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Pharology - How Lighthouses Work - H23

H23: The Optic (Often Called the Lens)

A lighthouse optic (above) is an assembly of lenses and prisms. It is used to gather as much light from the light source and send it out of the lighthouse in a beam with a characteristic flashing sequence. The photo below shows a panel of an optic. It is roughly circular with a single lens at its centre and is surrounded by complete and broken rings of prisms. The panel is designed to create a single beam of light from the light source. This optic actually has four panels, all similar (the other three cannot be seen) and arranged in a square geometry, so when the optic is rotated it will give out four flashes during the period of rotation.

The image above approximates to the panel in the photograph. On the left is the front elevation of the panel and on the right is the section through the panel, showing how the lens and prisms draw the light into a beam. For more on this see the later section "How do lenses and prisms work".

The image above shows a magnificent two-tier optic installed at Hartland Point, Devon, UK in the 1970s. Such an optic had two light sources, one for the upper tier and one for the lower tier. Similar optics were used at Eddystone and Bishop Rock until they were replaced with smaller ones during modernisation programmes.

Catadioptric Systems

The Catadioptric System is any combination of catoptric and dioptric, that is, it uses both refraction and reflection to bend the rays of light in the desired direction. These two processes may take place in a single element, for instance, in a reflecting prism as illustrated in the Figure, where refraction takes place at a and b and reflection at c. [Chance Bros and Co, 1910]

Horizontal divergence governs the amount of time for which a flash is visible. A narrow divergence would result in a quick flash, for example. Divergence is thus an important factor in optic design. Halving the height of an illuminant or doubling the focal distance of the optical apparatus halves the vertical divergence. If the illuminant is narrow horizontal divergence will be small and the flash may be too short in duration.

A Scots 21 inch reflector would decrease in luminosity from 350 times along the axis to 35 times at an angle of 8 degrees. When such reflectors were set in a circle in a lantern, to give a fixed light, it was considered reasonable to supply about seven reflectors for each 90 degrees of azimuth or horizon to be lighted. Though this arrangement of reflectors did not produce a light that was of equal power in all directions, in practice it proved satisfactory. The light from one reflector was reinforced by the light diverging from the sides of its neighbours and direct from their lamps, so that with the 21 ins reflectors the light transmitted in various directions would vary from about 1,100 to 2,450 candles.

Fresnel devised seven orders of optic. A first order optic was very large, produced a bright powerful light and was intended for use in the most strategic locations on the coast. A sixth order optic was smallest (there was an optic of order three and a half) and intended for harbours. In both Fixed and Flashing Lights the refractors are numbered from No. 1 upwards, starting with the belt or bull's eye as No. 1, and working outwards. The reflecting prisms start in the same way with the innermost prism as No. 1, and working outwards above and below. See Fig. 4, which gives a vertical section of each of the different "orders" of Apparatus, and shows the number of refractors and reflecting prisms, top and bottom, which are usually employed to cover the full vertical angle adopted in each case. [Chance Bros and Co, 1910]

The lenticular apparatus may be thus described :-It consists of a central and powerful lamp, of course emitting luminous beams in every direction. Around this is placed an arrangement of glass, so formed as to refract these beams into parallel rays in the required directions. [Findlay, 1862, Ch. III]

In 1822, Augustin Fresnel perfected a lens system that was to revolutionise the entire design and construction of lighthouse optics. He created a structure made of lenses and prisms that looked like a giant bee-hive of glass. this complex combination collected all rays of light emanating from a light source at the centre of the optic and produced a bright narrow sheet of light from its centre. This powerful apparatus being in the centre of the surrounding lenticular system, the ray impinging upon each lens is refracted into a series of parallel, or nearly parallel beams, whose section is the figure of the lens, in the ease of the revolving light, or into a continuous zone or band of light around the horizon in the fixed light. [Findlay, 1862, Ch. III]

"Nothing can be more beautiful," says Mr. Alan Stevenson, "than an entire apparatus for a fixed light of the first order. It consists of a central belt of refractors, forming a hollow cylinder, 6 feet in diameter and 30 inches high; below it are six triangular rings of glass ranged in a cylindrical form, and above a crown of thirteen rings of glass, forming by their union a hollow cage composed of polished glass, 10 feet high and. 6 feet in diameter. I know of no work of art more beautiful or creditable to the boldness, ardour, intelligence, and zeal of the artist." [Findlay, 1862, Ch. III]

In another method of producing this effect, constructed by M. Letourneau, the necessity for using two lenses is avoided; and, consequently, the loss of light inevitable in the absorption of a portion in its passage through the glass. The adjoining diagram will explain it. In the central portion of the apparatus B is one of the polyzonal lenses, similar to those figured on page 21; on either side of this is a portion of a fixed light apparatus, shown by the horizontal belts A A. For a fixed light, of course, these horizontal belts are carried all round; and the light appears as a vertical stripe of the breadth of the flame from the top to the bottom of the belt. In the polyzonal lens the light appears to cover its whole surface, and is only visible when in front. The whole apparatus is made to revolve by machinery, and the appearance is as above described: first, the fixed light from the portions on either side; then a short eclipse due to the light being diverted by the great lens; then the full blaze of the lens for 8 or 10 seconds; then another eclipse, and so on. [Findlay, 1862, Ch. III]

Divergence in the vertical plane is critical. If there were no vertical plane divergence a beam emitted from a lighthouse on a high cliff would travel over the head of the mariner on a ship. Insufficient vertical divergence in the design of optics has, in the past, led to numerous shipwrecks because the light was not visible to the seafarer.

One type of design of first order catadioptric is the bi-form optic where two similar polyzonal lens systems are mounted, one over the other. Such a system has been installed at Hartland Point for many years.

The Dioptric System is one in which the light is refracted or bent by a glass agent in the direction required. Such agent is termed a refracting prism or refractor. [Chance Bros and Co, 1910]