##  [Daylight Factor](/daylight-factor-1) 

 Definition

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

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

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

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

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

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

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

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

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.