The amount of direct light falling on a horizontal surface called Horizontal Illuminance. Illuminance falling on a vertical surface or imaginary vertical plane is called Vertical Illuminance. The Imperial measure of Illuminance is footcandles (FC), which is Lumens per square foot. The metric measure of illuminance is Lux (Lx), which is lumens per square meter. One footcandle equals 10.56 Lux or approximately a 1:10 (FC:Lx) conversion.