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.