Daylight

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 outdoors during the daytime. This includes direct sunlight, diffuse sky radiation, and (often) both of these reflected from the Earth and terrestrial objects. Sunlight scattered or reflected from objects in outer space (that is, beyond the Earth's atmosphere) is generally not considered daylight. Thus, moonlight is never considered daylight, despite being "indirect sunlight". Daytime is the period of time each day when daylight occurs.

Contents

Definition

Daylight is present at a particular location, to some degree, whenever the sun is above the horizon at that location. (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 such as a solar eclipse or very high levels of atmospheric smoke (See New England's Dark Day), dust,[1] or volcanic ash.[2]

Daylight intensity in different conditions

Artificial image showing a nightfall over Europe and Africa. The solar terminator is shown for UTC July 5, 2005 18.45.00
Illuminance Example
120,000 lux Brightest sunlight
110,000 lux Bright sunlight
20,000 lux Shade illuminated by entire clear blue sky, midday
10,000 - 25,000 lux Typical overcast day, midday
<200 lux Extreme of darkest storm clouds, midday
400 lux Sunrise or sunset on a clear day (ambient illumination).
40 lux Fully overcast, sunset/sunrise
<1 lux Extreme of darkest storm clouds, sunset/rise

For comparison, nighttime illuminance levels are:

Illuminance Example
<1 lux Moonlight[3]
0.25 lux Full Moon on a clear night[4][5]
0.01 lux Quarter Moon
0.002 lux Starlight clear moonless night sky including airglow[4]
0.0002 lux Starlight clear moonless night sky excluding airglow[4]
0.00014 lux Venus at brightest[4]
0.0001 lux Starlight overcast moonless night sky[4]

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

Effects

Daylight in January. At the polar regions there are extreme differences in the amount of daylight in summer and winter.

Daylight is widely accepted to have a positive psychological effect on the human being[citation needed], and consequently more cases of mental health problems are registered during the winter months than during the summer months due to the shortened periods of daylight[citation needed]. Cases of depression specifically linked to limited daylight are referred to as seasonal affective disorder.

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 films like Marianne, night scenes have to be shot during the daylight hours and digitally altered later.

See also

Notes

External links


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Look at other dictionaries:

  • daylight — ► NOUN 1) the natural light of the day. 2) dawn. 3) visible distance between one person or thing and another. 4) (the (living) daylights) life: he beat the living daylights out of them. ● see daylight Cf. ↑see daylight …   English terms dictionary

  • daylight — [dā′līt΄] n. 1. the light of day; sunlight 2. dawn; daybreak 3. full understanding or knowledge of something hidden or obscure 4. the approaching end of a task or an ordeal [to see daylight ahead] 5. [pl.] [Old Slang] Slang former the eyes 6 …   English World dictionary

  • Daylight — Day light ( l[imac]t), n. 1. The light of day as opposed to the darkness of night; the light of the sun, as opposed to that of the moon or to artificial light. [1913 Webster] 2. pl. The eyes. [Prov. Eng.] Wright. [1913 Webster] …   The Collaborative International Dictionary of English

  • Daylight — Daylight. См. Просвет. (Источник: «Металлы и сплавы. Справочник.» Под редакцией Ю.П. Солнцева; НПО Профессионал , НПО Мир и семья ; Санкт Петербург, 2003 г.) …   Словарь металлургических терминов

  • daylight — c.1300 (as two words from mid 12c., daies liht), from DAY (Cf. day) + LIGHT (Cf. light) (n.); its figurative sense of clearly visible open space between two things (1820) has been used in references to boats in a race, U.S. football running backs …   Etymology dictionary

  • daylight — [n] light part of 24 hours aurora, dawn, day, daybreak, daytime, during the day, light, light of day, sunlight, sunrise, sunshine; concept 810 Ant. darkness, evening, night, sunset …   New thesaurus

  • daylight — noun ADJECTIVE ▪ broad, full ▪ He was robbed in broad daylight. ▪ bright ▪ natural ▪ I prefer to work in natural daylight …   Collocations dictionary

  • Daylight — Filmdaten Deutscher Titel Daylight …   Deutsch Wikipedia

  • daylight — day|light [ˈdeılaıt] n [U] 1.) the light produced by the sun during the day in daylight ▪ They re shy animals and don t often come out in daylight. ▪ The park is open to the public during daylight hours . ▪ If possible, it s better to work in… …   Dictionary of contemporary English

  • daylight — [[t]de͟ɪlaɪt[/t]] 1) N UNCOUNT Daylight is the natural light that there is during the day, before it gets dark. It was still daylight but all the cars had their headlights on... Lack of daylight can make people feel depressed. Ant: night 2) N… …   English dictionary

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