In fluorescent lights, electrical current doesn’t run through a metal, but goes through gas confined in a tube. Ordinarily gas can’t carry an electrical current because all the electrons are tied to their atoms. So the first thing a fluorescent lamp does is inject some electrons into the tube, then speed them up with high voltage. That flow of electrons surges through the tube, hitting against other atoms.
Just as in a solid, some of the energy of the electronic current goes into heat, but the voltage puts the electrons into a narrow energy range, and because the gas is thin—not dense like a metal filament—the electrons stay in that narrow range as they fly along. The narrow range can be tuned to transfer just the right amount of energy to a specific atom, so very little is wasted.
In fluorescent bulbs the specific atom is mercury, and the “right amount” of energy puts a mercury atom into an excited state. That mercury electron returns to its lower energy state by emitting a photon. (That’s why it’s not so easy to dim a fluorescent bulb: turning down the voltage changes the speed of the electrons rushing through the gas, which messes up the way they’re “tuned” to the mercury atoms, which pretty much shuts the bulb down.) While in incandescent bulbs most of the photons are in the invisible infrared wavelengths, mercury atoms emit their photons in the invisible ultraviolet region of the spectrum. To turn those invisible photons into visible light the inside of a fluorescent tube is coated with phosphor.
Turning Invisible Light into Visible
A phosphor is a material that absorbs light at one wavelength and puts out light at a longer wavelength. The ultraviolet photons from the mercury atoms are highly energetic, which means they can deliver a lot of energy to the atoms in the phosphor material. Each invisible photon that gets absorbed by an atom in the phosphor ends up getting emitted as a visible light photon. Most of the mercury atoms floating around in the tube emit ultraviolet radiation at wavelengths of 254 and 185 nm, while visible light is at wavelengths between about 400 and 700 nm. (Wavelength is often measured in nanometers, abbreviated nm, with one nanometer being one-billionth of a meter.)
Shorter wavelengths have higher energy, so if one high energy ultraviolet photon is exchanged for one lower energy visible light photon, then some energy is getting lost along the way. In fact, about half the energy that goes into a fluorescent bulb is lost in this step—converted to heat or absorbed by the glass tube.
The wavelength range put out by a fluorescent light depends upon the composition of the phosphor. That is, the color they give out is determined by the composition of the phosphor. The familiar blue-white light associated with standard fluorescent tubes comes from a phosphor composed of calcium fluorophosphate activated by antimony and manganese. Whether that color combination strikes your fancy or not, it’s not blackbody radiation. That is, the color we see as a bluish white is made up of other colors put together to appear whitish.
Convenient Color Classification
In recent years many new phosphor combinations have been developed, to provide color options for different applications. Although it’s found wide acceptance in Asia, many Americans find the bluish-white light a little too harsh for most applications, but other colors are available. The newest fluorescents may have a “color temperature” printed on the label. The quoted color temperature represents the “closest” blackbody temperature that would put out about the same “white.” Paradoxically, the blue-white fluorescents are perceived as putting out “cool” white, even though the color temperature is higher, anywhere from about 4000 to 7000K. To mimic an incandescent bulb, a fluorescent lamp would have a color temperature of about 2800K.
The story for fluorescent bulbs gets even more complicated, because the gas inside the tube behaves differently before it’s turned on, when it’s just starting to emit light, and when it warms up. So a fluorescent light cannot just be turned on and left to its own devices. The current and voltage need to be controlled at each phase of operation. That means each bulb needs what’s called a ballast, an element to control the current and voltage.
Even though fluorescent bulbs are three to eight times more efficient than incandescent lighting, they’re not ideal light sources. Throw in the concerns about health and environmental effects of mercury, and there’s even more motivation to look at other technologies.
That’s where solid-state lighting comes in.
To complete the series on the physics of lighting, read about solid-state LED lighting in A New State of Lighting—Solid State Lighting from Light Emitting Diodes.
Or go back to the introduction, Not Your Grandpa's Bulb: The Physics of Lighting, or to Hot Bodies—How Incandescent Bulbs Work.
Sources
- Ronda, C. and Srivastava, A. (2003). Phosphors. The Electrochemical Society Interface. Retrieved from electrochem.org.
- Nave, R. (N.D.) Fluorescent Lighting. Retrieved from hyperphysics.phy-astr.gsu.edu.
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