Augustin Fresnel invented the graded lens used in lighthouses. In order to do this, he applied an idea which had already been developed by Buffon in 1850 and later commented on by Condorcet.

Fresnel had the idea of dividing the optic into concentric rings which would give an intensity equivalent to that of very large lenses, whilst at the same time removing the unnecessary parts of its thickness, which absorb light.

The first lens were used in 1819. It was made up of a collection of prisms glued onto a mirror. Each prism defracted and helped to return the beam following the same axis. Tests carried out in September 1821 and August 1922 demonstrated the indisputable superiority of the new lens. By comparison to the parabolic reflectors, the intensities were almost four times greater.

On 20th July 1823, the first optic composed of ringed elements replaced Cordouan’s reflecting device. Its eight lens gathered and horizontally returned a luminous beam coming from a large lamp with a 4-wick mechanism. The optic turned slowly over eight minutes, driven by a clock with weights made by the clockmaker Wagner.

Initially, due to the difficulties encountered in the manufacture of the glass and polishing of the layers, the optics were made up of distinct and different parts:

• the central dioptrical part formed by 80cm high panels,
• upper and lower mirrors which returned the furthest rays.
  This principle was applied until 1850.

From 1826, Augustin Fresnel undertook the manufacture of catadioptric ring optics, which used the whole reflection of the rays in the prisms in order to gather the furthest beams. An initial project was set up by Fresnel with the lighting of the Saint Martin canal in Paris in 1826, but he was unable to finalise it due to his death.

From 1848, the technical processes involved in glass grinding and machining on very large polishing lathes would allow the production of the catadioptric rings for large lighthouses. The first equipment of this kind was constructed in 1843 for the lighthouse at Gravelines (fixed light). In 1852, the lighthouse at Ailly would house the first revolving and flashing equipment of First Classification grading. The development of lenses in lighthouses is now complete, giving them their finished form.

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