A fluorescent lamp consists of a sealed glass tube. The tube contains a small amount of mercury and an inert gas, like argon, kept under very low pressure. In these electric-discharge lamps, a fluorescing coating on the glass—called phosphor powder—transforms some of the ultraviolet energy generated into light. Fluorescent lamps also require a ballast to start and maintain their operation.

Early fluorescent lamps were sometimes criticized as not producing enough warm colors, making them appear as too white or uncomplimentary to skin tones. A cool white fluorescent lamp has a CRI of 62. But today there are lamps available with CRIs of 80 and above that simulate natural day lighting and incandescent light. They also are available in a variety of CCTs: 2900 to 7000 (about CRI and CCT).

The "T" designation for fluorescent lamps stands for tubular—the shape of the lamp. The number after the "T" gives the diameter of the lamp in eighths of an inch. The T8 lamp—available straight or U-shaped—has become the standard for new construction. It also commonly serves as a retrofit replacement for 40-watt T12 lamps, improving efficacy, CRI, and efficiency.

In some cases, T10 and T9 lamps offer advantages over both T12 and T8 lamps, including higher efficacy, higher CRI values, a wider selection of CCTs, and compatibility with several ballast types.

Then there are T5FT fluorescent lamps. These lamps produce maximum light output at higher ambient temperatures than those that are linear or U-shaped.

Linear fluorescent lamps often are less expensive than compact fluorescent lamps. They can also produce more light, are easier to dim, and last longer.

Cold cathode fluorescent lamps are one of the latest technological advances in fluorescent technology. The "cold" in cold cathode means there is no heating filament in the lamp to heat up the gas. This makes them more efficient. Also, since there's no filament to break, they're ideal for use in rough service environments where a regular lamp may fail. They are often used as backlights in LCD monitors. They are used in exit signs too.

Compact fluorescent lamps (CFLs) are small-diameter fluorescent lamps folded for compactness. There are several styles of CFLs: two-, four-, and six-tube lamps, as well as circular lamps. Some CFLs have the tubes and ballast permanently connected. Others have separate tubes and ballasts.

Some CFLs feature a round adaptor, allowing them to screw into common electrical sockets and making them ideal replacements for incandescent lamps. They last up to 10 times longer than incandescent lamps, and they use about one-fourth the energy, producing 90% less heat.

However, typical 60-100 watt incandescent lamps are no more than 5.3-inches long, while standard CFLs are longer than 6 inches. Therefore, sub-CFLs have been developed. No more than 4.5 inches long, sub-CFLs fit into most incandescent fixtures.

Because of their energy efficiency, brightness, and low heat output, CFLs are also good replacements for halogen lamps in torchieres.

A standard incandescent lamp consists of a fairly large, thin, frosted glass envelope. Inside the glass is an inert gas such as argon and/or nitrogen. At the center of the lamp is a tungsten filament. Electricity heats the filament. The heated tungsten emits visible light in a process called incandescence.

Most standard light bulbs are incandescent lamps. They have a CRI of 100 and CCTs between 2600-3000, making them attractive lighting sources for many applications (about CRI and CCT). However, these bulbs are typically inefficient, converting only about 10% of the energy into light while transforming the rest into heat.

Another type of incandescent lamp is the halogen lamp. Halogen lamps also have a CRI of 100. But they're slightly more energy efficient, and they maintain their light output over time.

A halogen lamp also uses a tungsten filament. However, the filament is encased inside a much smaller quartz envelope. And the gas inside the envelope is from the halogen group. If the temperature is high enough, the halogen gas will combine with tungsten atoms as they evaporate and redeposit them on the filament. This recycling process lets the filament last a lot longer. In addition, it's now possible to run the filament hotter. This means you get more light per unit of energy. Because the quartz envelope is so close to the filament, it becomes about four times hotter than a standard incandescent lamp. As a result of this wasted heat energy, halogen lamps—popular in torchieres—really aren't too energy efficient. The exposed heat from halogen torchieres can also pose a serious fire risk, especially near flammable objects. Today, because of their inefficiency and risk, manufacturers have developed torchieres that can use other lamps, such as compact fluorescent lamps.

Lamps—commonly called light bulbs—produce light. When comparing lamps, it's important to understand the following performance characteristics:

• Color Rendering Index (CRI) — a measurement of a light source's accuracy in rendering different colors when compared to a reference light source with the same correlated color temperature. The highest attainable CRI is 100. Lamps with CRIs above 70 are typically used in office and living environments
• Correlated Color Temperature (CCT) — a measurement on the Kelvin (K) scale that indicates the warmth or coolness of a lamp's color appearance. The higher the color temperature, the cooler the color appearance. Typically, a CCT rating below 3200 K is considered warm, while a rating above 4000 K is considered cool.
• Efficacy — the ratio of light output (lumens) to input power (watts). The higher the efficacy, the more efficient the lamp.

Basically, you should try to use the most efficient lamp possible while maintaining the proper color rendering qualities required by a specific lighting application.