![]() When the Sun has set below the horizon at twilight ( θ s < 0°) and no longer directly illuminates the Earth, the light in the sky results in part from refraction and scattering of the Sun’s rays in the upper atmosphere. These were later accepted as the Commission Internationale de l’Éclairage (CIE) daylight model (henceforth called the ‘CIE daylight model’), and are widely used for modelling and synthesizing the spectral power distributions of daylight 3.Īmbient illumination intensity changes systematically and most rapidly around twilight, decreasing (at dusk) or increasing (at dawn) as a function of decreasing or increasing solar elevation ( θ s) 1, 4. 2 subjected a set of 622 measured daylight spectral power distributions to a dimensionality reduction technique and derived three basis functions (termed S0, S1 and S2 in the original work and below) which account for much of the variance in the dataset. ![]() In the 1960s, coordinated efforts were made to describe the spectral power distribution of daylight. The ambient light level also depends on the presence of clouds and haze, and may vary minute-to-minute due to cloud cover and atmospheric turbidity. In comparison, sunlight may be as intense as 100,000 lux 1. Starlight on a clear night has a brightness of ~0.001 lux, and moonlight ~0.2 lux 1. During the day and before twilight, the ambient illumination is between 1,000,000 (10 6) and 100,000,000 (10 8) times brighter than at night 1. ![]() During the 24-hour cycle, ambient illumination changes as a function of the Earth’s rotation in both intensity and spectral composition. ![]()
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