Which statement best describes a daylighting metric used to assess daylight availability?

• A. The aesthetics of daylight.
• B. The percentage of exterior daylight reaching the space.
• C. The indoor illuminance level relative to outdoor conditions.
• D. The energy cost of shading devices.

Answer: (C)

The essential idea is measuring how much daylight is actually available inside by comparing indoor light levels to outdoor light levels. This is the daylighting metric known as daylight factor: indoor illuminance expressed as a percentage of outdoor illuminance under reference sky conditions. It lets you compare spaces and design choices on daylight availability independent of the weather at a given moment, guiding decisions about window size, placement, glazing, and shading.

Aesthetics of daylight isn’t a quantitative availability measure, so it doesn’t describe how much daylight reaches the space. Simply counting exterior daylight reaching the space omits how much of that light makes it indoors and how bright the interior becomes. And shading costs relate to energy performance, not the daylight availability inside.