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 Guide\Chance fig3.jpg
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.
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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 |
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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
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"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 Guide\Findlay finlyp24.bmp
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.
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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 systemhas been installed at Hartland Point for many years. |
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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 |
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The French Authorities have installed several "twin" apparatus (Appareils jumeaux). These, instead of being superimposed, are placed alongside each other on a revolving iron plate, and are so arranged that their panels are parallel, and therefore send out parallel beams. Apparatus of comparatively small type only are used, and need very careful adjustment. Modern improvements have so increased the candle-power obtainable from a single apparatus, that the Bi-form and Twin types are not now often required. Chance Bros and Co, 1910 |
A modern optical apparatus consists of glass refracting and reflecting prisms, set in gun-metal framework. Each of these prisms has to be accurately adjusted, one at a time, so that the best light from the burner shall be sent to the horizon. Upon this adjustment depends the efficiency of the apparatus, and though every prism may be ground with absolute accuracy if they are not correctly adjusted, the whole utility of the Light is gone. It will therefore be understood that this adjustment is necessarily rather a lengthy process. We have given special attention to the fresh problems in this direction, which the general adoption of incandescent mantles has raised, and are confident our methods give as efficient a result as is possible. After the prisms have been adjusted as above, the optical apparatus is erected on its pedestal, and if of the revolving type, is accurately balanced, so that it floats evenly on the mercury, that is, so that the ring (j) to which it is attached, and therefore the focal plane, is absolutely horizontal. Chance Bros and Co, 1910
The apparatus for Sector Lights have of course to be specially designed for the particular conditions they have to fulfil, and the angles they have to cover, and it is therefore practically impossible to move them from their positions and employ them elsewhere, or to alter the angle over which they show, without considerable alteration, probably necessitating the re-setting of the glass in the frames. This is not the case with ordinary Lighthouse apparatus, for instance, a Double Flashing Apparatus may be moved from one Lighthouse to another without any difficulty. Chance Bros and Co, 1910
Mercury is a silvery liquid metal, most commonly used in thermometers and barometers. Mercury has an extremely high density and, being a liquid, it has been possible to design reservoirs filled with mercury so that a heavy optic floats in the liquid metal. In so doing, a bearing with a very low resitance due to friction is created and the optic can rotate easily with little consumption of energy. Mercury vapour is extremely toxic.
The case of a Hyper-radial Light of "Chance" design recently supplied by us to Cape Race, Newfoundland, is a still more striking example of the great weights which can be revolved at high speeds when supported on a mercury float. The figures in this case were: Weight of revolving parts (lenses with table, etc.) about 7 tons (7,112 kilos); Speed of revolution, 30 seconds, producing a flash of 0.3 of a second every 7 1/2 seconds Chance Bros and Co, 1910
1819 In 1819, 21 inch and 25 inch parabolic reflectors with an Argand lamp flame of 1.5 ins height and 1 ins diameter gave vertical and horizontal divergences sufficient for the needs of navigation, except from lighthouses that were particularly high above sea level. Owing to divergence the intensity of the light from a reflector faded off gradually at each side of the axis.
1822 Augustin Fresnel perfects his new lens design for use in lighthouse optics.
1860 A first order lenticular apparatus is one of the most beautiful objects in the world. It is a combination of elements, nearly 12 feet high and 6 feet in diameter, constructed with the utmost skill and refinement, and involving in its structure some of the highest principles of applied science. A first order light apparatus is 12 feet high and 6 feet in diameter; and the cost of the lenses alone varies from œ1,288 to œ1,536; or, with the cost of all apparatus, and light-room or lantern, œ2,488 to œ2,984. * These prices, which are common to nearly all manufacturers, are taken from the Tariff of Messrs. Chance, Brothers and Co., Birmingham (1860). Findlay, 1862, Ch. III
1860 A second order of light apparatus is 4 feet 7 inches in diameter; the lens costs from œ788 to œ1,131, or altogether, œ1,624 to œ2,187. * These prices, which are common to nearly all manufacturers, are taken from the Tariff of Messrs. Chance, Brothers and Co., Birmingham (1860). Findlay, 1862, Ch. III
1959 By the modern system of evaluating light intensity Smeaton's original twenty-four tallow candles are estimated to have produced something like 67 candle-power. The introduction in 1810 of Argand lamps and catoptric reflectors raised the strength of the lantern to 1,125 candle-power. In 1845 a non-revolving Fresnel dioptric frame-rather like a glass bee-hive made up of prisms and lenses-stepped it up to 3,216 candlepower. In 1872 a better lamp increased the candle-power figure to 7,325. Only ten years later Douglass's first apparatus jumped the figure to 79,250. In 1959 the same optic was producing a beam of 358,000 candle-power. The Eddystone light had come a long way. Majdalany, 1959, p199