Which statement about daylight factor is true?

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

Answer: (A)

Daylight factor is a ratio that compares how much light is inside a space to how much light is outside, using diffuse daylight conditions—typically an overcast sky. This makes it a stable, comparable measure for daylighting design because it reflects how well the building envelope and interior surfaces transmit daylight, independent of the exact outdoor light level or sun position. It’s expressed as a percentage and is used to evaluate and compare different designs, materials, and layouts.

This isn’t a rate per hour, so it doesn’t describe daylight entering a space over time. It also isn’t measured under a clear sky, since direct sun causes large, variable changes that aren’t as useful for a standard design metric. And it isn’t about total daylight available over a day, which would involve summing or integrating light over time. Therefore, the description that correctly defines daylight factor is the ratio under an overcast sky used for daylighting design.