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The Lights in Lighthouses

In this article we are going to look at what forms of lighting have been used and the different ways this light is then modified. We have two other articles looking at the working of a lighthouse that goes with this, Lighthouse Structure looks at what the parts of a lighthouse or light station are and some main features involved in planning or placing them, while The Development of Lighthouses looks at how lighthouses developed.

This article is in three sections, we look at the means of illumination, then the character of the light, what message or identification it gives, before looking at the optics that make this possible,

Even though it is a reasonable length article, it is still only an overview of this topic, the objective being to give an explanation of what happens and why.

I have bolded new terms when I first use them, so having read this if you want to get back to the position you can easily.

Point Lynas Lighthouse    Photo from Geograph

The Illumination

The first lights were beacon fires, lit on hillsides and later in towers or church towers, these burnt wood. The light was not constant and they produced a lot of smoke. In many cases the smoke rather than the fire was visible. Some wreckers built their own fires to drive boats onto rocks so they could be plundered.

Early lighthouses also used wood, and some wood fires continued in use until around 1800, but gradually they switched to coal, starting from around 1550 and developing faster in Northern Europe than elsewhere, which produced less smoke and gave a more consistent light. At that time a single lighthouse could consume 300 tons or more of coal a year. Given that many are not in easily accessible places just getting it to them presented a challenge.

In many cases this was on open top of the towers but sometimes with some shelter over. Some had pairs of towers such as the Lizard Lighthouse in Cornwall  or in the case of the Mumbles Lighthouse  in Glamorgan they had one fire above another, in both cases to differentiate one lighthouse from another.

Once in lantern rooms, the problem of sooting up of the glass and getting rid of fumes was a problem, but mariners preferred the light from coal to that of oil lamps or candles, and at that time a fully blazing coal fire was the best light available.

Others used whole collections of candles or small oil lamps together. A sort of candelabra approach.

A revolutionary development of the age in 1782, was the Argand lamp, invented by Swiss scientist, Aime Argand. This was an oil lamp that had a steady smokeless flame. This worked by having a circular wick and air going up the centre and outside, inside a glass chimney, allowing better combustion. Argand lamps with up to 10 concentric wicks, one inside another was developed that revolutionised lighthouse lighting. Initially these ran on fish oil, but vegetable oil and by 1860 mineral oil was in use. For over 100 years Argand lamps were the main source of lighthouse lighting, some lasting much longer.

In 1901 the vaporised oil burner was invented by Arthur Kitson, and later improved by David Hood and others. The light produced was 6 times the brightness of previous lights. This burner worked by burning vaporizing kerosene under pressure mixed with air through an incandescent mantle, similar to gas camping lights you an get now.

Gas produced from coal was used in a few lighthouses, most were too far from gas supplies so had their own gas production works. This was only practical in some locations.

Acetylene gas was easier to produce, being able to be produced onsite by mixing calcium carbide and water. A later process was developed that made it safe to make and store, leading to its wider use. Burnt with air through a mantle this produced a light as good as the argand lamp. Its main advantage that it was easier to control.

A range of developments occurred, mostly from the inventor of the system, Gustaf Dalen from Sweden, including a device to bring in a new mantle when one burnt out. Many of these devises worked from the pressure of the gas. He also invented the Aga Cooker.

Another device was the sun valve, this allowed the sun to turn on and off the gas. This had one black and one highly polished metal strip, that worked like a valve. This brought about the scope to automate some lighthouses, and they then only required an occasional visit to restock mantles and refill Acetylene supplies. It also opened up the scope for large numbers of small unmanned lighthouses to be deployed.

This, propane and other stored gas systems were in use up until recently.

From when oil lamps were in use, early electric lights were run, these were before light bulbs and involved a carbon arc lamp. The first place to use electricity was the South Foreland Lighthouses in Kent and the first lighthouse built for electricity was Souter Point Lighthouse in County Durham, which had its own generator, and this lighthouse although not is use, can be visited and the generator still runs toady. This did not generally catch on at this point, Acetylene was cheaper, more reliable and was easier to automate.

Electric light bulbs, or filament lamps came into general use in about the 1920's and many lighthouses eventually switched to this means, although when you read the details you often had a prime system as a fall back and still an Acetylene system as the final fall back if all else failed.

Sealed beam lamps and reflector systems are also used. With some smaller lighthouses and buoys the light is switched on and off by an electronic flasher and shines through a fixed drum-shaped lens.

More recently solar panels and wind driven generators, have been installed, and in some if not most  cases light output reduced, using more efficient electric lighting from recharged batteries.

Character of the Light

Initially it was just a single light, or two of them, from whatever source, the largest problem and often the need for more than one came about through an attempt to identify one light from another. At this point lights were fully on.

One solution was the occulting light, this involves bringing up a shade to cut the light for a short period, by using a clockwork mechanism and cams you could set up a pattern that was constantly repeated, giving the patterns of characters of lights.

Occulting has only short breaks in the light when longer breaks it was called eclipsing, and when the dark and light periods are equal its called isophase. You can have multiple signals then a constant period of light and this is called group occulting, flashing or eclipsing.

Rotating lights using optics had two advantages, the light in a beam could go further and the speed of the beam could be used to identify the light, plus with clever optics you could get the character of the light rather than a single beam. We look at this again under optics below.

Sector lights showing green and red angles allowed the replacements of a high and low light, with a single light to indicate a safe path. Later some also had lights set up so the rate of the flash increased or decreased as you moved away from the safe path.

To avoid confusion, by international agreement, there  is a limited number of defined flashing, group or other signals or characters of light in use.

When this all started there were not a lot of lights showing, while today we have a mass of lights. A flashing light however can easily be picked out from the rest.

Hartland Point Lighthouse     Photo by Steve Thoroughgood 

Optics

When they had a fire light it went in all directions, except downwards. Initially lantern rooms did not help much, a lot of the light was spread or lost up into the cap.

So as well as needing better light, as we have seen above, they also needed to make better use of the light they had.

Lighthouse optics really started with the argand lamp, which was the first reliable and predictable light form, with the first practical lighthouse system in 1777 developed by William Hutchinson from Liverpool. Known as the catoptric system, it comprised a parabolic reflector concentrating light into a beam. This was achieved by a large number of small pieces of silvered glass (mirrors), set in a plaster cast moulded to a parabolic shape. This lasted well. Next came polished metal versions that were brighter and cheaper to produce but deteriorated quickly. By 1820 the best reflectors were silver plated onto copper, but with 12 times as much silver as used on tableware of the time. These parabolic  mirrors increased the power of the lamp light by 400 times. They had to be regularly cleaned and the cleaning cloths were kept and processed to recover the silver they contained.

To get this to work, as you only had a beam, you needed to rotate the light, the first rotating light was in 1781. Initially seafarers did not like the change, saying they needed a steady light to take a bearing, but the increased brightness won them over, and changing the speed it rotated at gave the first light characters, as we saw above, for the first time allowing them to be sure which lighthouse it was.

The Fresnel catodioptic system was a step forward, The fresnal lens developed first in 1828 by Augustin Fresnel, from France, and was the first Dioptric system, is based on the refraction available through glass. This is effectively a series of prism rings around a bull's-eye, with triangular prisms above and below and more than one set of these around the light, allowing a number of beams,  that can vary, to be generated from one light. This is very heavy up to 5 tones, more than half of this being glass, and be up to 12 foot high, in the case of South Foreland Lighthouses, in Kent and Bishop Rock Lighthouse, in Cornwall, at that time, although there were a range of sizes smaller. Usually you find the unit floating in a cast iron container of mercury to remove friction, this could then be driven by a clockwork mechanism or by acetylene gas pressure. This is the arrangement we think of now, when thinking of the light unit inside a lighthouse, and what we will often see at the top of the climb up a tower. He also developed a barrel system based on the same idea, that sends light out in all directions but a small vertical height, and these were used in some bouys. Later with electric lamps the unit could be reduced in size to around 30 inches high and made of Perspex or plastic, allowing it to be driven by a small electric motor

Photo by Chris Allen from Bishop Rock Lighthouse i