What Is Artificial Light?

Artificial light is optical radiation produced by engineered sources rather than by the sun, sky, or fire occurring naturally, and it includes both the illumination intended for a task and the stray emission, called light pollution, that escapes the area it was meant to serve. The subject covers the physics of light generation, the photometric and colorimetric quantities used to specify it, the luminaires and control systems that distribute it, and the effects of nighttime illumination on vision, ecology, and astronomy. It draws on optics, semiconductor device physics, radiometry, and human vision science, since the quantities that matter are defined relative to the response of the eye rather than to radiant power alone.

Electric lighting consumes a significant share of global electricity, so efficiency has driven the technology's history: from the incandescent filament, through low-pressure and high-pressure discharge lamps, to the semiconductor sources that now dominate new installations.

Sources and Technologies

Light is generated by three broad mechanisms. Incandescence heats a filament until it radiates in the visible band, an approach that wastes most of its input as infrared. Gas discharge excites atoms in a vapor, either directly as in low-pressure sodium and metal halide lamps, or indirectly through a phosphor that converts ultraviolet emission to visible light, as in fluorescent tubes. Electroluminescence in a semiconductor junction produces light in light-emitting diodes and organic light-emitting diodes, where a blue emitter combined with phosphors typically yields white output. The solid-state lighting program at the U.S. Department of Energy tracks the resulting device efficacy along with lighting quality factors such as spectrum, glare, and flicker that raw efficiency measures do not capture.

Photometry and Specification

Artificial light is specified in photometric units weighted by the standard photopic luminosity function: luminous flux in lumens, illuminance in lux, luminance in candelas per square meter, and luminous efficacy in lumens per watt. Color is described by chromaticity coordinates, correlated color temperature, and color rendering metrics that quantify how faithfully a source reproduces the appearance of surfaces compared with a reference illuminant. Spatial distribution is captured in photometric files that record intensity as a function of angle, which lighting designers use to predict illuminance across a road, a workplane, or a building facade. Spectral power distribution has gained importance as evidence accumulated that short-wavelength content affects circadian regulation independently of visual brightness.

Artificial Light at Night

Outdoor lighting that reaches the sky produces skyglow, and its extent has been mapped globally: an atlas of artificial night sky brightness found that the Milky Way is hidden from more than a third of humanity, including 60 percent of Europeans and nearly 80 percent of North Americans. Satellite radiometry has since shown that the artificially lit outdoor surface of Earth grew in both area and radiance through the period when efficient sources were being adopted, a rebound effect in which cheaper light led to more of it. Measurement itself is an active problem, since orbital sensors such as the visible infrared imaging radiometer suite are blind to the blue emission that LEDs added to the night sky, and a review of current approaches to monitoring light pollution covers the ground-based photometers, all-sky cameras, and citizen science networks used to fill that gap.

Applications

Artificial light is central to work in a range of fields, including:

• Architectural, roadway, and industrial lighting design
• Display, projection, and automotive lighting systems
• Horticultural lighting and controlled-environment agriculture
• Machine vision, photography, and optical instrumentation
• Circadian and occupational health research
• Astronomical site protection and dark sky policy