Principle of Operation
A conventional convex lens bends light at its two curved surfaces. The glass in between contributes nothing optically; it merely holds the surfaces apart. As a lens grows, that dead volume grows with the cube of its dimensions, so a large lens quickly becomes too heavy to mount, too thick to cast without flaws, and too slow to cool evenly.
The insight behind the design is that only the surface curvature matters. If the lens is divided into concentric annular zones and each zone is collapsed towards a flat plane while keeping its own local curvature, the assembly refracts light almost as the original solid lens would, at a small fraction of the thickness and mass.
The result is a stepped profile of prismatic rings. Outer rings deviate light through larger angles than inner ones, so the design captures a wide cone of light from a source near the focus and redirects it into a narrow beam. In a lighthouse installation the beam is collimated horizontally, which is where a mariner is, and light that would otherwise have been thrown into the sky or the ground is recovered.
The trade-off is image quality. The discontinuities between rings scatter light and the stepped surface cannot form a sharp image, which is irrelevant for a beacon and disqualifying for a camera lens.
Invention and Early Deployment
The design is named for Augustin-Jean Fresnel, the French civil engineer and physicist who developed it for the Commission des Phares in the 1820s. Fresnel was already known for the wave theory of light, and the lens was an applied consequence of that work rather than a lucky piece of tinkering. Earlier proposals for stepped lenses had been sketched by the Comte de Buffon and by Sir David Brewster, but Fresnel produced a manufacturable design and had it built.
The first installation was at the Cordouan light at the mouth of the Gironde estuary in 1823. Contemporary reports credited it with a dramatic increase in range over the parabolic reflectors then in use, because reflectors returned only the light that happened to strike them and absorbed a significant share of that.
Adoption was rapid but uneven. France and Britain re-equipped their major stations over the following decades. The United States, whose lighthouse administration was at the time notoriously parsimonious, persisted with inferior reflector systems for years, a lag that became a subject of congressional inquiry before the Lighthouse Board was reorganised in 1852.
Orders and Classification
Installations were graded by order, a classification based on focal length. A first-order lens, with a focal distance of about 920 millimetres, stood several metres tall, weighed several tonnes, and was reserved for major landfall lights that had to be seen from far offshore. Sixth-order lenses, small enough to carry, marked harbour entrances and river channels.
Assemblies were built as a drum of refracting rings around the focal plane with reflecting prisms above and below to recover light escaping at steep angles. Rotating the whole assembly produced a flashing characteristic; a stationary drum with vertical panels produced a fixed light. Rotation initially ran on chariot wheels driven by clockwork and falling weights, and later floated the assembly on a mercury bath, which nearly eliminated friction at the cost of a serious occupational hazard for the keepers who maintained it.
Because each station could be given a distinct flash interval, the classification system turned individual lights into an identifiable network rather than a set of anonymous glows.
Later Applications
Glass Fresnel assemblies were displaced in navigation by compact sealed-beam and LED optics, and surviving lenses are now largely museum pieces, some of considerable value. The principle, however, outlived the artefact.
Moulded acrylic made thin Fresnel surfaces cheap, and they are now ubiquitous in places where light must be gathered or spread but no image is required: the condenser in an overhead projector, the fly-eye arrays in theatrical and film lighting, the wide-angle patch stuck to the rear window of a van, the collimator in front of a traffic signal, and the lens over the pyroelectric sensor in a domestic motion detector, which divides its field of view into fan-shaped zones so that a moving body crosses them in sequence.
Solar concentrators use large Fresnel sheets to focus sunlight onto photovoltaic cells or thermal receivers, returning the design to the problem of collecting as much light as possible with as little material as possible, which is what Fresnel set out to solve.