The accuracy with which the 3-component CIE daylight model captures variability in spectral composition at twilight, or more generally as a function of solar elevation, is not known. This period is informally termed the “blue hour” in photography. With ever decreasing solar elevations, the short-wavelength content of ambient light becomes enriched due to the increasing amount of ozone absorption caused by the increased path length of solar light through the atmosphere and so-called Chappuis filtering of green/yellow light 9. Twilight is classified into three distinct phases according to solar elevation and the prevailing visibility conditions due to the illumination level: a) civil twilight (−6° < θ s < 0°), when terrestrial objects can still be distinguished by human observers, b) nautical twilight (−12° < θ s < −6°), when only object outlines are visible, and c) astronomical twilight (−18° < θ s < −12°), when the illumination level is low enough such that stars and other astronomical objects are available for observation 5.ĭuring twilight not only does the intensity of the illumination change, but so does the spectral composition (colour), giving rise to vivid phenomena visible to the human eye at and before twilight 6, 7, 8, such as the yellowish twilight arches during civil twilight and the purple and red sky during nautical twilight. 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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