Daylight factor

Light can reach a room via through a glazed window, rooflight, or other aperture via three paths: The sum of the three components gives the illuminance level (typically measured in lux) at the point considered:

Architects and engineers use daylight factors in architecture and building design to assess the internal natural lighting levels as perceived on working planes or surfaces.

The design day for daylight factor calculations is based on the standard CIE overcast Sky for 21 September at 12:00pm, and where the Ground Ambient light level is 11921 Lux.

[3] To assess the effect of a poor or good daylight factor, one might compare the results for a given calculation against published design guidance.

In the UK this is likely to be CIBSE Lighting Guide 10 (LG10-1999), which broadly bands average daylight factors into the following categories:[4]

A study of daylight factors within a single storey building resulting from different perimeter glazing and rooflight designs and glass types. Undertaken using the IES Radiance software Module.