The average horizontal illuminance Eh of daylight fluctuates over broad ranges. With a clouded sky at noon – depending on season, geographic location and the amount of clouding or mist – the illuminance is between 6.000 lx and 20.000 lx (see figure). With sunshine and a clear sky, it is even over 100.000 lx. By moonlight, it is around 0,1 lx. This dynamic of available light is perceived as normal and natural and makes people feel comfortable.
Daylight
In work places and common rooms, however, the illuminance levels installed serve to fulfil visual tasks and usually do not exceed 500 lx – without any significant dynamic in lighting levels, and around the clock, where required. The light colour of daylight also fluctuates on a broad scale. In the early morning, the daylight is mostly determined by the "warm", meaning long-wave component of the spectrum of the rising sun. This is referred to as red morning sky. Soon after, the daylight is increasingly influenced by blue, colder colour tones towards noon, which are especially pronounced when the sky is blue. In the evening, it is once again the warm, long-wave radiant component of the setting sun which creates a red evening sky.
The cause for red skies in the mornings and evenings is a stronger refraction of blue radiant components and their travelling reflection into space on the elongated optical path from the sun to earth. The colour temperature of daylight composed of blue sky and sunlight runs the gamut of ca. 10.000 Kelvin (K) at noon to 3.000 K (sunset). The blue sky itself has a colour temperature of around 20.000 K, the sun has only around 5,000 K. The moon has a colour temperature of around 4.200 K.
Figure 3.46: Yearly and daily progression of average, horizontal illuminance outdoors with a uniformly cloudy sky for locations at 51° latitude north. The parameters of the curves state the true local time.