All You Need to Know About Daylight

A daylight zone or daylight area is defined by the ASHRAE/IES 90.1-2010 Energy standard as “the floor area substantially illuminated by daylight.”

In other words, it should consistently receive significant quantities of daylight.

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Daylight-area-in room
Courtesy - 123rf


It is the superficial area on the working plane illuminated to not less than a specified daylight factor, that is, the area within the relevant room.

Basically, day lighting is necessary for every room as it is the admission of natural light into the building to reduce electricity and save energy. To achieve it, skylights and windows are designed as per requirements. The regions in a building that are close enough to a source of daylight where daylight harvesting is possible are considered within a “daylight zone.”

There are 3 types of daylight zones:

01. Top lighting Zone: It is an area which is illuminated by one or more skylights.


02. Primary Side lighting Zone: It is an day lighting area – directly adjacent to one or more windows.

Daylight-Skylight zone-area

03. Secondary Side lighting Zone: It is an area which is not directly adjacent to a window that still receives some daylight through its proximity to the window.

Different paths from where daylight enters in a room (through windows):

The three possible paths along which light reach at point inside a room through glazed windows are as follows:

  • Light from the patch of visible sky at the point considered, is expressed as the sky component (SC),
  • Light reflected from opposing exterior surfaces and then reached the point, is expressed as the externally reflected component (ERC),
  • Light entering through the window but reaching the point only after reflection from internal surfaces, is expressed as the internally reflected component (IRC)

The sum of the three components gives the daylight factor i.e:


SC – Sky Component

ERC – Exterior Reflected Component

The amount of daylight that penetrates a room depends upon the window orientation, size and its glazing characteristics.

The distance to which adequate daylighting will penetrate into a room usually depends upon window location and interior surfaces. There is a direct connection between the window head height and the depth of daylight penetration.

Usually adequate daylight will penetrate one and one-half times the window head height, although it may penetrate a distance of twice the height under direct sunshine.

In other words, it can be explained as a maximum distance to which a given daylight factor contour penetrates into a room.

  • Day Light Factor:

It is a ratio in percent, of workplane illuminance (at a given point) to the outside illuminance on a horizontal plane which is evaluated under cloudy sky conditions only (no direct solar beam).


DF= (Ei/Eo) x 100%


Ei = illuminance due to daylight at a point on the indoor’s working plane.

Eo = simultaneous outdoor illuminance on a horizontal plane from an unobstructed hemisphere of overcast sky.

Floor plans and sections indicate average daylight area with daylight factor. Where red colour indicates maximum daylight area because that area is very adjacent to windows and blue colour indicates very less amount of natural light as these area is far from window.

There are three components for estimation of total amount of daylight:

01. Direct light of the sun

02. External Reflected Component

03. Internal Reflected Component

Day lighting is generally considered on a horizontal plane.

What are the Aspects of Day Lighting?

Daylight is a full-spectrum source of light to which human vision is adapted. Daylighting has two general benefits:

  • It can improve the quality of light in a space, and
  • Reduce the amount of electrical lighting energy required.

The clear design sky concept for daylighting takes care of the worst possible situation; orientation is not a major problem for daylighting in buildings, except that direct sunshine and glare should be avoided. However, due allowance should be given to the mutual shading effects of opposite facades.

More importantly, daylight provides tremendous psychological benefits to building occupants; this should be a main goal of day lighting rather than the simple reduction of electrical lighting requirements

Also Read: All You Need to Know About Glare

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