H26: Magnification

Q: How do you make the light even brighter?

You use a combination of lenses and prisms because prisms help you collect more light that would be otherwise lost around the edges of the lens. Thus, the light is even brighter and more efficient.

Lighthouses frequently magnify the light from the source to give extra brightness. The principle is simply to collect light that would otherwise be wasted (remember, light is emitted from a source in all directions) and concentrate it into a bright beam or parallel rays of light. Let's suppose we have a light source emitting 100 watts every second. It could be that only 5% of it (5 watts) travels off in the direction of the mariner's eye. If we can gather all 100 watts into a single beam and focus that beam in the direction of the mariner, then we could have a light that was 20 times brighter and could be seen at a greater distance.

To achieve this, optics are used which are built from complex arrangements of lenses and prisms. However, the first method used involved highly polished metal reflectors, in the shape of a parabola. You will have seen this often in such places as electric fires, car headlights and battery torch lights. Copper was frequently used for these reflectors because, being more corrosion resistant than iron-based alloys, it is easy to keep clean and shiny.

When glass lenses and prisms are used, rather different principles are involved. The polished and shaped pieces of glass can be used to change the direction of the light rays, lenses changing the direction by a little and prisms changing it by a lot. Of course, any material transparent to light could be used, but traditionally glass has been the material of choice. The two main properties involved are reflection and refraction of light. In simple terms, we can think of the light passing through the material and finding atoms and molecules of the solid "in the way". This results in both a slowing down and a change of direction of the light rays.

There are some really complicated (and very beautiful) devices built to make lighthouses work as well as possible. They are big, heavy assemblies of glass lenses, prisms and metal frames to hold it all together. This is generally called an optic. If you have a system that only uses reflection (usually the older parabolic metal reflector type), it is called catoptric. If a system uses refraction only it is called dioptric. The most common modern system (because it is generally the best) is called catadioptric.There are some really complicated (and very beautiful) devices built to make lighthouses work as well as possible. They are big, heavy assemblies of glass lenses, prisms and metal frames to hold it all together. This is generally called an optic. If you have a system that only uses reflection (usually the older parabolic metal reflector type), it is called catoptric. If a system uses refraction only it is called dioptric. The most common modern system (because it is generally the best) is called catadioptric.

Q: Where do you find the brightest lights?

Naturally, the size of the whole system (in combination with the brightness of the light source) determines the overall brightness and the largest optics were installed in lighthouses called landfall lights. These were lighthouses located at the extremities of the major ocean crossings. Thus, a ship that had been crossing oceans and major seas for many days would catch first sight of a landfall light and this was a very popular and comforting thing for both sailors and passengers to see. The greatest brightnesses and visible ranges were called for here. Examples of landfall lights are Bishop Rock (England), Cape Race (Newfoundland) and Cape Ottway (Australia).

Q: Is there a method of classifying the different sizes of optics?

Yes. It is called the order of the light. The biggest lights are called hyper-radial, but from then on, in decreasing size, it goes from first order to second, third, fourth ... through to seventh order.

Q: How do you go about designing these systems?

The design of optics is based on relatively simple physics but, in reality, is extremely difficult and specialised. There are few people capable of professional optic design today. The first designs were by a Frenchman call Augustin Fresnel. His methods and designs were used all over the world and are still in use today.