| illuminance produced by direct sunlight on a surface on the Earth that is perpendicular to the direction of illumination by the Sun Note 1 to entry: Normal solar illuminance is not the same as direct solar illuminance, which is measured on a horizontal surface on the Earth, not necessarily perpendicular to the direction of illumination by the Sun. Note 2 to entry: The normal solar illuminance is expressed in lux (lx = lm·m−2).
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