Lighting, Illuminated: A Glossary of Key Terms.

We get a lot of questions (sometimes from each other) about what lumens, color rendering indexes and ballasts are. So, we got to thinking that you (and we) might find a glossary of lighting terms awfully handy. Remember, we're here to help. Got other questions? Send them our way.

Ballast: An electrical device used with fluorescent and other light bulbs (a.k.a. lamps) that supplies the voltage they need to start and run efficiently.

Base: The part of the light bulb (or lamp) that physically screws or plugs into a socket or lamp. Normal screw-in light bulbs have a "medium" base; chandeliers usually have a "candelabra" base.

Color Rendering Index (or CRI): A measurement of how accurately a lamp renders colors. Sunlight scores a perfect 100, but we're very happy with bulbs that have a CRI of 80 and up.

Color Temperature: A light source's color helps us describe it as "warm" (redder) or "cool" (bluer). Most folks consider light below 3200K to be "warm," while those over 4000K are "cool." Sunrise and sunset are about 1600K, candlelight is about 1800K, a cool white fluorescent bulb is 4100K and heavily overcast sky is 6500K. Our skin looks lovely in warm light, and pale in cooler light. If you're looking for the warmth of incadescent light, we recommend 2700K. Sometimes cooler temperatures are labelled "sunlight" or "bright white," which sound great. But they're probably not right for most household applications. The letter K stands for Kelvin (more on that later).

Compact Fluorescent Lamp (or CFL): Small fluorescent lamps with folded, spiral or bridged glass tubes and high color rendering (CRI). CFLs are both efficient and long-lived.

Dimmer: An electrical control that lets you increase or decrease a lamp's brightness. When used to dim incandescent and halogen lights, dimmers also increase the life and decreases the energy they use. CFL compatible dimmers make the most of dimmable CFLs. CFLs are about 5 to 10 times more efficient than incandescent lights.

Efficacy: Efficacy is the measure of light output in lumens per watt of energy consumed. The higher the better: The efficacy of a traditional incandescent bulb may be somewhere in the 15-20 range, while compact fluorescent bulbs and LEDs can range anywhere from 50-100, or higher.

Filament: A tungsten wire that glows white hot (or incandesces) when an electric current runs through it.

Four-Way Switch: A wall switch that lets three switches control one lighting device. Flip one four-way switch in a given circuit, and the light turns from on to off (or from off to on).

Halogen Lamp: A kind of incandescent bulb that contains halogen gases (think iodine, chlorine, bromine, and fluorine), which help the tungsten filament last longer. Touching the glass exterior with bare hands shortens their life; if you do, remove your fingerprints with denatured alcohol.

Incandescence: The warm (expensive, and energy burning) glow of a heated filament.

Incandescent Lamp: Thomas Edison's original bright idea. Light is produced when the metal filament inside the bulb is heated to the point of incandescence by an electric current.

Light emitting diode (or LED): A tiny, solid-state light source lit up by electrons moving through a semiconductor material. LEDs used to be synonymous with cold light because of their color temperature was often above 5000 Kelvin (K). Nowadays, LEDs with color temperatures ranging from 2600K to 3500K cast a warm glow. LEDs are about 20 times more efficient than incandescent lights.

Lumens: Lumens measure how much light a bulb gives off. The soft light of an traditional 60-watt bulb gives off 840 lumens; so does a 15W CFL.

Photometry: The measurement of visible light, based on how bright it seems to the human eye.

Three-Way Switch: A wall switch that allows two switches to control one light. Whenever you flip one of the three-way switches in a given circuit, the light changes from on to off, or back again.

Voltage: The flow of electricity, expressed in volts, measures the electrical pressure in a circuit.

Wattage: The amount of electrical power consumed by a lamp or light fixture is measured in watts (W). If you have a 4-Watt LED, or a 14W CFL, or a 100W incandescent bulb, they all give the same amount of light (see Lumens). Sometimes CFL and LED bulbs are described as "100W Equivalent;" that means that, using just 4W or 14W, they are as bright as your 100W incandescent.

Watt-Hour: If you leave your 4-Watt LED on for one hour, you use 4 Watt-hours of energy. It all adds up, and it's the measurement your electricity bill is based on. By that point, your Watt-hours are usually measured in the thousands, when they become known as kilowatt-hours, or kWh.