DaylightFactor

Daylight factor is a metric used in architectural daylight design to quantify how much daylight from the outdoors reaches the interior of a space. It is defined as the ratio of interior illuminance to exterior illuminance under a standardized sky condition, typically expressed as a percentage. The common definition uses E_in, the illuminance on a specified interior plane (often a horizontal plane at the working height, about 0.8 meters above the floor), and E_out, the illuminance on a horizontal exterior plane under a standard overcast sky. The formula is DF = E_in / E_out.

Because the calculation is based on a standardized outdoor sky, the daylight factor is largely independent of the actual time of day or weather. It is determined by the building’s geometry, the transmittance of glazing, interior surface reflectances, and any shading devices. In practice, DF reflects how effectively daylight is transmitted through the enclosure and redistributed by surfaces, rather than the instantaneous brightness of the sky.

Daylight factor is used in early design stages to compare alternatives and set daylighting targets. It provides a simple, area- and reflectance-dependent measure that can be applied without detailed hourly lighting simulations. However, it has limitations: it does not account for sun position, temporal variation, glare, occupant comfort, or the interaction with artificial lighting.

Typical values for many spaces under standard overcast conditions range from about 2% to 5%. Higher values indicate brighter daylighting, while lower values suggest limited natural illumination. Daylight factor is one of several metrics used to evaluate and optimize indoor daylight performance.