Light is emitted at the outer layer of the sun, the photosphere. Due to the earth’s atmosphere, we also receive indirect sunlight that is scattered from microscopic particles in the earth atmosphere which makes the sky bright and bluish. The daylight varies with the sun’s position which is dependent on the time of the year and day and on the location on earth. Weather conditions influence the sky condition and therefore also the daylight level and spectrum.
CIE has defined spectra for a series of standard illuminants that represent the spectra of daylight of different correlated colour temperatures. CIE also defined different standard luminance distributions of skies for the purpose of daylight calculations. The CIE standard overcast sky is usually used to determine the daylight factor that predicts the potential of daylight in buildings depending on exterior obstructions, fenestration and interior inter-reflection. At many locations on earth, daylight measurements have been made that give quantitative data on the amount of daylight. By combining these location-specific data with daylight factors, detailed insight is obtained in how much of the time and where in the lit space sufficient daylight will be present.
The flow of daylight through vertical windows is much more horizontal than that of electric lighting from the ceiling. This has consequences for the quality of a day-lit space.
For the designer of electric lighting installations to understand the interaction between “his or her” lighting installation and daylighting in “his or her” building, a basic knowledge of daylight is indispensable.
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