Jul 5, 2018


A measure of daylight illuminance at a point on a plane, expressed as the ratio of the illuminance at that point to the simultaneous exterior illuminance on a horizontal plane from an unobstructed sky of assumed or known luminance distribution (typically overcast). Direct sunlight is excluded from both interior and exterior illuminance values.

NOTE: This definition was modified in ANSI/IES RP-16-17 Addendum 2.

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