Daylight Calculations

DAY LIGHT CALCULATIONS

The amount of daylight received into an indoor space or room is defined as a daylight factor. It is the ratio between the measured internal and external light levels.

Daylight Factor

The Daylight Factor (DF) is given as:

where 

E₁ - illuminance due to daylight at a point on the indoors working plane.

E_{0 -} simultaneous outdoor illuminance on a horizontal plane from an unobstructed hemisphere of overcast sky

There are three possible paths along which light can reach a point inside a room through glazed windows (Figure 3.7).

They are:

(a) Sky Component (SC): Light from visible at the point considered.

(b) Externally Reflected Component (ERC): Light reflected from opposing exterior surfaces and then reached the point.

(c) Internally Reflected Component (IRC): Light entering through the window but reaching the point only after reflection from internal surfaces.

The day light can reduce the usage of artificial lighting. This process is known as daylight harvesting.

In recent years, work has taken to recreate the effects of daylight artificially.

This is however expensive in terms of both equipment and energy consumption. It is applied in specialist areas such as film making.

The sum of the three components gives the illuminance level (Lux) at the point considered as

LUX SC + ERC + IRC

Daylight factor is used in building design in order to assess the natural lighting level as received on the working plane.