To choose the best energy-efficient lighting options for your home, you
should understand basic lighting principles and terms.
- The distribution of light on a horizontal surface. The purpose of
all lighting is to produce illumination.
- A measurement of light emitted by a lamp. As reference, a 100-watt
incandescent lamp emits about 1750 lumens.
- 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.
- 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).
- 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 light bulb. A light
source with a CRI of 80 or higher is considered acceptable for most
indoor residential applications.
- The excessive brightness from a direct light source that makes it
difficult to see what one wishes to see. A bright object in front of
a dark background usually will cause glare. Bright lights reflecting
off a television or computer screen or even a printed page produces
glare. Intense light sources – such as bright incandescent lamps
– are likely to produce more direct glare than large fluorescent
lamps. However, glare is primarily the result of relative placement
of light sources and the objects being viewed.
- Ambient lighting
- Provides general illumination indoors for daily activities, and outdoors
for safety and security.
- Task lighting
- Facilitates particular tasks that require more light than is needed
for general illumination, such as under-counter kitchen lights, table
lamps, or bathroom mirror lights.
- Accent lighting
- Draws attention to special features or enhances the aesthetic qualities
of an indoor or outdoor environment.