History of the daylight criteria conditions influencing new methods for the determination of interior daylighting

Determination of sunlight and skylight availability and utilization may be based only on the characteristics of the sun and sky as natural sources of daylight. Sun as a primary source of light has been studied for centuries with gradually accumulating knowledge concerning the sun position on the sky vault in different geographical locations and various time periods. Extraterrestrial solar energy flow towards the Earth's atmosphere was determined in the form of the Solar Constant and horizontal sunlight illuminance under various changes in time, solar altitude and atmospheric conditions. Skylight from the whole sky vault is the largest spacious source of diffuse daylighting in building interiors. Historic achievements of its determination by sky luminance patterns was first defined by Lambert with unity uniform sky luminance as the simplest standard with the possibility of determining relative Sky Factor criterion. Modelling of the whole range of sky types is possible by applying gradation luminance patterns and determination of their gradation functions, with scattering of sun beams in different space directions and is determined by indicatrix functions. Relative luminance sky patterns of different sky types is now standardized and normalized by zenith luminance. However, it is possible to define zenith luminance and horizontal illuminance in absolute physical units, thus to change daylight criteria from relative values of the Daylight Factor to interior physical illuminance in lux. This paper shows the historical progress, developments in measuring sunlight and skylight sources and the current state of art.