Quantifying natural light availability indoors
Daylight factor is the ratio of indoor illumination to outdoor illumination, expressed as a percentage. A desk 3 meters from a north-facing window on an overcast day might receive only 1-2% of the exterior light, while the same desk 1 meter from a large south-facing window could reach 4-5%. The metric simplifies a complex variable (time of year, cloud cover, window transmission, interior reflectances) into a single dimensionless number. Design targets range from 2% for task areas (sufficient for reading without electric light on typical overcast days) to 0.5% for ambient background lighting.
Geometry and orientation shape daylight distribution
Window size alone does not determine daylight penetration. A large window reveals only its head height and width, so tall narrow windows deliver light deeper into a space than short wide ones. Distance from the window matters exponentially: doubling distance from a window roughly quarters its illumination. Orientation is equally critical: north-facing windows provide steady, diffuse light; south-facing windows in cold climates maximize solar gain; east and west exposures create glare in mornings and afternoons. Overhang depth and angle determine how much sun reaches windows seasonally. Professional daylighting design combines window sizing, placement, reflective surfaces, and shelves to distribute natural light evenly across work areas while minimizing glare and overheating.