Understanding Lighting Lingo

Forget the days of referring to light bulbs as "40 watt", "60 watt" or "100 watt". The new nomenclature focuses on light output in “lumens”.
Light Output/Lumens: Technically, a lumen is a unit of luminous flux derived by measuring the time rate of flow of light. As a point of reference, a traditional 40-watt incandescent light bulb produces a light output of 500 lumens.

Energy Used in Watts: Different types of light bulbs use vastly different amounts of energy to produce the same lumens, so an important bit of information is the energy used in watts, i.e. how many units of active electric power are used by a particular light bulb.

Lumens Per Watt/Efficacy: This measurement expresses the total light output of a light source divided by the total power input. The higher the efficacy rating of a light bulb, the less it will cost you to use it. Calculating a light bulb's efficacy is a simple mathematical process using the information now required on the front of a light bulb package - lumens divided by energy used in watts equals lumens per watt, or efficacy.

For example, the traditional 40 watt incandescent light bulb that produces 500 lumens has an efficacy of 12.5 (500 lumens = 40 watts = 12.5 lumens per watt). The 450 lumens LED light bulb has an efficacy of 56.25 (450 lumens = 8 watts = 56.25 lumens per watt); and the 450-lumens CFL light bulb has an efficacy of 50 (450 lumens = 9 watts = 50 lumens per watt).

Life Hours: Feel like you're always replacing light bulbs? You'll have more information now that the life hours must be labeled on the front of every light bulb package. The life hours are the rated average life of a light bulb as determined by engineering testing and probability analysis.

For example, a traditional 40watt incandescent, 500 lumens light bulb typically provides 1,000 life hours, whereas a 450 lumens LED typically provides 25,000 life hours, and a 450 lumens CFL typically provides 8,000 life hours.

Color Accuracy/Color Rendering Index: Known as CRI, the color rendering index is a description of the effect of a light source on the color appearance of objects being illuminated. CRI is expressed on a scale of 1-100, with a CRI of 100 being the maximum possible. On a good-better-best scale, a good CRI is 60-79, better CRI is 80-89, and best CRI is 90-100.

For example, incandescent light bulbs typically are 100 CRI (no matter what the light output or lumens rating), while CFL light bulbs typically are 82-88 CRI, and LED light bulbs typically are 65-85 CRI.

Light Color/Correlated Color Temperature: Known as CCT, the correlated color temperature describes the light's appearance in terms of its perceived warmth or coolness. CCT is expressed as a temperature measured on the Kelvin temperature scale, where a low CCT (4,000 Kelvin and lower) indicates a warm light, while a higher CCT (4,000 Kelvin and higher) indicates a cool light. Although it seems counter-intuitive for a higher temperature to correlate with cooler light, it is more understandable when picturing a piece of iron that glows red hot at a lower temperature than when it becomes blue hot; the human eye perceives red as warm and blue as cool.