Daylight Factor (DF)
What it is
The daylight factor is the ratio of the illuminance at a point indoors to the illuminance on an unobstructed horizontal surface outdoors, under a CIE standard overcast sky, expressed as a percentage:
DF = Eindoor / Eoutdoor × 100 %
Because the overcast sky has no sun and a fixed luminance distribution, the DF does not depend on orientation, location or time of year. It is a static measure of how well a room's geometry, windows, obstructions and surface reflectances let diffuse daylight in. A DF of 2 % means the point receives 2 % of the outdoor diffuse light — roughly 200 lx when it is 10 000 lx outside.
DF is the oldest and most widely used daylight metric, and it is the metric behind most national building codes.
Scale and colours
DF values run from 0 to 100 in theory, but indoor values above 10 are very rare, so LUX visualises results on a 0–10 % scale. The critical value is 1 %: below it a point is generally considered under-lit.
- Blue — DF near 0 %; little diffuse daylight reaches the point.
- Purple / magenta — around the 1 % compliance threshold used in Sweden.
- Red to orange — 2–4 %; comfortably daylit for most habitable rooms.
- Yellow to near-white — 5 % and above; very well daylit, close to a window.
Download the exact stops: DF colour scale (JSON).
Where DF is used
DF (or a target DF derived from it) is the compliance metric in:
- Sweden — BFS 2024:8 — DF ≥ 1.0 % over > 50 % of the habitable-room area
- Sweden — AFS 2023:12 — same DF ≥ 1.0 % criterion, applied per room
- Norway — TEK17 §13-7 — average DF ≥ 2.0 % per room
- Denmark — BR18 — Route A: DF ≥ 2.1 % over ≥ 50 % of the area
- EN 17037:2018 — daylight provision, Method 1: target DF DT
See Regulations for the full standard-by-standard comparison, and Sweden — BFS 2024:8 for how LUX applies the Swedish criterion.
Related
- Daylight Autonomy (DA) — the climate-based counterpart
- Simulation modes — DF Default vs DF Custom
- Reflectance and Transmittance — the material inputs that move DF