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Pharology - How Lighthouses Work - H21

21: Intensity Reduction

In the late nineteenth and early twentieth centuries as technology was continuously improving and enhancing the scope for ever brighter and more powerful lights, it was common practice to do just that, i.e. commensurate with appropriate budgeting, to build a lighthouse with the brightest light possible and visible for the greatest possible distance. In the end of the twentieth century, this has become less common and the brightness of many lighthouses has been actually reduced.

For example, the brightest light in the UK for many years was at the Lizard lighthouse with an intensity of 3 million candlepower. Indeed, this light was considered to be brighter than it needed to be and when a new light source was installed, a saving of energy was obtained by reducing the intensity of the light source so that today, even though this is still a landfall lighthouse, its intensity is only x candlepower. This has occurred at many other lighthouses.

The original light at the Lizard was so bright that it could be seen well beyond the horizon. how is this possible? Well, on a clear night you could see the glare of the beams from over the horizon. The glare is caused by the refraction of light by water vapour in the atmosphere which bends the light and makes it visible over the horizon. In any case, if a light of 250,000 candle power can be seen for 25 miles, why use a light any more powerful than that?

This type of approach is what comes from more careful engineering design and has impacts in other areas too. Instead of the biggest, brightest, tallest, etc, as was common in old engineering designs (just because it was possible or because the engineer wanted to be famous), recent decades have seen the refinement of designs where economics has played an ever increasing part. After all, accepting that modern shipping does require a system of lights for navigational purposes, does it require those lights to be seen at such great distances when they now have so many other forms of navigation equipment, not least of which is GPS?