The meteorological visibility is an alternative way to describe the extinction by the atmosphere of an object being viewed. This can be quantitatively characterised by the atmospheric transmissivity.
Meteorological visibility is the greatest distance at which a black object of suitable dimensions can be seen and recognized by day against the horizon sky, or, in the case of night observations, could be seen and recognized if the general illumination were raised to daylight level.
Allard's Law allows the calculation of the illuminance E as a function of distance d, luminous intensity I and meteorological visibility v.
In the case of a light that appears as a point source, the luminous range D is defined as the maximum distance at which a light can be seen, as determined by the luminous intensity I of the light, the meteorological visibility V and the illuminance E at the eye of the observer. At this maximum distance, the illuminance E at the observer's eye is reduced to the value Et.
It is possible therefore to derive the intensity I, required for a given luminous range D at a given visibility V.
