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Lighthouses

see http://www.trinityhouse.co.uk/interactive/gallery/portland_bill.html

The distance at which a light of a given height may be seen depends on 3 factors:

1. Meteorological Visibility
2. Intensity (Brightness)
3. Geographic Range

see also Dipping distance of a lighthouse

The Actual Range at which a light will be visible at any time is the lesser of the Luminous Range and the Geographic Range. This is called the Computed Range of the light.

Nominal Range
Each light has a Nominal Range, which is the luminous range when the Meteorological Visibility is 10 Miles.

Visible Range
The distance at which a light may be seen is dependent on 3 factors:

Meteorological Visibility - The weather conditions at any time can have a dramatic effect on the visibility of a light. Thick fog can totally obscure a light, whilst thermal gradients in the atmosphere can give rise to lens effects which can make a light appear brighter than it should under normal conditions. Meteorologists classify the effect of weather conditions on visibility in terms of meteorological visibility, expressed in Sea Miles (M)

Intensity (Brightness) - The intensity of a light determines the maximum theoretical distance at which it can be seen, the brighter the light the further away it is visible. This is governed by the inverse square law, i.e. the light is reduced to one quarter the original intensity if the distance between it and the observer doubles. Some modern lights have light intensities measured in millions of candelas (lumens), with theoretical (luminous) ranges of 40-50M in perfectly clear conditions. Each light has a Nominal Range, which is the luminous range when the Meteorological Visibility is 10M. The navigator can use a Luminous Range table to convert the reported Meteorological Visibility and the Nominal Range of a light into the Luminous Range of the light at that time.

Geographic Range - The earth is a sphere, and so its surface is curved. This curvature limits the effective range of a light. There are two elements which dictate the geographic range of a light for a particular observer, the height of the light above sea level and the height of the observer. For a light of a particular height there is a fixed distance to the horizon beyond which an observer at sea level will not see the light, the higher the light the greater is this distance. For observers above sea level there is an additional range between the observer and the horizon in which the light might be visible with no obstruction by the earth. The sum of these ranges gives the Geographic Range of the light.

Computed Range

The actual range at which a light will be visible at any time is the lesser of the Luminous Range and the Geographic Range. This is called the Computed Range of the light.

Posted 21 November 2010