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.
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, and volcanic ash.
|120,000 lux||Brightest sunlight|
|111,000 lux||Bright sunlight|
|20,000 lux||Shade illuminated by entire clear blue sky, midday|
|1,000 - 2,000 lux||Typical overcast day, midday|
|<200 lux||Extreme of thickest storm clouds, midday|
|400 lux||Sunrise or sunset on a clear day (ambient illumination)|
|40 lux||Fully overcast, sunset/sunrise|
|<1 lux||Extreme of thickest storm clouds, sunset/rise|
For comparison, nighttime illuminance levels are:
|<1 lux||Moonlight, clear night sky|
|0.25 lux||A full Moon, clear night sky|
|0.01 lux||A quarter Moon, clear night sky|
|0.002 lux||Starlight, clear moonless night sky, including airglow|
|0.0002 lux||Starlight, clear moonless night sky, excluding airglow|
|0.00014 lux||Venus at brightest, clear night sky|
|0.0001 lux||Starlight, overcast moonless night sky|
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.