Month: December 1969

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.

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.

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.

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.

Is having a black light in your room dangerous?

It depends on how bright the light is an how long you are exposed to it. If it is simply a normal lamp, coated with some filter that absorbs all the visible light, then it is no worse than having the visible light around. It will be a very dim ultraviolet light. However, if it is a serious ultraviolet lamp, emitting several watts or even tens of watts of ultraviolet light, then it is not a great toy. Long wavelength UV is less dangerous than short wavelength UV, but neither is great. Sunlight itself contains a far amount of both long and short ultraviolet. Fortunately for us, the small amount of ozone gas in the earth’s upper atmosphere absorbs much of the short wavelength UV. But long exposure to sunlight is dangerous, too.

Is a neon light actually a mercury/phosphor tube?

Most “neon” lamps are mercury lamps with a colored phosphor coating on the inside. However the true neon lamp (that special red glow) is really neon gas glowing directly. Take a close look at an advertising lamp that contains a variety of colors. The mercury/phosphor ones will seem to emit light from their frosted glass walls. You are seeing the phosphors glowing. But the real neon lamp will emit light from its inside. The glass will be clear and you will see the glow originate in the gas itself.

How does radiation trapping work?

Each atom has certain wavelengths of light that it is particularly capable of absorbing and emitting. For mercury, that special wavelength is about 254 nanometer (ultraviolet). For sodium, it is about 590 nanometer (orange-yellow). If you send a photon of the right 590 nanometer light at a sodium atom, there is a good chance that that atom will absorb it, hold it for a few billionths of a second, and then reemit it. The newly reemitted light will probably not be traveling in the same direction as before. Now if you have a dense gas of sodium vapor and send in your special photon of light, that photon will find itself bouncing from one sodium atom to another, like the metal ball in a huge pinball game. The photon will eventually emerge from the gas, but not before it has traveled a very long distance and spent a long time in the gas. It was “trapped” in the sodium vapor. This radiation trapping makes it hard for high-pressure gas discharges to emit their special wavelengths because those wavelengths of light become trapped in the gas.

How does an ultraviolet (“black light”) fluorescent tube work?

Some ultraviolet fluorescent tubes are simply the mercury discharge tubes (as in a normal fluorescent tube) but without any phosphor coating on the inside of the tube and with a quartz glass tube that transmits 254 nanometer light. In such a bulb, the 254-nanometer light emitted by mercury vapor in a discharge is emitted directly from the tube without being converted into visible light. A filter somewhere in the system absorbs the small amount of visible light emitted by a low-pressure mercury discharge. For the longer wavelength black light used in most applications, other gases that emit lots of 300-400 nanometer light are used. Again, these tubes have no phosphor coatings to convert the ultraviolet light into visible light. One other way to make longer wavelength black light is to use a mercury discharge but to coat the inside of the tube with a phosphor that fluoresces ultraviolet light between 300 and 400 nanometer.

How does a fluorescent light work?

A fluorescent lamp consists of a gas-filled glass tube with an electrode at each end. This lamp emits light when a current of electrons passes through it from one electrode to the other and excites mercury atoms in the tube’s vapor. The electrons are able to leave the electrodes because those electrodes are heated to high temperatures and an electric field, powered by the electric company, propels them through the tube. However, the light that the mercury atoms emit is actually in the ultraviolet, where it can’t be seen. To convert this ultraviolet light to visible light, the inside surface of the glass tube is coated with a fluorescent powder. When this fluorescent powder is exposed to ultraviolet light, it absorbs the light energy and reemits some of it as visible light, a process called “fluorescence.” The missing light energy is converted to thermal energy, making the tube slightly hot. By carefully selecting the fluorescent powders (called “phosphors”), the manufacturer of the light can tailor the light’s coloration. The most common phosphor mixtures these days are warm white, cool white, deluxe warm white, and deluxe cool white.

The only other significant component of the fluorescent lamp is its ballast. This device is needed to control the current flow through the tube. Gas discharges such as the one that occurs inside the lamp are notoriously unstable—they’re hard to start and, once they do start, tend to become too intense. To regulate the discharge, the ballast controls the amount of current flowing through the tube. In most older lamps, this control is done by an electromagnetic device called an inductor. An inductor opposes current changes and keeps a relatively constant current flowing through the tube (although that current does stop and reverse directions each time the power line current reverses directions — 120 times a second or 60 full cycles, over and back, in the United States). Some modern fluorescent lamps use electronic ballasts—sophisticated electronic controls that regulate current with the help of transistor-like components.

How do phosphors change the light from ultraviolet to visible?

They absorb the light and light energy by transferring electrons from low energy valence levels to high-energy conduction levels. These electrons wander about inside the phosphors briefly, losing energy as heat, and then fall back down to empty valence levels. Since they have lost some of their energy to heat, the light that they emit has less energy than the light they absorbed. Incoming ultraviolet light is converted to outgoing visible light.