Lighting Glossary


A LampColor Temperature Comparison, via Yerocus of Wikimedia Commons
Color Rendering
Color Rendering Index (CRI)
Color Temperature
Incandescent Light
Line Voltage Bulb
Low Voltage Bulbs
Pin-Based Lamp
PAR lamps
Reflectorized Bulb
Screw-Based Bulb


A Lamp: The shape of a standard household bulb. The A stands for arbitrary.

Ballast: an electrical device for fluorescent light sources that is typically located in the fixture. The ballast provides the high initial current that starts the fluorescent lamp (called "striking an arc") and then delivers the correct current to maintain the arc in the lamp. Ballasts must match the lamp type and wattage to assure proper operation.

Color Rendering: The presence or absence of color in a light source, which determines how it renders light. Therefore, two sources of the same color temperature will render objects differently, depending on the spectrum of color that makes up each source. For example, colors appear different under a "cool white" fluorescent than they do outdoors at five in the afternoon, although the color temperature is similar.

Color Rendering Index (CRI): Measures how well a lamp illuminates an object's color tones. The index runs from 0 – 100 and a perfect 100 score is daylight. 

Color Temperature: Refers to how "warm" (reddish) or cool (bluish) a tint of white appears. Color temperature is a number that quantifies the appearance of light with the terms "warm" and "cool," which refer to subjective experiences like a warm flame or a cool winter sky. Color temperature is measured in degrees Kelvin. See the above photo for a comparison of color temperatures: the incandescent bulb is at 2700 K, the right-hand side fluorescent bulb is at 3500 K and the bottom fluorescent is 5500 K. 

Diffusion: Process of softening light by spreading it out, making it more comfortable. Tinted and gently textured materials (such as frosted glass or fabric shades) diffuse light.

Efficiency: Relates light output (lumens) to the electric power required to produce it (watts). Measured as lumens per watt. Light source efficiency is the best measure of the lifecycle cost.

Fluorescent: Light source that creates light through a 3-step process. First, the lamp generates an arc (electric current through gas). Then, the arc stimulates the gas inside the tube, which emits ultraviolet energy. This in turn stimulates a phosphor coating insure the lamp.

Fluorescent lights create light very efficiently and last a long time (up to 30,000 hours), thus dominating commercial, industrial and institutional applications. Due to improvements in color rendering, fluorescent sources are becoming more popular in homes. For tips on using fluorescent lighting, please see Fluorescent Lighting's Role in Eco-Friendly Lighting.

Foot-candles: A measurement of how much light falls on a surface. One lumen of light falling on one square foot equals a single foot-candle.

Halogen: Type of incandescent lighting offering longer life, higher efficiency and a whiter light. The lamp contains halogen gas that recycles tungsten, helping to keep the bulb clean and maintain lumen output. 

Incandescent Light: The most common source of light. Incandescent bulbs create illumination by causing a filament to glow by heating it with electricity. See our Incandescent Lighting article for more information.

LED (Light Emitting Diode): a form of solid state lighting that is at the leading edge of evolving lighting technology. Known for longevity and versatility, companies are also starting to produce A bulb-shaped LED bulbs for residential applications.

Line Voltage Bulb (or lamp): Operates directly off the household current at the North American standard of 120 volts. Most incandescent light sources for general and decorative lighting are line voltage. Available in 120 volt or 130 volt (the latter is typically used in commercial applications). You can dim line voltage bulbs lights with basic, inexpensive dimmers.

Low Voltage Bulbs (or lamps): Operates at a reduced voltage, most often at 12 volts. A transformer is required as part of the fixture or the current to change the 120 volt household current to the lower 12 volts. The chief benefit is the ability to make the light source very small and deliver a precise beam of light. Low voltage systems are most often used in accent and landscape lighting.

Lumens: The raw quantity of light given by a light source. All common electrical sources are rated in lumens. 

Pin-Based Lamp: Type of compact fluorescent light bulb in which the ballast is separated from the bulb and housed in the fixture.

PAR Lamps: Parabolic-shaped hard glass with an aluminized coating and a molded glass lens. Low voltage PAR lamps are popular for sophisticated accent lighting and landscape lighting. Their thick glass makes them ideal for outdoor use.

Reflectorized Lamps: Light bulbs that have a glass envelope shaped and finished to direct light into a beam. Primarily used for accent and task lighting, as well as throwing light from long distances, like in a high ceiling.

Screw-Based Lamps: Type of bulb with a screw-on base, compatible with most fixtures. Many compact fluorescent bulbs have screw bases and can fit light fixtures designed to accept incandescent bulbs.

Wattage: The power required to operate electric lighting is measured in watts. The wattage of a light bulb does not tell you how much light it produces. Instead, wattage refers to how much power the light will consume and therefore how expensive it will be to operate.

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