Daylight

World map showing the areas of the Earth receiving daylight around 13:00 UTC, April 2nd.

Daylight, or the light of day, is the combination of all direct and indirect sunlight during the daytime. This includes direct sunlight, diffuse sky radiation, and (often) both of these reflected by the Earth and terrestrial objects, like landforms and buildings. Sunlight scattered or reflected by objects in outer space (that is, beyond the Earth's atmosphere) is generally not considered daylight. Thus, daylight excludes moonlight, despite it being indirect sunlight. Daytime is the period of time each day when daylight occurs. Daylight happens because Earth rotates, and either side on which the Sun shines is considered daylight.

Definition

Daylight is present at a particular location, to some degree, whenever the Sun is above the local horizon. (This is true for slightly more than 50% of the Earth at any given time. For an explanation of why it is not exactly half, see here). However, the outdoor illuminance can vary from 120,000 lux for direct sunlight at noon, which may cause eye pain, to less than 5 lux for thick storm clouds with the Sun at the horizon (even <1 lux for the most extreme case), which may make shadows from distant street lights visible. It may be darker under unusual circumstances like a solar eclipse or very high levels of atmospheric particulates, which include smoke (see New England's Dark Day), dust,[1] and volcanic ash.[2]

Intensity in different conditions

IlluminanceExample
120,000 luxBrightest sunlight
111,000 luxBright sunlight
109,870 luxAM 1.5 global solar spectrum sunlight
20,000 luxShade illuminated by entire clear blue sky, midday
1,000 - 2,000 luxTypical overcast day, midday
<200 luxExtreme of thickest storm clouds, midday
400 luxSunrise or sunset on a clear day (ambient illumination)
40 luxFully overcast, sunset/sunrise
<1 luxExtreme of thickest storm clouds, sunset/rise

For comparison, nighttime illuminance levels are:

IlluminanceExample
<1 luxMoonlight,[3] clear night sky
0.25 luxA full Moon, clear night sky[4][5]
0.01 luxA quarter Moon, clear night sky
0.002 luxStarlight, clear moonless night sky, including airglow[4]
0.0002 luxStarlight, clear moonless night sky, excluding airglow[4]
0.00014 luxVenus at brightest,[4] clear night sky
0.0001 luxStarlight, overcast moonless night sky[4]

For a table of approximate daylight intensity in the Solar System, see sunlight.

Effects

Daylighting is lighting an indoor space with openings such as windows and skylights that allow daylight into the building. This type of lighting is chosen to save energy, to avoid hypothesized adverse health effects of over-illumination by artificial light, and also for aesthetics. The amount of daylight received into an indoor space or room is defined as a daylight factor, being the ratio between the measured internal and external light levels. Artificial lighting energy use can be reduced by simply installing fewer electric lights because daylight is present, or by dimming/switching electric lights automatically in response to the presence of daylight, a process known as daylight harvesting.

In recent years, work has taken place to recreate the effects of daylight artificially. This is however expensive in terms of both equipment and energy consumption and is applied almost exclusively in specialist areas such as filmmaking, where light of such intensity is required anyway. In some filmmaking locations, such as Sweden, there is too much light due to long summer days. As a result, in location films like Marianne, night scenes have to be shot during the daylight hours and digitally altered later.

See also

Notes

  1. ↑ Cox, Clifford. "Dust Bowl". perryton.com. Retrieved 2017-08-01.
  2. ↑ "Volcanic Ash Impacts & Mitigation". USGS. Retrieved 2017-08-01.
  3. ↑ Bunning, Erwin; Moser, Ilse (April 1969). "Interference of moonlight with the photoperiodic measurement of time by plants, and their adaptive reaction" (Scholar search). Proceedings of the National Academy of Sciences of the United States of America. 62 (4): 1018–22. Bibcode:1969PNAS...62.1018B. doi:10.1073/pnas.62.4.1018. PMC 223607. PMID 16591742. Retrieved 2006-11-10.
  4. 1 2 3 4 5 Schlyter, Paul (2006). "Radiometry and photometry in astronomy FAQ".
  5. ↑ "Petzl reference system for lighting performance". Archived from the original on 2008-06-20. Retrieved 2007-04-24.
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