Glossary of Terms

The following is a list of basic lighting terms:

CANDELA: Unit of luminous intensity, demonstrating the intensity of a light source in a specific direction.

CANDELA DISTRIBUTION: A curve based on plotted coordinates that illustrates the luminous intensity of a lamp or luminaire in a plane through the light center.

COLOR RENDERING INDEX (CRI): A scale demonstrating the effect of a light source on the color appearance of an object when compared to the same object’s color appearance under a reference, or control light source. CRI is expressed on a scale of 1 to 100, where 100 indicates no color difference; a low CRI rating indicates that an object’s color will appear unnatural under the specific light source.

COLOR TEMPERATURE: A specification of the color appearance of a light source, where color is related to a reference source heated to a particular temperature, measure by thermal units called Kelvin. Color Temperature is often described as “warm”, “neutral”, and “cool”, with “warm” considered to be below 3500K, “neutral” between 3500 and 5000K, and “cool” above 5000K.

EFFICACY: A measurement comparing light output to energy consumption, typically measured by Lumens per Watt. For example, if a 50 watt light produces 5000 lumens, the efficacy is 100 lumens per watt.

FOOTCANDLE (FC): The English unit of measurement of the amount of light on a surface. One footcandle is equal to one lumen per square foot.

IES: Abbreviaton for Illuminating Engineering Society – IES prescribes various standards used to assess lighting products.

ILLUMINANCE: Describes the amount of light incident on a surface or plane. Illuminance is commonly called light level. It is expressed as lumens per square foot (footcandles), or lumens per square meter (lux).

LED: Abbreviation for light emitting diode. See FAQs for more information.

LUMEN: A measurement of light flow; A lumen rating measures the total light output of a light source.

LUX: Metric unit of measure for illuminance of a surface – One lux is equal to one lumen per square meter. A lumen is equal to one foot-candle falling on one square foot of area.

OPTICS: The components of a light fixture (reflectors, refractors, lenses, louvers, etc.) that control how much light and in which direction light is emitted.

PHOTOMETRIC: Measurement of the properties of light; photometric data is compiled into charts or spreadsheets and is used describe the illumination characteristics of a light source.

WATT (W): The unit for measuring electrical power. It defines the rate of energy consumed by an electrical device while it is operating.