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.
World map showing the areas of the Earth receiving daylight around 13:00 UTC, April 2nd.
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  or volcanic ash.
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
||Shade illuminated by entire clear blue sky, midday
|10,000 - 25,000 lux
||Typical overcast day, midday
||Extreme of darkest storm clouds, midday
||Sunrise or sunset on a clear day (ambient illumination).
||Fully overcast, sunset/sunrise
||Extreme of darkest storm clouds, sunset/rise
For comparison, nighttime illuminance levels are:
||Full Moon on a clear night
||Starlight clear moonless night sky including airglow
||Starlight clear moonless night sky excluding airglow
||Venus at brightest
||Starlight overcast moonless night sky
For a table of approximate daylight intensity in the Solar System, see sunlight.
Daylight is widely accepted to have a positive psychological effect on the human being, 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. Cases of depression specifically linked to limited daylight are referred to as seasonal affective disorder.
Daylight in January. At the polar regions there are extreme differences in the amount of daylight in summer and winter.
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.
- Daylight saving time
- Daytime (astronomy)
- Right to light
- Day length
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