» There is no substitute for daylight «
Daylight is described as the combination of all direct and indirect light originating from the sun during daytime. Of the total solar energy received on the surface of the earth, 40% is visible radiation and the rest is ultraviolet (UV) and infrared (IR) wavelengths, as shown in Figure 1.1.1
Daylight availability outside varies for different locations due to different sun paths and sky conditions through the course of the day, the season and the year. Put simply, the amount of light on the ground depends on the solar elevation; the higher the sun, the greater the illuminance on the ground. Daylight levels vary significantly on horizontal and vertical surfaces by time of day and season, directly related to the local sun paths and sky conditions.
While certain electric light sources can be constructed to match a certain spectrum of daylight closely, none have been made that mimic the variation in the light spectrum that occurs with daylight at different times, in different seasons, and under different weather conditions (Boyce et al., 2003).