Most of the lighting designs are based on the far-field characteristics of light sources. It is important to know whether the measurement distance is far enough to simulate the light source as a point source. In this paper, we deduce the far-field conditions for surface light sources with continuous luminous area. The relative far-field distance for several typical flat and curved surface light sources, such as the round, rectangular, annular, hemispherical, and semi-cylindrical sources are calculated. Results show that the classical five times rule is not applicable to surface light sources with hemispherical, arc-shaped semi-cylindrical, and narrow annular luminous areas.
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