Daylight Calculations
Daylight or natural light is provided directly by the sunlight and/or the sky. Sunlight is so
variable that it is not of much use in lighting design of interiors. Illuminance from the sky
is also variable; it varies during the day and through the year. In countries like Britain
where the climate is not reliable, an overcast sky is considered to be the main source of
daylight. It also represents the worst type of daylight.
Daylight factor
The daylight factor is defined as the ratio of the natural illuminance at a particular point
on a horizontal plane to the simultaneously occurring external illuminance of the
unobstructed overcast sky. In Britain, the standard sky is assumed to give at least 5000 lx
of illuminance on the ground.
Daylight factor (DF) =
Internal illuminanc e
 100
External illuminanc e
The daylight reaching any point inside a room is usually made up of three components:
Sky component
Externally reflected component
Internally reflected component
If there is no external obstruction like trees, buildings etc. the externally reflected
component is omitted. Several techniques, manual as well as computerised, may be used
to calculate these components for a building.
In side-lit rooms, the maximum DF is near the windows, and it is mainly due to the sky
component. In the early stages of building design, the average daylight factor may be
used to assess the adequacy of daylight:
Average DF =
W
Tθ
2
A (1 - R )
where: W is the area of the windows (m2)
A is the total area of the internal surfaces (m2)
T is the glass transmittance corrected for dirt
θ is visible sky angle in degrees from the centre of the window
R is the average reflectance of area A.
The values of these quantities are determined from the given data and W, T and R are
corrected by using factors given in the BS Daylight Code and other publications.
Example:
Find the average daylight factor for a room – 5.0 m × 4.0 m × 2.4 m high with one
2.4 m × 1.3 m high double-glazed window. Assume that the surfaces of the room are
light in colour; there is an obstruction as shown in Figure App 9.1.
Solution:
The emphasised figures are approximate correction factors
Average DF =
W
Tθ
2
A (1 - R )
Window area = 2.4 × 1.3 = 3.12 m2
W = 3.12 × 0.7 = 2.18 m2
T = 0.6 (approx.) for double-glazed windows in clean environment
θ = 73º
A = 2 × (5.0 × 2.4 + 4.0 × 2.4) + 5.0 × 4.0 + 5.0 × 4.0 = 83.2 m2
R = 0.6 (considering light coloured room surfaces)
5.0 m
2.4 m
PLAN
4.0 m
Visible sky angle
°
73
SECTION
Obstruction
Figure App 9.1
Average DF =
0.6  73
2.18

83.2 (1  0.6 2 )
=
95.484
83.2  0.64
= 1.8 %