THE visibility of a light depends, on a clear, dark night when the effect of atmospheric absorption can be neglected, on the inverse square of the distance at which it is observed, while the ultimate range at which it becomes invisible depends on the visual acuity of the observer.
Toulmin‐Smith, A.K. and Green, H.N. (1931), "The Range of Air Line Beacons: A Determination of the Limit of Visibility for Use in the Design of Aircraft Lights", Aircraft Engineering and Aerospace Technology, Vol. 3 No. 1, pp. 12-12. https://doi.org/10.1108/eb029356
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