Lighthouses may seem like quaint artifacts of a bygone era and now mostly used as tourist attractions, but as marine-based commerce expanded in the 18^th and 19^th centuries, they were indispensable for ship navigation and guidance to an extent we probably can’t imagine (this was before the gyrocompass, radar, and GPS world, of course). As a result, there was a need for ever-more-powerful lighthouses that could cast their beams for tens of miles, a need which drove the development of increasingly powerful light sources and their “projectors.”

Lighthouses were so vital to marine navigation that they became common structures (Figure 1). By the early-to-middle part of the 1800s, there were thousands of them along coastlines, both for navigation/location and to indicate to ships of where the often-rocky shore — a dangerous place for a sailing ship — was located.

In the earliest seafaring era thousands of years ago, before formal lighthouses were built, simple wood fires were used on mountaintops. Over the centuries, the idea of shore-based beacons was upgraded, with towers constructed at key locations, use of coal- and then oil-fired lamps for much brighter and consistent illumination, the addition of reflectors positioned behind the light, and eventually a lens in front to direct and focus the beam. The on-site “lighthouse keeper” has a vital role and lifetime job. A mechanism was added to rotate the reflector and lens at a known rate, giving each lighthouse a unique “signature” so sailors could determine which one they were seeing.

But there was one problem: the optical efficiency of the standard light and its focusing lens was low, so the resultant brightness and useable range were also low. It’s not a matter of height versus the curvature of the Earth; it’s a matter of intensity versus distance. To reach further, brighter lamps were needed along with larger, more efficient lens assemblies to capture the light and direct it. The largest lenses in were about 2 meters in diameter, about a meter thick at the center, and weighed about 1000 kg. Even at these dimensions, they didn’t do a very good job, and they had reached the limits of fabrication technology.

Even if a bigger lens could be made, there were many undesirable consequences. The cost was prohibitive, transporting it to the site and hoisting it up was difficult, the lens thickness was too great for enclosure against the weather, and having such a large, heavy assembly at the top of a long, skinny tower was a construction challenge (it could even be unstable). There were some attempts to cast a large lens in subsections and then glue these together at the factory or on-site, but this only solved some of the problems, and that idea was abandoned.

Part 2 will look at how Fresnel addresses the problem within the technical and materials limitations of this time period.

EE World Online References

Flat-pack lens boosts solar power
Minuscule, Flexible Compound Lenses Magnify Large Fields Of View
Novel Nanotechnology Technique Makes Table-top Production Of Flat Optics A Reality
Enable Machine Learning with an Advanced Motion Detector Using PIR Sensors
Physicists Create Star Trek-Style Holograms

Related Content and References:

1. “A Short Bright Flash: Augustin Fresnel and the Birth of the Modern Lighthouse,” Theresa Levitt, W.W. Norton & Co., 2013
2. “Fundamentals of Physics,” Halliday and Resnick, John Wiley & Sons, Inc.
3. Encyclopædia Britannica , “Augustin-Jean Fresnel”
4. Wikipedia, “Fresnel lens”
5. “Fresnel Lens
6. Hyperphysics/Georgia State University, ” Fresnel Lens”