The primary source of daylight
is the sun. The sun and its rays which are scattered over
the sky produces light in the sky and on the ground. The daily
and seasonal movements of the sun are predictable. Even with
clouds modulating the light from the sun and from the sky,
the main patterns of the daylight luminance are discernable
and solicited.
Spectral Distribution
of Daylight
The following figure illustrates the patterns
of distributions of spectral-power-density functions of beam
and diffuse solar radiation.
Figure 3.1.1 Three power-density spectral distribution of
daylight.
Sunlight
This is the light which travels directly from the sun. On
earth, the diameter of the sun subtends an angle less than
one degree. But the illuminance of sunlight on a horizontal
plane can be much larger than that from the light from sky.
Direct light from the sun is intense. Its luminance is in
the range of billions of Candela per square meter. Sunlight
is associated with heat and glare due to excessive intensity
and luminance. In a normal situation, sunlight is not used
directly for lighting in building interior. The efficacy of
sunlight is in the range of 80-130 lm/W.
Skylight
This is the part of daylight which reaches a point from the
sky, and excludes sunlight. Scattered and reflected sunlight
in the sky produces skylight. The sky dome is often taken
to be equivalent to a light-emitting hemisphere. Any patch
of area on the hemisphere is a diffusive light source. The
luminance levels of the different parts of the sky dome usually
differ. In clearer sky, those areas close to the sun possess
much higher luminance than areas further from it. Total light
flux from skylight (of the whole sky dome) reaching a point
on the horizontal plane is smaller than or comparable to that
from sunlight, and is in the range of 5-70 klux . The luminance
of a clear sky is of the order of 5,000 cd/m2
. The efficacy of skylight is higher than that of sunlight.
Daylight from
Ground and Other Reflecting Surfaces
For a point on a vertical plane, or on a vertical window of
a building, the view factor from the point to the ground in
front of it equals to that from the point to the sky in front
of the plane. For a vertical plane, half of the sky dome as
well as the ground in front of the plane are seen equally
by the plane. Light from the ground possesses high potential
contribution to daylighting through window. The ground is
generally assumed to comprise diffusive surfaces with reflectances
in the range 0.1 to 0.9 (with the lower value appropriate
for grass-covered ground and the upper value corresponding
to snow-covered ground). Daylight from the ground thus comprises
reflected light from skylight and that from sunlight, and
is taken to be diffuse.
A large surface of the façade
of a building in front of a window may block a part of the
skylight from the window, but the reflected daylight from
the surface can contribute to total daylight reaching the
window.
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