Define daylighting metrics such as daylight factor or illuminance and their relevance to design.

• A. Daylight factor expresses indoor illuminance as a percentage of outdoor illumination; used to gauge daylight availability and guide design strategies.
• B. Daylight factor is the same as lumens per square meter.
• C. Illuminance levels are irrelevant to daylighting design.
• D. Daylight factor measures color temperature.

Answer: (A)

Daylight factor is a ratio that shows how much daylight reaches an indoor space compared with the daylight available outdoors, expressed as a percentage. It’s calculated using indoor horizontal illuminance divided by outdoor horizontal illuminance under a standard sky (often overcast) and then multiplied by 100. This metric is useful because it provides a simple, comparable measure of daylight availability that guides design decisions like window size and placement, glazing choices, shading strategies, and interior reflectance to balance adequate daylight with glare and energy efficiency. It’s not the same as lumens per square meter (illuminance in lux) and it doesn’t measure color temperature; focusing on illuminance levels alone wouldn’t capture how much daylight actually penetrates a space. Daylight factor helps set targets and compare different layouts or façades to ensure meaningful daylight without over-reliance on weather conditions.