Compact Fluorescent Lamps (CFLs) have been much in the news lately, especially with Wal-Mart’s decision to promote them. Many environmentalists would like to see incandescent light bulbs (the old-fashioned kind of light bulb with the glowing filament) banned outright, because they are quite inefficient — most of the electricity they consume produces heat, not light.
Occasionally mentioned, but mostly in a dismissive way, is that CFLs (or any fluorescent lamp, for that matter) produce light that is quite visibly different than that produced by incandescent bulbs, or by the sun. Personally, I find the light from most fluorescent bulbs to be unpleasant — harsh and color-distorting — though I have seen some fluorescent lighting whose light is less awful than most. What exactly is the difference in the light the various kinds of bulbs put out?
The difference is in the spectrum of light that the bulbs emit. The sun and incandescent bulbs both produce what scientists call “warm-body” light, by which they mean the light emitted when something gets really, really hot. Cooler warm-body light is reddish to our eye, such as the light emitted by a red-hot piece of metal. Hotter warm-body light is bluer to our eye, such as the light emitted by the sun. But in all cases, warm-body light emitters put out some light of every possible color — the difference in their apparent color is caused by the relative intensity of the various colors. So the red-hot piece of metal puts out a lot of red light, some orange light, a bit of yellow light, a small amount of green light, and very little blue or violet light. The sun, on the other hand, puts out a lot of every color — which is why it’s light appears white to our eyes.
CFLs (and all fluorescent light bulbs) work by a completely different phenomenon: fluorescent emission, which of course is how they get their name. The inside walls of CFLs are coated with phosphors — substances which emit light in the visible range of colors when they are illuminated by ultraviolet light. Unlike incandescent light bulbs, the inside of a CFL is not a vacuum — instead it contains (usually) mercury vapor at a very low pressure. There is so little mercury required for this that the bulbs are not dangerous individually. When CFLs are turned on, an arc of electricity flows through the mercury vapor, which then produces high intensity ultraviolet light (which we cannot see). This light strikes the phosphors, which then emit visible light.
But here’s the catch: phosphors emit light of a distinct color. Any given phosphor will emit light of a specific color — green, red, blue, etc. This is very, very different than warm-body light emission. Of course none of us want to light our homes with some pure colored light — so the CFL manufacturers combine multiple phosphors in every bulb to simulate whiter light. It is exactly what happens with stage lighting when spot lights of multiple colors are aimed at the same point, and the colors mix. For example, if a blue spot and a yellow spot are aimed at the same point, we perceive green lighting. With three colored spots (red, green, and blue, for example), we can simulate white light. However, our eyes are being “tricked", and this simulated white light is not the same as the white light from a warm-body emitter. The “harsh” look of such simulated white light comes from the fact that the individual colors have to be much brighter than that same color would be in white light from a warm-body emitter. The color distortion is caused by objects that reflect colors that are different than the colors actually in the simulated white light. With the simulated white light, such objects appear much darker and less colorful than they would under light from a warm-body emitter.
Scientists use an instrument called a spectrograph to measure the spectrum of light. Most of us don’t have access to a “real” spectrograph — but I’ll bet you’ve got many of them lying about your home. These are what I call “dual-purpose” spectrographs: not only can they be used as a spectrograph, but they will also play music or video. I’m talking about CDs or DVDs — the side with the music or video makes a very nice diffraction grating, which is the heart of a real spectrograph. I’m sure you’ve noticed the rainbow effect they produce — that’s the diffraction grating at work.
Grab a CD or DVD, and take it to a lamp that uses an incandescent light bulb (this includes halogen lamps, if you have them). Hold the CD or DVD about chest-height, while you’re standing, and experiment with different orientations until you see a strong rainbow of light. You won’t have much trouble doing this, I’m sure. Now note the nature of that rainbow — because you’re reflecting light from a warm-body emitter (that incandescent light bulb), you’ll see a broad rainbow with every possible color in it — very pretty, isn’t it?
Now take that CD or DVD over to a CFL lamp, and do the same thing. Whoa! Not the same thing at all! What you see now is bright reflected images of the lamp in each of the colors emitted by the phosphors it contains. Most CFLs use three phosphors: red, green, and violet. Better (whiter, “full spectrum") CFLs will use four, five, or even more phosphors. But in all cases, you will see something very different than you see with a warm-body emitter.
Here’s the bottom line: it is not possible to make a CFL that accurately simulates natural lighting from a warm-body emitter. Our eyes evolved to use natural lighting, so lighting from CFLs (or any fluorescent lamp) looks unnatural. The harshness and color distortion can be improved — by using more phosphors — but it cannot be made perfect.
So… If you like the energy savings of a CFL (and who wouldn’t?), and you’re willing to trade off lighting quality to get the savings, by all means go for the CFL. But I’m going for the halogen (a very hot, very white, warm-body emitter)…
