Author: Lou Bloomfield

What’s the difference between fluorescent, phosphorescent, and triboluminescent? – DS

Fluorescence is the prompt emission of light from an atom, molecule, or solid that has extra energy. For example, when some of the dyes used in modern swimwear and clothing are exposed to ultraviolet light, they absorb the light energy and promptly reemit part of that energy as visible light—typically brilliant greens and oranges. In contrast, phosphorescence is the delayed emission of light by an atom, molecule, or solid that has extra energy. Glow-in-the-dark objects are phosphorescent—they are able to store the extra energy they obtain during exposure to light for remarkably long times before they finally release that stored energy as visible light. Systems that exhibit phosphorescence rather than fluorescent are those that have special high-energy states that have enormous difficulty radiating away energy as light. Finally, triboluminescence is the emission of light from a surface experiencing sliding friction. Since sliding friction introduces energy into the surfaces that are sliding across one another, it’s possible for that energy to be emitted as light.

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.