First, they are fluorescent bulbs. That means that they produce light in a very different fashion from Thomas Edison's lil' bulbs: Incandescents make light from electrical currents heating a metal (tungsten alloy) filament to white-hot.
Instead, a Compact Fluorescent bulb consists of two stages: the opaque white ceramic base with the Edison mount contains lots of electronics, and the bulb contains gas, a phosphor coating, and two electrodes. The electronics part is actually a miniaturized ballasted AC-AC converter supply. The gas is harmless enough, but it has mercury, a required part of the process of making the light. This fact has ramifications which may be hard for the uninitiated to realize, so I'll make them plain here.
- Phosphors are not good for you. They are more not good for you than Lead oxides.
- Fluorescents produce light that your eyes don't like as much as light from incandescent filaments. Why is this so? Because the gas in the CFL tube emits invisible light, which is used to pump the phosphors. The phosphors emit numerous (5-7 or so) specific wavelengths of light, rather than a smooth spectrum, like the sun and heated metals (like the tungsten alloy used in incandescents.) Those lines "look" like white to our eyes, but they do it by beating on your eye's sensors with two or three wavelengths in the vicinity of each sensor (Cone), rather than a broader spectral set of wavelengths. And they also use the wall power almost directly, which means the generated light varies as the wall power does, at 60 or 120 cycles per second. Many people can detect light like that with their eyes, and it is very annoying.*
- The way CFL's make light is a lot like how neon signs make light. The difference is mercury. The amount of mercury has been reduced from the original fluorescent tubes of our youth, but it's still there, and when you throw away a CFL or break it, there's mercury with the phosphors to be cleaned up.
- Those electronic ballasts are more complicated than the ballasts for straight, large tubes: the care that goes into CFL electronics is notorious.
- In addition to marginal designs, the electronics in CFL bulbs is limited by cost, and failures are usually due to cheap parts.
- CFLs with electronic failure are easily repaired, if you know the failed part and can get a replacement, although it's fiddly: the problem is that the designs are kept secret by the manufacturer, and no one wants to bother with the extra work required to fix them.
To light a CFL, an electrical plasma is induced in a low-pressure gas (such as Neon) to produce photons (light). The light's wavelength is determined by the makeup of the gas in the tube. A beautiful and well-written explanation, accompanied by images of spectra and lamps using the gasses adorn the wikipedia.org entry for "Gas-Discharge lamp".
Fluorescent lamps start with a low-pressure Nobel gas: Neon, Argon, Krypton or Xenon. (Radon is not used, because it is rare, and also radioactive.) A small amount of mercury is used in the tube, and the starting process vaporizes it so it can mix with the gas. The plasma formed when the electrons flow through the gas excites the gas and mercury vapor to produce ultra-violet (and some green) light. The ultra-violet light is mostly invisible to your eyes, so to provide more visible light, rare-earth phosphors, which coat the inside of the glass, accept the UV photons and emit the energy in other wavelengths.
The base of the CFL contains a line-power (120VAC)-to-DC converter. This powers an oscillator which produces a large voltage which drives the plasma formation. Once the plasma is formed, the circuitry changes to simply maintain the arc. As long as the power is on, the arc continues to excite the plasma, providing those initial photons. The phosphors are "passive": they don't require anything more than the photons from the plasma in order to make light. That keeps the outside of the tube fairly cool... but the extra work required to make the plasma and maintain it creates a lot of heat: this is trapped in that small white base, and shortens the life of the electrical components. If those components are cheap, it shortens the life of the bulb, considerably, over a similar set up that was vented or actively cooled.
The photons that the plasma produces are "discrete" wavelengths. That means they are identifiable by their wavelength, and don't have a large spread of numbers. (Can I make that any more opaque? I thought not. OK, slight rabbit trail: The color of light relates to wavelength. A "pure" color, like green, may be a range of wavelengths, all happening at the same time. In such a case, we'd identify the wavelengths that make the color as something like "500-550nm" (a band of wavelengths which fill the space between 500 nanometer-long and 550 nanometer-long wavelengths). We might say "525nm green", but in reality, that particular wavelength might appear quite rarely. But from a mercury plasma, we can say "546.1nm" for the green line, and 435.8nm for the blue line: they are pretty exact! And between those lines might be other lines, but not bands. An incandescence produces bands, while fluorescence produces lines. The sun's light is continuous, with all the spaces filled with active wavelengths, but a fluorescent bulb's light is not: there are specific lines with big gaps between.)
The phosphors are chosen to respond to the discrete wavelengths that the mercury vapor produces, and to emit new colors in tight lines which your eye will combine to make "white" or "cool white" or "warm white" (etc.) But it is important to remember that you are not looking at a spread of wavelengths like the sun produces, but a handful of discrete lines. You can see this by looking at the reflection of fluorescent light on a DVD, compared to the sun's light on the same surface: the tracks split the light, analogous to a prism, into component colors. Fluorescent reflections split into discrete colors, the sun provides a wash of colors, gradually changing from one to another.
*That 60-cycle-per-second variation is also pronounced in a single LED: it's the nature of the beast. A diode passes current in one direction only, so only the positive half of the cycle produces light. Now it is possible to wire two LEDs butt-to-face, and then each one produces light for each half cycle, but there's a dead spot when the incoming voltage is between the turn-on-voltage of one LED and the turn-on-voltage of the other. That means that there are sixty very short periods of zero light output, compared to the CFL's longer period of darkness, which is smoothed over by the Phosphors slow response...if they are formulated properly. And, in fact (and this footnote is being written in 2023), LED lights are producing much smoother (no dark-dropouts) output using phosphors themselves, and are producing much less UV output than they did when this article was first written.
