Illuminance is the area density of luminous flux incident at a point on a surface. In the International System of Units (SI), illuminance is measured in lux (lx). In the United States Customary System, the unit is footcandle (fc). One lux is one lumen per square meter. One footcandle is one lumen per square foot. It is important to note that the human eyes respond to luminance, they do not see illuminance. The predominant use of illuminance is partly due to the fact that it is easier to measure and calculate.
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