As sunlight is scattered through the atmosphere, it turns the entire sky dome into a daylight source, which makes the sun and sky the main daylight sources. Indoor daylight levels, however, are also greatly affected by reflected daylight from the sun and the sky off exterior and interior surfaces, resulting in three main daylight components in interior spaces:

1. The direct component, referring to direct daylight from the sun and the sky,
2. The externally reflected component, referring to reflected daylight from the sun and the sky off exterior surfaces, and
3. The internally reflected component, referring to contribution of inter-reflections of the direct and externally reflected components off interior surfaces.

The Sun
Under clear sky conditions the sun is a very high-intensity, almost-point light source (.5 degree solid angle), producing very high luminance and illuminance levels and sharp shadows.
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The Solar Spectrum
The sun emits a continuous spectrum of electromagnetic radiation from about 100 to about 3,000 nm. About 40-45% of the solar radiation is in the visible part of the spectrum, from about 360 nm to about 800 nm.
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The Sky
As sunlight is scattered through the atmosphere, the whole sky dome becomes a hemispherical diffuse light source, with varying luminance distribution depending on sky conditions and sun position.
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Reflected Daylight
The light from the sun and the sky may find its way into spaces either directly or indirectly, through reflection off exterior surfaces, such as the ground and facades of neighboring buildings.
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Daylight Availability
Successful daylighting is a function of daylight availability, which varies significantly for different locations due to different sun paths and sky conditions through the course of a year. Daylight availability data in terms of sun and sky light intensity are available in weather data records and climate summaries for multiple locations in the U.S. and many other countries.
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Daylight Characteristics
The use of daylight for illumination of building interiors begins by understanding the intensity of direct sunlight, the contribution of skylight, their shifting spectral qualities and overwhelming potential for glare, and the effects of reflectivity and transmittance through buildings materials.  This section reviews these and other primary characteristics of daylight.
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