energy. But we can’t see that light because it’s in the ultraviolet region of wavelengths, so it has to be converted into light that humans can see. This is accomplished by that white coating on the inside of the tube. It consists of chemicals (calcium and strontium phosphates and silicates) that absorb ultraviolet light and re-emit it as visible light; this wavelength-shifting process is called fluorescence.

Fluorescent lamps are cooler than incandescent lamps because they have only those two little mildly heated filaments at the ends and the fluorescence process itself doesn’t produce any heat. But they’re hard to start, because the filaments’ electrons first have to blast their way through the gas in the entire length of the tube. That requires several hundred volts of push, but our household voltage is only 115 volts. So something has to provide an initial voltage kick to the electrons.

That’s what the starter does, or the ballast. And here’s where it gets really confusing, because there are several kinds of fluorescent lamp systems and circuits. Some have ballasts, those heavy little iron transformers, while others have starters, those little aluminum cans. And some have both. Fuhgeddaboudit. You don’t hafta know.

What to do when your fixture of unknown breed won’t light up? First, replace the tube with one that has identical code numbers on it. You can’t even substitute a different wattage, as you can with incandescent bulbs; that can cause dangerous overheating of the ballast, which was designed for the other wattage. The only freedom you have is to swap a cool white for a warm white or vice versa, or to substitute one of the many other “deluxe” colors. If your fixture has one of those little starter cans in it, you may as well replace that too; they’re cheap and they simply twist in and out of the socket.

If you’re still in the dark, both literally and figuratively, buy a whole new fixture.

Oh, and an F40CWT10 is a 40-watt, cool white, one-and-a-quarter-inch straight fluorescent tube.

About Karen Hill

Karen Hill is a freelance writer, editor, and columnist. Born in New York, her work has appeared in the Examiner, Yahoo News, Buzzfeed, among others.

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