LUX: Overcast Sky

These graphs represent raw illumination data measured at the horizontal workplane. Illumination is the most common measure of light and is defined in lay terms as the amount of light falling on a surface. On an overcast day in North America, outdoor illumination values can range from 1,000-30,000 lux (100-3,000 footcandles) depending on time of day, time of year, and location. Electric lighting criteria are almost exclusively defined in terms of illuminance. Common space types such as classrooms and offices have workplane illumination requirements that range from 200-500 lux (20-50 footcandles). These graphs are shown with 300 lux highlighted using a red dashed line. Not surprisingly, the same spaces perform very differently under overcast skies compared to sunny skies.