What is daylight autonomy and how is it used in architectural design?

• A. The percentage of annual operating hours when interior illumination relies on artificial light.
• B. The average daylight illuminance in a space.
• C. The daylight factor under a clear sky.
• D. The percentage of annual operating hours when interior illumination meets a target level with daylight alone.

Answer: (D)

Daylight autonomy measures how much of the year the interior lighting level can be met using daylight alone, without turning on artificial lighting. It’s expressed as a percentage of annual operating hours, based on a target illuminance on the working plane (for example, the level you want for comfortable tasks). In practice, this tells you how well a space can rely on natural light under typical conditions, helping guide decisions about window size and placement, glazing types, shading, and interior layout to maximize daylight benefits while managing glare and heat gain.

The best answer captures this time-based performance: it’s the percentage of annual operating hours when interior illumination meets a target level with daylight alone. This is different from just an average daylight illuminance, which doesn’t say how often that level is reached. It’s also different from the daylight factor concept under a clear sky, which is a ratio used to estimate indoor illuminance from outdoor light and isn’t about hours of sufficient daylight. And it differs from the period when artificial lighting is needed, which would be the complement of daylight autonomy.