The Daylight Factor (DF) is the ratio, expressed as a percentage, of the interior horizontal illuminance at a specific point inside a building to the simultaneous unobstructed exterior horizontal illuminance under standard overcast-sky conditions; it is measured without artificial lighting and characterizes the relative availability of natural daylight at that point.

Principle

DF is a relative, geometry- and material-dependent metric: for a given exterior sky luminance distribution, DF depends on the window and opening geometry, glazing and surface transmittances, internal surface reflectances, and obstructing geometry, not on the absolute exterior illuminance.

Demonstration

Illustrative scenario — Situation: A workspace receives no artificial light during a uniformly overcast midday sky. Recognition: Exterior horizontal illuminance (unobstructed) is measured at 10 000 lux and the interior horizontal illuminance at a desk point is 500 lux. Action: Compute DF = (interior / exterior) × 100 = (500 / 10 000) × 100 = 5%. Consequence: A DF of 5% indicates moderate daylight availability and suggests that designers may reduce artificial lighting needs or consider smaller glazing changes than for lower DF values.

Misapplication

Treating a measured interior-to-exterior ratio taken on a sunny day, or while artificial lighting is operating, as a valid DF. The semantic error is failing to control for the required overcast-sky condition and zero artificial lighting; such measurements do not isolate the geometric/material determinants DF is intended to represent.

Consequence

DF provides a standardized, comparable indicator for early-stage daylight design and compliance checks: low DF values typically motivate increasing sky access (larger or additional openings), enhancing transmittance or internal reflectances, or introducing light wells; however DF alone does not predict occupant satisfaction, glare risk or annual useful daylight performance.

Reversal

DF’s relevance declines for climate-based, dynamic performance assessments (e.g., daylight autonomy, useful daylight illuminance) and for glare analysis — in those contexts, time-varying sky luminance, direct sun, and occupant metrics supersede a single steady-state percentage.

Boundary

Clearly within: point illuminance on a horizontal work plane inside a room measured under an overcast sky. Boundary case: DF measured near a deep window reveal where sky view is partially blocked — DF applies but is highly sensitive to local geometry. Clearly outside: metrics of annual daylight availability, glare indices, vertical surface luminance or exterior site daylighting behaviour are not DF.

Semantic Tension

Increasing glazing area or transmissivity raises DF but usually worsens thermal transmittance and potential solar gains, creating a trade-off between daylighting and thermal performance that must be resolved by integrated design.

Synthesis

DF is a simple, repeatable relative metric that isolates the geometrical and material determinants of instantaneous daylight availability under overcast conditions; it is most useful for comparative and early-design decisions but must be complemented by dynamic climate-based and occupant-centred metrics for final performance assessment.