Daylight factor is used for daylighting design; which option describes it?

• A. The ratio of indoor light level to outdoor light level under clear sky; used to design daylighting.
• B. The rate of daylight entering a space per hour
• C. The total daylight availability in a space per day
• D. The ratio of indoor light level to outdoor light level under overcast sky; used to design daylighting.

Answer: (D)

Daylight factor is a way to quantify how much daylight actually reaches inside a space under diffuse sky conditions, so designers can compare spaces and plan daylighting approaches consistently. It is the ratio of indoor illuminance to outdoor illuminance, expressed as a percentage, measured under a standard overcast sky. This overcast condition emphasizes diffuse daylight rather than direct sun, giving a steady metric that helps determine appropriate window size, interior finishes, and shading to achieve a desired level of daylighting. It’s not about the rate of daylight entering per hour, nor the total daylight arriving over a day, and it specifically uses the overcast sky scenario, not clear sky.