Lighting Design Glossary

Daylight Factor

(Term of lighting design/daylighting)

The ratio of interior illuminance at a given point on a given plane (usually the workplane) to the exterior illuminance (reference) under the same overcast sky conditions, eg. the CIE overcast sky distribution.

This is one of the key values when analyzing the quantitative aspects of daylighting. Since the outside illuminance varies a lot with weather conditions (between 8'000 and 25'000 lx), the interior illuminance alone does not provide much useful information. The acceptability of the amount of daylight reaching an interior space for a specific task can only be determined in relation to the situation outside.

Very often, lighting designers assume an average outside illuminance of 10'000 lx, in order to estimate the expected interior illuminance values for comparison with illuminance from electrical lighting. This conservative assumption is especially practical for calculation, as every percent of daylight factor will result in 100 lx of interior illuminance.

There is little use in computing the relation of outside and indide illuminances under sunny sky conditions.

Unit of daylight factor is percent (%).