The equation [I(γ)=d*d*E(γ)] only applies when the distance d is large enough to view the lamps or luminaries as a point source approximately. EN 130-1:2004 specifies the requirements of measurement distance: The measurement distance shall be at least 5 times the largest luminous dimension of the test source.
However for luminaries with distributions significantly different from a cosine, the above ratio will give rise to errors in excess of 1%. For such luminaries a measurement distance ratio in excess of 10:1 may be needed.
The minimum measuring distance for a floodlight with reflector can be determined by the distance of “beam cross over” point as [D min=a*a*(2a/5+1)/4f+2af/s].
However, the measuring distance can not be too large either, it is firstly limited by the space of the darkroom, and secondly it is limited by the sensitivity of the photometer detector. Generally said, for large luminaries, the measuring distance of 20-30 meters is reasonable.
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