Definitions

electric lights

Electric light

Most of the industrialized world is lit by electric lights, which are used both at night and to provide additional light during the daytime. These lights are normally powered by the electric grid, but some run on local generators, and emergency generators serve as backups in hospitals and other locations where a loss of power could be catastrophic. Battery-powered lights, usually called "flashlights" or "torches", are used for portability and as backups when the main lights fail.

Types

Types of electric lighting include:

Different types of lights have vastly differing efficiencies.

Name optical spectrum nominal efficiency
(lm/W)
Lifetime (MTBF)
(hours)
Colour temperature
(kelvins)
Colour Color
rendering
index
Incandescent light bulb Continuous 12-17 1000-2500 2700 Warm white (yellowish) 100
Halogen lamp Continuous 16-23 3000-6000 3200 Warm white (yellowish) 100
Fluorescent lamp Mercury line + Phosphor 52-100 8000-20000 2700-5000* White (with a tinge of green) 15-85
Metal halide lamp quasi-Continuous 50-115 6000-20000 3000-4500 Cold White 65-93
Sulfur lamp Continuous 80-110 15000-20000 6000 Pale green 79
High pressure sodium broadband 55-140 10000-40000 1800-2200* Pinkish orange 0-70
Low pressure sodium narrow line 100-200 18000-20000 1800* Yellow, virtually no color rendering 0
*Color temperature is defined as the temperature of a black body emitting a similar spectrum; these spectra are quite different from those of black bodies.

The most efficient source of electric light is the low-pressure sodium lamp. It produces an almost monochromatic orange light, which severely distorts color perception. For this reason, it is generally reserved for outdoor public lighting usages. Low-pressure sodium lights are favoured for public lighting by astronomers, since the light pollution that they generate can be easily filtered, contrary to broadband or continuous spectra.

Vendors

Public lighting

The total amount of artificial light (especially from street light) is sufficient for cities to be easily visible at night from the air, and from space. This wasted light is the source of light pollution that burdens astronomers and others.

Human-made lights highlight particularly developed or populated areas of the Earth's surface, including the seaboards of Europe, the eastern United States, and Japan.

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