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Light

This article sheds some light on the concepts of "luminous flux" and "illuminance", qualifying and quantifying the difference between light and dark.

Luminous flux, a measure of light quantity, in the SI system is measured in lumens. Luminous flux differs from radiant flux a.k.a. power because only wavelengths that are visible to the human eye are taken into account. The amount of radiation per wavelength varies with the temperature of the light source: the higher the temperature, the more the balance shifts towards higher wavelengths.
Laser light has a very narrow spectrum, but normal light a very wide one. This causes interference, making normal light beams spread out while they travel from their source. This spreads the energy and also the brightness over an ever increasing area, reducing illuminance. In the SI system, illuminance is measured in luxes. 1 lux = 1 lumen / square meter.

sunlight The primary source of light on Earth is the Sun. It has a very high luminous flux, though only a small part of that reaches our planet because we are 150 million kilometers away. Because our eyes are attuned to sunlight, a high proportion of the solar radiance falls into the visible spectrum and the ratio of luminous flux to radiant flux is high. By the time that sunlight reaches the Earth, its illuminance is about 128,000 lux. Part of that light is absorbed by the atmosphere, so that by the time it reaches sea level, illuminance is down to 100,000 lux. Of course at higher altitudes the difference is less.
The numbers above are for a clear sky, with the sun in the zenith. Latitude, time of the day and season and weather tend to decrease these numbers significantly. At 52° latitude, sunlight is 50% less on average. In midwinter this loss increases to about 90%; in midsummer it drops to only 20%. These numbers are for the time of the day when the sun is at its highest; earlier and later on the day they are lower and at nighttime of course zero.
Clouds and fog dampen sunlight too. Light cloud cover reduces it by about 1/3; heavy clouds by 2/3; dark thunderstorms by 9/10.
So for example on a cloudy December morning in London illuminance may be as low as 5,000 lux.

artificial lamps At night, but also during daylight hours, we are often surrounded by artificial lights. Because these operate at lower temperatures than the Sun, which is 6000° Kelvin at its surface, their ratio of luminous flux to radiant flux is lower. Much of their energy is radiated not as visible light, but infared, which is usually experienced as heat. The efficiency of lamps varies a lot with type. Incandescent light bulbs typically achieve 10 - 20 lumens / watt; compact fluorescent bulbs 50 - 70; light-emitting diodes (LED) 70 - 90, up to 150. Over many types power consumption varies, but luminous flux is quite constant, ranging from 200 to 1000 lumens for most commercial lamps. The illuminance of lamps also depends on their distance. For example, at 2 meters distance a lamp has to illuminate a sphere with a surface area of 4πr² = 50.3 square meters, so that its illuminance is decreased by a factor of 50 to 4 - 20 lux. But usually much of this light is reflected by walls, so effective illuminance is significantly higher. A typical living room is lighted with 50 lux; corridors 100 - 150; large rooms 100 - 150; offices 300 - 500; workplaces that require bright light up to 2000.

At night, there is but little natural light. The light of a full moon is about 1 lux at the time that it reaches the surface of the Earth. Of course the luminous flux is less for other phases of the Moon. Starlight is far dimmer: around 0.0001 lux. Again latitude and weather can dim this extraterrestial light further.