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.

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.

Can you get a tan from an ultraviolet light bulb?

Yes. Tanning appears to be your skin’s response to chemical damaged caused by ultraviolet (high energy) light. Each photon of ultraviolet let carries enough energy to break a chemical bond in the molecules that make up your skin. Exposure to this light slowly rearranges the chemicals in your tissue. Some of the byproducts of this chemical rearrangement trigger a color change in your skin, a change we call “tanning”. Any source of ultraviolet light will cause this sequence of events and produce a tanning response. However, the different wavelengths of light have somewhat different effects on your skin. Long wavelength ultraviolet (between about 300 and 400 nanometers) seems to cause the least injury to cells while evoking the strongest tanning response. Short wavelength ultraviolet (between about 200 and 300 nanometers) does more injury to skin cells and causes more burning and cell death than tanning. However, all of these wavelengths have enough energy to damage DNA and other genetic information molecules so that all ultraviolet sources can cause cancer.

When the temperature is sub-zero (e.g., -40°), is it necessary to heat the electrodes or the gas or both for the tube to light? What is the optimum tube temperature with respect to efficiency?

Fluorescent lamps do not operate well in extreme cold. Below about 15° C (59° F), the density of mercury atoms in the tube’s vapor is too low to produce efficient light. While the tube also contains inert gases that allow it to start at almost any temperature, the scarcity of mercury atoms leads to a reduced light output. In any case, the electrodes must be heated to make them emit electrons to sustain the discharge.

The optimal internal temperature for a fluorescent lamp is about 60° C (140° F). The tube reaches this internal temperature when its outside is about 40° C (104° F). When the surrounding temperature exceeds 40° C, the tube begins to waste energy again because the density of mercury atoms in the vapor becomes too large.

Do fluorescent light fixtures emit magnetic fields? If so, would they be intense enough to affect diskette magnetic media?

While fluorescent light fixtures do emit magnetic fields, those fields are far too weak to affect magnetic media. Any electric current produces a magnetic field, even the current flowing through the gas inside a fluorescent tube. However, that field is so weak that it would be difficult to detect. Nearby iron or steel could respond to that weak magnetic field and intensify it, but the field would still be only barely noticeable. The only strongly magnetic component in a fluorescent fixture is its ballast coil. The ballast serves to stabilize the electric discharge in the lamp and relies on a magnetic field to store energy. However, the ballast is carefully shielded and most of its magnetic field remains inside it.

As for affecting diskette magnetic media, that’s extraordinarily unlikely. Even if you held a diskette against the ballast, I doubt it would cause any trouble. Modern magnetic recording media have such high coercivities (resistances to magnetization/demagnetizations) that they are only affected by extremely intense fields.