Evaluate how much daylight reaches building façades from the sky to assess design quality and check compliance with local daylight guidelines.
- Analyze tab > Forma Analyses panel >
(Daylight Potential)
What the analysis evaluates
Daylight potential analysis measures how much light reaches a building's façades from the sky, expressed as a percentage score called the Sky Component. For points on vertical facades, this is referred to as the Vertical Sky Component (VSC). The score indicates how much light would shine through a window placed at that point, and it considers shading from buildings and terrain in the model.
How the analysis works
Daylight potential analysis predicts illumination on building surfaces using the Vertical Sky Component (VSC) method. VSC is the ratio of illuminance at a point on a vertical plane, received directly from a CIE Standard Overcast Sky, to the illuminance on an unobstructed horizontal plane under the same sky. The CIE overcast sky is three times brighter at the zenith than at the horizon, so an unobstructed horizontal surface has a Sky Component of 100%, and an unobstructed vertical surface has a Sky Component of almost 40%.
The overcast sky model is independent of the sun and weather conditions. It represents a heavily overcast day and is based on the CIE Standard Overcast Sky. It uses the Moon and Spencer (1942) sky luminance model, in which sky luminance at a given angle from the zenith is proportional to the zenith luminance. This model is a conservative, commonly used representation of light conditions for daylight simulations.
The analysis runs using WebGPU locally on your computer. If WebGPU is not enabled, see How to enable WebGPU.
Vertical Sky Component vs. Daylight Factor
Daylight Factor (DF) is a measure commonly used for evaluating daylight conditions. It assesses illuminance inside the rooms of a building, using window details and material reflective properties to account for direct and reflected light.
Vertical Sky Component (VSC) assesses illuminance outside on the building façades. It considers only direct light from the sky.
Because direct light from the sky through a window is typically the main contributor to a DF score, VSC can provide a good early predictor of indoor daylight performance. DF and VSC are both usually defined using the CIE overcast sky model.
Vertical Sky Component thresholds
Analysis results display as a color-coded map on building façades, using threshold values suggested by the Building Research Establishment (BRE, UK) to predict daylight performance. These thresholds are targeted to predict an average daylight factor of 2%. Analysis results are divided into color bins using the thresholds listed in the following table. The darkest areas are where achieving reasonable daylight is often impossible, while the lightest areas are where daylight conditions are usually satisfactory. The maximum Sky Component score for vertical faces is roughly 40%; for non-vertical faces, scores can go up to 100%.
| Vertical Sky Component (VSC) | Daylight conditions |
|---|---|
| VSC >= 27% | Conventional window design is usually satisfactory. |
| 15% < VSC < 27% | Larger windows or changes in layout are usually needed. |
| 5% < VSC < 15% | It is difficult to provide adequate daylight. |
| VSC < 5% | Achieving reasonable daylight is often impossible. |
Assumptions and limitations
- Terrain outside the defined map area for the site is not considered.
- The analysis does not consider window sizes, building materials, room sizes, or room functions; it is intended as an indicator of daylight potential rather than an exact room-by-room daylight calculation.
More information
For more details about daylight potential analysis, see these resources.