Originally developed for lighthouses, the design enables the construction of lenses of large aperture and short focal length without the weight and volume of material which would be required in conventional lens design. Compared to earlier lenses, the Fresnel lens is much thinner, thus passing more light and allowing lighthouses to be visible over much longer distances.
Dioptric Lens Diagram from the book A Few Notes on Modern Lighthouse Practice, Chance Brothers and Co, 1910.
Fresnel's lighthouse lenses originally fell into six orders based on their focal length. The order of a Fresnel lens is approximately the dioptre or optical power of the lens. The dioptre is the reciprocal of the focal length of the lens in meters. A Fresnel lens with a focal length of 50 cm or 0.5 m would be classified as a third order lens.
Lens Order Diagram from the book A Few Notes on Modern Lighthouse Practice, Chance Brothers and Co, 1910.
Lighthouse Apparatus are divided into various “orders,” as follows, according to their Focal Distance, that is, half the diameter of the Apparatus on the Focal Plane:
|0 Hyper-radial||1,330 mm|
|0.5 Meso-radial||1,125 mm|
|1st Order||920 mm|
|2nd Order||700 mm|
|3rd Order||500 mm|
|3rd Middle Order||375 mm|
|4th Order||250 mm|
|5th Order||187.5 mm|
|6th Order||150 mm|
From 1851, Chance Brothers became a major lighthouse engineering company, producing optical components, machinery, and other equipment for lighthouses around the world.
James Timmins Chance
Sir James Timmins Chance pioneered placing lighthouse lamps inside a cage surrounded by fresnel lenses so as to increase the available light output; these cages, known as optics, revolutionised lighthouse design. Another important innovation from Chance Brothers was the introduction of rotating optics, allowing adjacent lighthouses to be distinguished from each other by the number of times per revolution that the light flashes.
Lighthouse optical lenses were commonly made from 2 distinct glass groups, crown and flint glass, crown glass commonly used up to the 19 century.
Crown glass is a type of optical glass used in lenses and other optical components. It has relatively low refractive index (≈1.52) and low dispersion. Crown glass is produced from alkali-lime (RCH) silicates containing approximately 10% potassium oxide and is one of the earliest low dispersion glasses.
As well as the specific material named crown glass, there are other optical glasses with similar properties that are also called crown glasses. Generally, this is any glass with Abbe numbers in the range 50 to 85. For example, the borosilicate glass is an extremely common crown glass, used in precision lenses. Borosilicates contain about 10% boric oxide, have good optical and mechanical characteristics, and are resistant to chemical and environmental damage. Other additives used in crown glasses include zinc oxide, phosphorus pentoxide, barium oxide, fluorite and lanthanum oxide.
Flint glass is optical glass that has relatively high refractive index and low Abbe number. Flint glasses are arbitrarily defined as having an Abbe number of 50 to 55 or less. The currently known flint glasses have refractive indices ranging between 1.45 and 2.00.
The term flint derives from the flint nodules found in the chalk deposits of southeast England that were used as a source of high purity silica by George Ravenscroft, circa 1662, to produce a potash lead glass that was the precursor to English lead crystal.
Pharology A Study of Lighthouses by Dr Ken Trethewey
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A website containing the collected writings of Dr Ken Trethewey
Author of Lighthouses - Race to Illuminate the World
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