Explain the concept of daylighting factor and its use in design.

• A. The measure of solar heat gain coefficient for glazing.
• B. The ratio of interior illuminance to exterior illuminance under diffuse sky; used to quantify daylighting performance.
• C. The percentage of daylight that is reflected by interior surfaces.
• D. The difference between daytime and nighttime lighting levels.

Answer: (B)

Daylight factor is a simple, practical measure of how effectively daylight penetrates a space. It expresses interior daylight levels as a ratio of what you actually get inside to what’s available outside, under a diffuse sky. In formula form, it’s the interior illuminance at a point due to daylight divided by the exterior illuminance on a horizontal plane, usually shown as a percentage. This standard reference condition lets designers compare options without worrying about the sun’s position or weather.

Several things influence the daylight factor: window size and placement, glazing transmittance, and the reflectance of interior surfaces. Higher transmittance glazing, more sky exposure, and lighter, more reflective interiors raise the interior illuminance for a given outside daylight level, increasing the daylight factor. Designers use this metric early in design to evaluate and compare layouts, window configurations, and finishes to meet targeted daylighting performance. It’s not about heat gain, daylight reflected alone, or the day-night difference; it specifically gauges usable daylight inside relative to outside under diffuse sky conditions.