Lighting Terminology & Definitions

When you’re trying to find the perfect light bulb or light fixture, you might come across some unfamiliar or confusing lighting terms. We’ll break down the definitions for you:

Ampere (amp) - the International System of Units (SI) base unit of electrical current. In other words, it measures electric current.

Candela - also called candlepower, candela is the measurement for describing luminous intensity. This measures the value of light intensity in a single direction from the light source.

Color Rendering Index (CRI) - Measures the ability of a light source to accurately render the color of an object. This rating ranges on a scale of 0-100. Lower numbers, between 0-55, are poor rendering. 60-85 means that the light source is good or acceptable at rendering the color. A rating of 90-100 means this light source is the most accurate at rendering color.

Correlated Color Temperature (CCT) - describes the color of a visible light source and is measured in Kelvin (K). It describes only the color of the light emitted by a light source and ranges on a scale. Common color temperatures used in residential applications range between 2200K-6500K. Lower temperatures are warmer, or more yellow, while higher temperatures are cooler, or bluer.

Efficacy - the amount of visible light output from a light compared to the amount of electrical power the light source consumes. This is measured in lumens per watt (lm/W).

Footcandle - a measurement of light intensity. A footcandle is one lumen per square foot. It measures the amount of light which falls on a surface that is one foot away from a single candle.

Kelvin (K) - the SI base unit of thermodynamic temperature. This scientific measurement is for the color of the light.

Lumen (lm) - the standard unit of luminous flux. Indicates the overall amount of visible light you receive from a light source.

Lumen depreciation - the decrease of lumen output from a light source over time.

Luminaire - a complete lighting unit, together with parts designed to distribute the light, to position and protect the lamps, and the parts and wiring necessary to connect the lamps to a power source.

Luminous flux - the measure of the total amount of visible light emitted by a light source. Measured in lumens (lm). It measures only the visible portion of the light and not any light, such as ultraviolet (UV) or infrared (IR) radiation, which are not visible to the human eye.

Lux - a measure of illuminance. It measures the total amount of light that falls on the surface. Lux indicates how bright a surface will be or the intensity of the light in an area. One lux equals one lumen per square meter.

Power factor - the ratio of working, or actual, power compared to apparent, or total, power.

Volt (V) - unit of electrical potential.

Voltage - the difference in electrical potential between two points, expressed in volts (V).

Watt (W) - a unit of power equal to one joule per second.

Wattage - measure of electrical power, expressed in watts (W).

The definitions of lumens, lux, and candela are all pretty similar. They all deal with the light or intensity of the light, but they all have a slightly different meaning.

Lumens measures the total amount of visible light in all directions, or the total amount of visible light produced by the light source.

Lux measures illuminance, which means it measures the amount of light output in a given area. It indicates the total amount of light that falls on a surface.

Candela measures the amount of light that a light source emits in a single direction.