Daylight factor is used in design to estimate daylight availability and guide artificial lighting needs. Which statement correctly defines daylight factor?

• A. The ratio of interior illuminance to exterior illuminance
• B. The ratio of exterior illuminance to interior illuminance
• C. The luminous efficacy of daylight
• D. The amount of daylight reaching interior in lumens per square meter

Answer: (A)

Daylight factor is a simple, comparable measure of how much daylight actually makes it indoors. It’s defined as the ratio of interior illuminance to exterior illuminance on the same plane, usually expressed as a percentage: inside light level divided by outside light level. This makes it possible to estimate how well a space is daylighted and to predict how much artificial lighting will be needed, independent of lamp output. It’s not about luminous efficacy or total daylight flux; it’s about the interior-to-exterior illuminance ratio under standardized sky conditions, typically overcast, to keep comparisons fair.