Part 2 looked at the background to Fresnel along with the principle and implementation of his innovative lens. This part extends that investigation by looking at closely related innovations which he developed to maximize the system performance.
Improving it further
The lens was only part of his solution since the optics needs a near-ideal “point source” for efficiency. Fresnel did tests which showed that multiple small wicks produced more light than a single large one for the same amount of fuel, but the multi-wick source was too large to be a point-like source. He then worked with other engineers to develop an innovative circular multi-wick design, which was smaller yet brighter than a single large wick or a conventional multi-wick assembly.
After many delays and setbacks in procurement and fabrication, he had his first test model, a single square panel 60 cm on edge, made of 97 pieces of glass laced into a bulls-eye configuration, with a fairly short focal length of about 70 cm (a telescope of the period had a focal length of several meters).
His next step was a lens assembly of multiple panels, arranged in a circle-like pattern. The first public tests were on February 13, 1821, and his lens was estimated by judges to be 20 times brighter than a mirror-backed unit of the same size (Figure 1). Even so, this efficiency was not enough for Fresnel.
Due to light escaping through the top and bottom of the assembly, a lens only captured about half the source’s emissions. Some designs added prisms above and below to capture and re-direct the light, but these could not capture light at more than 45⁰ off axis. Others tried mirrors to try to capture and direct the light to the main lens, but much was still wasted. Fresnel then used another optical technique, which he knew from his studies of light as a wave: total internal reflection. He sized and arranged prisms oriented for internal reflection around the top and bottom assembly, and was able to capture and redirect light to the lens with almost no loss (Figure 2).
Despite the delays in producing the first lenses, the advantages of Fresnel’s approach were so dramatic and obvious that it quickly became the standard. Within a decade of the first demonstration, all new lighthouses in France and much of Europe employed it, while many existing lighthouses were retrofitted. The technology spread worldwide within a few years. (One estimate is that there were tens of thousands of lighthouses of all sizes situated worldwide, they were that important.)
There were other outcomes, as well. The need for such a high volume of his lenses drove advances in glassmaking materials, technology, casting, and assembly. It also drove a “standards” program, as Fresnel developed a formal four-grade classification system for lenses and sizes so that each group could use a common set of drawings, tooling, and techniques: “first order” (2-meter diameter) for lighthouses positioned for ships coming from the open sea; “second order” (140 cm) for near to shore; “third order” (50 cm) for river entries and bays; and “fourth order” (30 cm) for harbors. Each increase in “order” was also matched with a smaller, simpler lamp assembly for further standardization.
Part 4 will show how while lighthouses are no longer needed and are functionally obsolete. The Fresnel lens has found new roles in both low-cost, low-end magnifiers as well as high-performance lenses for some of our modern, sophisticated products.
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Related Content and References:
- “A Short Bright Flash: Augustin Fresnel and the Birth of the Modern Lighthouse,” Theresa Levitt, W.W. Norton & Co., 2013
- “Fundamentals of Physics,” Halliday and Resnick, John Wiley & Sons, Inc.
- Encyclopædia Britannica , “Augustin-Jean Fresnel”
- Wikipedia, “Fresnel lens”
- “Fresnel Lens
- Hyperphysics/Georgia State University, ” Fresnel Lens”