##  [Daylight Factor](/daylight-factor-0) 

 Definition

The ratio, expressed as a percentage, of the interior horizontal illuminance at a point on a reference plane under an overcast sky to the simultaneous unobstructed exterior horizontal illuminance under the same sky; used as a standardized, steady‑state indicator of natural lighting performance independent of direct sunlight.

 

 

 

 

 

 





## Principle

Principle

Daylight Factor is a normalized, geometry‑and‑transmittance‑based metric: it depends on sky luminance distribution, window and glazing transmittance, room geometry and surface reflectances; because it is defined for sky‑only conditions it represents relative daylight availability rather than dynamic occupant exposure to sunlight.

 

 

 

 

 





## Demonstration

Demonstration

Illustrative calculation — For a small office with a single glazed façade, known window area, glazing visible transmittance and specified internal reflectances, a daylighting analysis under an assumed standard overcast sky produces a DF at the desk plane of, for example, 2.5%. Consequence: designers use the DF to assess whether glazing area and internal finishes provide baseline natural illuminance for the task.

 

 

 

 

## Misapplication

Misapplication

Using Daylight Factor to predict occupant experience under sunny conditions or to assess annual daylight performance. Plausible error: conflating a steady‑state, sky‑only percentage with dynamic metrics like daylight autonomy or useful daylight illuminance. Semantic error: treating DF as a standalone indicator of overall daylighting adequacy across variable climates and times.

 

 

 

 

 





## Consequence

Consequence

Applied correctly, DF supports early design decisions about window sizing, room depth and internal reflectances; misapplied, it can under- or over‑estimate daylight performance in climates or conditions where direct sun dominates, leading to poor glazing choices or inadequate glare control strategies.

 

 

 

 

## Reversal

Reversal

In climates and designs dominated by direct sun and seasonal variation, dynamic metrics (daylight autonomy, climate‑based daylight modelling, useful daylight illuminance) better predict occupant exposure and glare risk; DF remains useful as a simple standard reference and for comparisons under overcast conditions.

 

 

 

 

 





## Boundary

Boundary

Clearly within: steady‑state interior horizontal illuminance ratio under a defined overcast sky at a point on a workplane. Boundary case: locations immediately adjacent to windows where direct sun contributions may dominate instantaneous illuminance; these require complementary metrics. Clearly outside: metrics of daylight glare, thermal solar gains, or annual daylight availability that account for direct sun and temporal variability.

 

 

 

 

 





## Semantic Tension

Semantic Tension

Simplicity and standardization of DF versus climatic and temporal realism required for occupant comfort: DF is easy to compute and compare but omits direct solar contribution and time variation that drive real occupant experience.

 

 

 

 

 





## Synthesis

Synthesis

Daylight Factor is a compact, standardized baseline metric for comparing natural light provision under sky‑only conditions; it informs early design choices about glazing and interior reflectance but must be combined with dynamic, climate‑based metrics where direct sunlight, glare or temporal performance matter.