Lighting Basics: Principles & Terms
It’s tough to choose the best energy-efficient lighting options for your home. To better understand the basic lighting principles and terms we have provided a guide below.
Light Quantity
Illumination
The distribution of light on a horizontal surface. The purpose of all lighting is to produce illumination.
Lumen
A measurement of light emitted by a lamp. As reference, a 100-watt incandescent lamp emits about 1600 lumens.
When you’re shopping for lightbulbs, compare lumens and use the Lighting Facts label to be sure you’re getting the amount of light, or level of brightness, you want.
Footcandle
A measurement of the intensity of illumination. A footcandle is the illumination produced by one lumen distributed over a 1-square-foot area. For most home and office work, 30–50 footcandles of illumination is sufficient. For detailed work, 200 footcandles of illumination or more allows more accuracy and less eyestrain. For simply finding one’s way around at night, 5–20 footcandles may be sufficient.
Energy Consumption
Efficacy
The ratio of light produced to energy consumed. It’s measured as the number of lumens produced divided by the rate of electricity consumption (lumens per watt).
Light Quality
Color temperature
The color of the light source. By convention, yellow-red colors (like the flames of a fire) are considered warm, and blue-green colors (like light from an overcast sky) are considered cool. Color temperature is measured in Kelvin (K) temperature. Confusingly, higher Kelvin temperatures (3600–5500 K) are what we consider cool and lower color temperatures (2700–3000 K) are considered warm.
Cool light is preferred for visual tasks because it produces higher contrast than warm light. Warm light is preferred for living spaces because it is more flattering to skin tones and clothing. A color temperature of 2700–3600 K is generally recommended for most indoor general and task lighting applications.
Color rendition
How colors appear when illuminated by a light source. Color rendition is generally considered to be a more important lighting quality than color temperature. Most objects are not a single color, but a combination of many colors. Light sources that are deficient in certain colors may change the apparent color of an object.
The Color Rendition Index (CRI) is a 1–100 scale that measures a light source’s ability to render colors the same way sunlight does. The top value of the CRI scale (100) is based on illumination by a 100-watt incandescent lightbulb. A light source with a CRI of 80 or higher is considered acceptable for most indoor residential applications.
(sourced from energy.gov)