Reflectance
Reflectance is how much of the light hitting a surface bounces back off it, on a scale from 0 (all absorbed) to 1. Light colours have a higher reflectance (a white wall is around 0.8) and dark colours a lower one (asphalt is around 0.07).
Most surfaces in a room — walls, floor, ceiling — are treated as matte, so one reflectance number per surface is enough for a daylight simulation; it doesn't need to account for shiny or mirror-like reflections.
How much reflectance actually matters
Reflectance determines how much daylight reaches a point indirectly — the reflected component — after bouncing off walls, floor, ceiling and the surfaces outside the window, as opposed to light reaching the point directly from the sky. In most rooms the reflected component is less than 20 % of total illuminance, so tweaking materials will not work miracles for a non-compliant room.
This is also why it's good practice to run simulations — especially preliminary ones during design — with default values: compliance shouldn't depend on whether a tenant changes the materials later, and keeping that margin in hand means a slightly lighter material can still be applied at the final-simulation stage if the room needs it.
Where reflectance does move the result more:
- The facade of the building across the street — usually outside the architect's control, unless it's part of the same project — can have a significant effect on rooms with heavy obstruction, such as lower floors in a deep urban canyon.
- Large interior surfaces (floor, ceiling, walls) have more potential impact than small ones simply from their area — though ceiling and wall defaults are already fairly high, leaving little margin for improvement there; floor reflectance is usually where there's more room to work with.
Typical values
The values below are the ranges commonly seen in daylight practice and the fixed defaults LUX uses in its Default simulation modes. Plausible range is the outer bound for the surface/material category — values outside it should not be used unless backed by documentation for the specific material. Typical range is the tighter band that won't look out of place to a reviewer; staying inside it is the safer choice when in doubt.
| Surface | Plausible range | Typical range | LUX default |
|---|---|---|---|
| Ceiling | 0.50 – 0.90 | 0.75 – 0.85 | 0.80 |
| Interior walls | 0.30 – 0.90 | 0.60 – 0.80 | 0.70 |
| Floor | 0.05 – 0.60 | 0.15 – 0.40 | 0.30 |
| Facade / exterior walls | 0.10 – 0.60 | 0.20 – 0.50 | 0.30 |
| Terrain / ground | 0.05 – 0.40 | 0.15 – 0.30 | 0.20 |
| Window frame | 0.10 – 0.80 | 0.10 – 0.80 | 0.50 |
| Balcony underside | 0.30 – 0.90 | 0.50 – 0.70 | 0.70 |
| Context buildings | 0.10 – 0.60 | 0.20 – 0.40 | 0.30 |
Guide values for common finishes: white paint 0.70–0.85, light grey 0.40–0.60, exposed concrete 0.20–0.40, wood panelling 0.25–0.50, dark carpet 0.05–0.15, grass 0.20–0.25, asphalt 0.05–0.10, snow cover 0.60–0.80.
Standards rarely mandate a reflectance set. EN 17037 and the national building codes that adopt it leave the surface properties to the modeller; some voluntary schemes publish a reference set for reproducibility.
How LUX uses reflectance
In DF Default (the validated, in-production mode) the values in the table above are fixed. DF Custom, which accepts a value per surface, is developed but not yet deployed — see Simulation modes for the current model status. Values outside the ranges shown are clamped by the backend.
LUX Certify runs a full Radiance simulation and takes the project materials directly.