Allard's Law

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A formula relating the illuminance produced on a normal surface at a given distance from a point source of light, the intensity of the light and the degree of transparency of the atmosphere (assumed uniform).

The law may be expressed as follows


where EIlluminanceEnergy is the resulting illuminance on this surface

Icharge currentIntensity is the intensity of the incident light in the direction of measurement.

TTime periodroll periodTransmissivity is the atmospheric transmission factor (assumed uniform) for specified Unit: distance.

x is the distance of the surface from the light source, measured in the specified units of distance.

Note: Another form of Allard'ssecond Law is


where VVolumeVoltVisibility = meteorological visibility (2-1-280 and 2-1-290, Note:).

Please note that this is the term as it stands in the original IALAInternational Association of Marine Aids to Navigation and Lighthouse Authorities - AISM Dictionary edition (1970-1989)