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
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
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.
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 :
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
room3. (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
Determining the Sky Component using the Daylight Factor Protractor in SECTION.
Find the average altitude(a) Read the angles of elevation of lines AH and AS, on the inner degree-scale.(b) Add the two angles and divide the sum by 2.
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
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.