2. Components of the Natural Light :
There are three separate
components of the natural light that
reaches any point inside a building:
1. Sky Component (SC) -
Directly from the sky, through an
opening such as a window.
2. Externally Reflected
Component (ERC) - Reflected
off the ground, trees or other
buildings.
3. Internally Reflected
Component (IRC) - The inter-
reflection of 1 and 2 off surfaces
within the room.
Fig: Components of the Natural Light
3. Sky Component (SC):depends on:
1. Width of the window
2. Distance between the point and
window
SC varies from 0.01 to 15%
Externally Reflected Component
(ERC) is small. The luminance of
obstrction is taken as 10-20% that of the
sky
Internally Reflected Component
(IRC): is a half of SC and is therefore
significant
4. DF = SC + ERC + IRC
Separate consideration of these three components is justified by the fact that
each is affected by different elements within the design. The daylight factor
is thus given as a percentage and is simply the sum of each of these three
components.
5. Daylight Factor
where
Ei = illumiance due to daylight at a point on the indoors
working plane
Eo = simultaneous outdoor illuminance on a horizontal plane
from an unobstructed hemisphere of overcast sky
The daylight factor is defined as :
6. Determining Initial Sky Component
The procedures to use BRE
Protractor are as follows:
1. Determine the initial sky component
(infinite length of window)
2. (a) Refer to the elevation view of a
room
3. (b) Mark the work plane and the
point considered (A).
4. (c) Connect this point to the window
sill (AS) and the window head (AH).
5. (d) Make a tracing of protractor
(Figure 7).
6. (e) Place the tracing with its centre
on point (A) and its base along the
working plane, with scale 1 upright.
7. (f) Read the values where lines AS &
AH intersect the protractor scale.
8. (g) The difference between the two
values gives the initial sky
component Determining Initial Sky Component
7. Determining the Sky Component using the Daylight
Factor Protractor in SECTION.
8.
9. Determining the Sky Component using the Daylight
Factor Protractor in SECTION.
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13. ERC Component tables for Vertical Glazing
the same calculation as
for the sky component
is used but with the
result multiplied by 0.2
to represent the
average reflectance of
external surfaces
15. The internally reflected component (IRC)
Where:
W = Window area (m2),
A = Total internal surface area, wall, floors ceilings and windows (m2),
p1 = Area weighted average reflectance of area A, (using 0.1 as reflectance for
glass).
p2 = Average reflectance of surfaces below working plane,
p3 = Average reflectance of surfaces above working plane,
C = Coefficient of external obstructions.
