These graphs represent raw illumination data measured on 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 a sunny day in North America, outdoor illumination values can range from 20,000-100,000 Lux or more depending on time of day, 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 20-50 footcandles, or approximatly 200-500 Lux. These graphs are shown with 300 Lux highlighted using a red dashed line.