Daylight Factor

To assess the performance of a rooflight daylighting system, the CIE recommended method is the ‘Daylight Factor’ - also a useful tool for estimating the amount of glazing required to provide adequate daylight to a space.

The Daylight Factor uses a ration between indoor and outdoor lighting levels which, of course, are dependent on both rooflight design and location. The Daylight Factor does not use a specific value of illuminance - it is defined as ‘the ratio of interior illuminance on a horizontal surface to the exterior illuminance on a horizontal surface from the CIE Overcast Sky definition’.

Essentially, the greater amount of light entering through a rooflight, relative to the outside illuminance, the higher the Daylight factor will be - the illuminance in a day lit interior will vary depending on sky luminance, the Daylight Factor remains the same.

Please note, the final Daylight Factor can also be affected by external factors such as maintenance and cleanliness of the rooflight, internal decorative finishes and surface distance from the opening.

Practical Examples of Daylight Factors

Daylight Factor
less than 2%

Room looks gloomy

Often needs full artificial lighting during the day

Decor is dominated by the appearance of artificial lights

Daylight Factor
between 2 – 5%

The optimum range of daylighting for efficient energy use

Room appears to be predominantly lit by daylight

Artificial lighting is required away from rooflights and on dull days

Daylight Factor
greater than 5%

Room appears to be strongly lit by daylight

Artificial lighting rarely required during the day

Potential solar gain is a
consideration and therefore careful specification is required


During the earliest stages of rooflight design and specification, it may be necessary to calculate the area of glazing required to deliver a set daylight factor. Daylight can be predicted with the Daylight Factor equation - this uses the following formula to calculate the Daylight Factor at a single point in the centre of the room.


DF is the daylight factor (%).
Aw is the glazed area of the windows (m²).
A is the total area of all the surfaces in the room (including windows).
T is the transmittance of the glazing to diffuse light; it includes the maintenance factor (to account for dirt on the glass.
Ø Is the angle of visible sky, measured in section from a point in the centre of the window (degrees).
R is the area-weighted mean reflectance of the room surface.

Additional practical information can be found in CIBSE's Lighting Guide LG10:1999 - 'Daylighting and window design.'