WILLIAM & DEBORAH HILLYARD

Travel -

Lighthouses

Lighthouse Lighting

Essentially, all early US lighthouses utilized versions of the Lewis Lamp, which was a poor copy of the Swiss Argand light.  Eventually, these were replaced by Fresnel lenses of various powers, or "orders", until the use of aerobeacons like the early DCB-224.  More recently, the Vega VRB-25 (see below) rotating beacon has been used extensively.  This lamp provides visibility up to 22 nautical miles (nearly 42 km) with a 100 watt bulb!  The future is likely to reside with LED lamps like the Vega VLB-44 and VLB-48 or Pharos FA-250 lamps. 

There is an excellent history of lighthouse illumination at the "Seeing the Light" website.  The largest Fresnel lens ever put in working lighthouses was the "hyperradiant" which towered more than 12 feet tall; even larger than the first order
This diagram below shows how a Fresnel lens ("A") works to concentrate light from a lamp (oil lamp "L" here) into a beam.  Augustin-Jean Fresnel invented the lens in 1822.  It increased the light projecting power enormously without the weight penalty of a conventional lens of its size.  Also shown in the diagram are mirror strips (labeled "m" to "n") mounted above and below the lens that gather more of the lamp's light.  The diagram to the right shows a typical installation; click the image to see a real 4th order Fresnel lens. 
Many lighthouses, including the Highland, have been upgraded to use aerobeacons, and this is a typical model; the Vega VRB-25, made in New Zealand by Vega Industries.  It is much smaller and lighter than traditional lights.  Generally, an array of six acrylic Fresnel lenses are located around a bulb. 

For flashing lights, the lenses rotate driven by a microprocessor controlled motor.  For simple, single flashes, the array rotates at an appropriate rate; for example, rotating at 1 rpm, six flashes are created every minute or one every ten seconds.  Some lights produce more complex sequences, so some of the lenses are blanked off, and an array with eight lenses is also available for even more complex arrangements.  Incandescent bulbs are not suited to being switched on and off, so the revolving prisms are a necessity.  Six bulbs are housed in a mechanism that changes the bulb automatically in the event of failure. 

The VLB-44 and VLB-48 are more recent versions that use LED bulbs that do not deteriorate with constant switching, so the revolving prisms are not required, making the whole lamp even simpler.  Depending on the vertical spread chosen, and the number of units stacked (up to eight), they can cover ranges up to 15 nautical miles (over 25 km).  It seems likely that they will replace the VRB-25 over time. 
VRB-25 Aerobeacon
Fresnel Lens
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