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

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

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

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

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

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

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

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

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.