DAYLIGHT CALCULATIONS

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.

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.

Determining the Sky Component using the Daylight Factor Protractor in PLAN

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 Minimum ERC of DF (percent)

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.