Category: fluorescent lamps

Can the light from a fluorescent lamp be collimated into a beam of parallel rays?

While a converging lens or a concave mirror can always direct light from a bright source in a particular direction, the degree of collimation (the extent to which the rays become parallel) depends on how large the light source is. The smaller the light source, the better the collimation. Spotlights and movie projects use extremely bright, very small light sources to create their highly collimated beams. Since fluorescent lamps tend to be rather large and have modest surface brightnesses, I’m afraid that you would be disappointed with the best beam that you could create from that light. The ultimate collimated light source is a laser beam. In effect, the identical photons of light in a laser beam all originate from the same point in space, so that the collimated beam is as close to perfectly collimated as the nature of light waves will allow.

What is the difference between the magnetic and electric ballasts used in fluorescent lights?

Fluorescent lights work by sending an electric current through a vapor of mercury atoms in what is known as an electric discharge. Unfortunately, electric discharges are very unstable—they are hard to start and, once started, tend to draw more and more current until they overheat and damage their containers and power sources. Thus a fluorescent light needs some device to control the flow of current through its discharge. Since normal fluorescent lamps are powered by alternating current—that is, the current passing through the discharge stops briefly and then reverses direction 120 times each second in the United States and 100 times each second in many other countries (60 or 50 full cycles of reversal, over and back, each second respectively)—the current control device only needs to keep the current under control for about 1/120 of a second. After that the current will reverse and everything will start over.

Older style fluorescent lights use a magnetic ballast to control the current. This ballast consists essentially of a coil of wire around a core of iron. As current flows through the wire, it magnetizes the iron. Because energy is required to magnetize the iron, the presence of the iron inside the coil of wire slows down the current when it first appears in the wire by drawing energy out of that current. This effect, typical of devices known to scientists and engineers as “inductors”, prevents the current passing through the ballast and then through the discharge from increasing too rapidly once it starts. The magnetic ballast is able to slow the current rise through the fluorescent lamp long enough for the alternating current to begin reversing directions. In fact, as the current in the power line begins to reverse, the ballast begins to get rid of the energy stored in its magnetized core. This energy is used to keep the discharge going longer than it would on its own. The ballast thus smoothes out the discharge so that it stays under control and emits an almost steady amount of light.

Modern electronic ballasts still control the current through the discharge, but they use electronic components to achieve this control. Just as an electronic dimmer switch can control the current through an incandescent light bulb in order to adjust the bulb’s brightness, such electronic devices can control the current passing through the discharge in a fluorescent lamp to keep that current from growing dangerously large.

How does the pressure inside a mercury vapor lamp affect its spectral distribution, particularly as a source of ultraviolet light?

At low pressure, a mercury vapor lamp emits mostly short wavelength ultraviolet light at a wavelength of 254 nanometers. This light comes from the dominant atomic transition in the mercury atom, between its first excited state and its ground state. However, as the pressure and density of mercury atoms inside the lamp increase, two things happen. First, the high density of mercury atoms in the lamp makes it difficult for the 254-nanometer light to escape from the lamp. Each time a 254-nanometer photon (particle of light) is emitted by one mercury atom, a nearby mercury atom absorbs it. As a result, the 254-nanometer light becomes trapped inside the lamp and diminishes in brightness. With so much energy trapped inside the lamp, the mercury atoms are able to reach more highly excited states than at low density. Second, frequent collisions between the now highly excited mercury atoms allow those mercury atoms to emit wavelengths of light that are normally forbidden in the absence of collisions. The mercury atoms begin to emit light at a wide variety of wavelengths, including substantial amounts of visible light. That’s why a high-pressure mercury lamp is a brilliant source of visible light—most of the ultraviolet light is trapped by the mercury vapor and a substantial fraction of the light emerging from the lamp is visible light.

I’ve heard (from observations recorded in an office environment) that fluorescent light bulbs “emit” their energy at a certain frequency. If this frequency is at or below the rate at which our eyes blink/scan, this will cause eye fatigue and other health “problems.” What would be the best light system for the office environment?

Fluorescent light bulbs flicker rapidly because they operate directly from the alternating current in the power line. The light that you see is emitted by a coating of phosphors on the inside surface of the glass tube. These phosphors receive power as ultraviolet light and emit a good fraction of that power as visible light. The ultraviolet light comes from an electric discharge that takes place in the mercury vapor inside the tube. Since this electric discharge only functions while current is passing through the tube, it stops each time the current in the power line reverses. Thus, with each reversal of the power line, the discharge ceases, the ultraviolet light disappears, and the phosphors stop emitting visible light. So the tube flickers on and off. However, the alternating current in the United States reverses 120 times a second in order to complete 60 full cycles each second. The fluorescent lamps flicker 120 times a second. Even the very best computer monitors don’t refresh their images that frequently because our eyes just don’t respond to such rapid fluctuations in light intensity. In short, you can’t see this flicker with your eyes. If you get eye fatigue from fluorescent lamps, it’s the color or intensity of the light that’s bothering you, not the flicker. It’s just too fast to affect you.

We have some problems with a “fluorescent lamp igniter”, the device that turns on the lamp. I would like to know what is necessary for the fluorescent lamp to turn on?

A fluorescent lamp produces light as the result of an electric discharge that takes place inside the lamp tube. Electrons, emitted from hot filaments at each end of the tube, are pulled through the tube by electric fields and collide violently with mercury atoms inside the tube. These mercury atoms then emit ultraviolet light, which is converted to the visible light you see by the phosphor coating inside the glass tube.

To emit the electrons needed to sustain the discharge, the filaments at each end of the fluorescent tube must be heated. In the “preheat” style of fluorescent lamp, these filaments are heated red-hot for a few seconds by sending current directly through them. There are two pins at each end of the tube and current is sent to the filament through one pin and extracted through the other pin. Once the filaments are hot enough, the lamp turns off this current flow and tries to send current through the tube itself. If the discharge starts, the discharge is able to keep the filaments hot enough to emit electrons continuously. But if the discharge fails to start, the filaments are heated some more to try to release enough electrons to initiate the discharge.

The “igniter’s” job is to preheat the filaments for a few seconds and then to test the main discharge. If you see no red glow from the filaments at each end of the tube or you see no attempt by the igniter to start the main discharge, then the igniter should be replaced. It could also be that the tube itself is bad—that its filaments have burned out. If you see only one end of the tube glowing red or you see the igniter trying repeatedly to start the discharge, the tube is probably bad. I’d suggest replacing both the igniter and the tube and seeing if that fixes the problem. The only other component of the lamp, other than wiring, is the ballast—the device that controls the amount of current flowing through the discharge. It, too, could be bad.

What actually causes a fluorescent bulb to burn out?

The electrodes age, particularly during start up. The endless bombardment of charged particles gradually chips or “sputters” material off of the electrodes until they are no longer able to sustain a steady discharge. The final blow usually occurs when the heater filament breaks and the lamp cannot be started at all.

Are flood lights incandescent or fluorescent? Why are they so bright?

Most modern commercial and industrial floodlights are fluorescent lamps. Fluorescent lamps are so much more energy efficient than incandescent lamps that they quickly pay for their higher cost by saving electricity. Fluorescent lamps also last much longer than incandescent lamps, particularly if they are left on for long periods of time. Fluorescent lamps age most during their start-up cycles. Even around the house, fluorescent floodlights are becoming popular. Fluorescent lamps using about 150 W of power are as bright as incandescent lamps using 500 W. Both are bright, but one is much more energy efficient.

What happens when a fluorescent lamp flickers during start-up but doesn’t fully light?

Sustaining the discharge in a gas lamp requires the steady production of charged particles. Even if a lamp contains many negatively charged electrons and positively charged ions, these particles will quickly migrate to the electrodes once electric fields are present in the tube. If they don’t produce more charged particles as they fly across the tube, these charged particles will quickly disappear and the discharge will stop. It takes a critical number of charged particles in the tube to ensure a steady production of new charged particles. Thus the tube may not always start, even if it has a brief flicker of light.

As a kid, we’d shake streetlights. They’d get real bright and then explode. Then we’d run away. Why’d they get brighter and explode?

I’ll have to guess at this one. If the lamps you are talking about are mercury vapor, then they contain a reservoir or droplet of liquid mercury. If shaking these lamps would cause the mercury to flow out of the cooler reservoir and into hotter regions of the bulb, the mercury would boil and raise the pressure inside the lamp. The current passing through the lamp would increase and the bulb would get very bright. It would also get hotter and hotter, so its pressure would rise still further. Eventually the pressure would become so high that the bulb would explode.

What is the correct way to dispose of fluorescent lamps? Do they really have mercury inside them? Is the powder that covers the inside of them dangerous? Is there a simple way to get rid of a burned fluorescent lamp without pollution? – Augusto

While there is mercury in a fluorescent lamp, the amount of mercury is relatively small. There are only about 0.5 milligrams of mercury in each kilogram of lamp, or 0.5 parts per million. In fact, because fluorescent lamps use so much less energy than incandescent lamps, they actually reduce the amount of mercury introduced into our environment. That’s because fossil fuels contain mercury and burning fossil fuels to obtain energy releases substantial amounts of mercury into the environment. If you replace your incandescent lamps with fluorescent lamps, the power company will burn less fuel and release less mercury. That’s one reason to switch to fluorescent lamps, even if you must simply throw those lamps away when they burn out. Nonetheless, there are programs to recycle the mercury in fluorescent lamps. Last year, the University of Virginia recycled 31 miles of fluorescent lamps. They distilled the mercury out of the white phosphor powder on the inner walls of the tubes. Once the mercury has been removed from that powder, the powder is not hazardous. The university also recycled the glass. One last note: the mercury is an essential component of the fluorescent lamp—mercury atoms inside the tube are what create ultraviolet light that is then converted to visible light by the white phosphor powder that covers the inside of the tube.