# 3 Major Components of Daylight Factor

Daylighting is more than just adding skylights or windows to a particular space. It is a balance of heat gain and loss carefully with glare control and variation in availability of daylight. The art & science for daylight is about how to provide enough natural light to an occupied space without any undesirable side effect. Successful daylighting is about the use of shading device to reduce excess contrast & glare in the workspace.

The sum of the three components gives the daylight factor:

DF = SC + ERC + IRC

• SC – Sky Component
• ERC – Externally Reflected Component
• IRC – Internally Reflected Component

#### Sky Component:

Basically the light that directly comes from the sky is called sky component or we can say it is ratio of daylight falling on vertical surface to the daylight available under an unobstructed sky.

The sky component normally refers to the diffused sky; i.e. it is not used to describe direct sunlight.

In other words, it can define as the ratio (or percentage) of that part of the daylight illuminance at a point on a given plane which is received directly from the sky as compared to the simultaneous exterior illuminance on a horizontal plane from the entire hemisphere of an unobstructed clear design sky.

#### External Reflected Component (ERC):

Sunlight reflected from an exterior surface (building/walls) and then reaching the point considered, known as the externally reflected component (ERC).

OR

In other words, it is the ratio (or percentage) of that part of the daylight illuminance at a point on a given plane which is received by direct reflection from external surfaces as compared to the simultaneous exterior illuminance on a horizontal plane from the entire hemisphere of an Unobstructed clear design sky.

#### Internal Reflected Component (IRC):

The usual internally reflected component of the daylight factor represents the daylight reaching a reference point after reflections and inter-reflections from the surfaces inside a room as shown in figure.

The value of the inter-reflected light will vary all over the room, but for the most purposes it is sufficient to calculate an average value to offer to the most of the room and a minimum for points distant from the window.

In other words, it is the ratio (or percentage) of that part of the daylight illuminance at a point in a given plane which is received by direct reflection or inter-reflection from the internal surfaces as compared to the simultaneous exterior illuminance on a horizontal plane due to the entire hemisphere of an unobstructed clear design sky.

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