How Do You Make a Huge Lens Without the Weight? Throw Away the Middle

Light changes direction when it crosses the boundary between air and glass and travels straight in between, so the bending a lens performs is entirely determined by the angles of its two surfaces at each point. The thickness between those surfaces does nothing optically and only adds weight, absorbs light and makes the piece harder to cast and cool without flaws. A conventional large lens is therefore mostly dead material. Dividing the surface into concentric rings and sliding each ring back so that all of them lie in one plane preserves every surface angle and removes almost all the bulk, which is the whole design.

The advantages are large and the drawbacks are specific:

• •A fraction of the weight and material of an equivalent lens
• •Large apertures become practical where a solid lens would be impossible
• •Thin even sections cool without the stresses that crack thick castings
• •The rings introduce visible steps, so image quality is poor
• •Light scatters at every ring edge, reducing contrast
• •Excellent for gathering and directing light, unsuited to fine imaging

Augustin-Jean Fresnel developed the design for lighthouses in the 1820s, and the problem he was solving was severe. A light source at the focus of a reflector sends out only the fraction of its light that the reflector catches, and metal reflectors of the period absorbed much of it. A lens large enough to capture a useful cone would have been a metre or more across and unliftable, and casting glass that thick without flaws was beyond the technology. His arrangement made such an aperture possible, and the improvement was dramatic, with a well-equipped light visible more than thirty kilometres away. The designs added prisms above and below the central rings that catch light at steep angles and bend it into the beam by reflecting inside the glass.

The same reasoning applies to curved mirrors and produces a device most people have stood in front of without noticing. A parabolic reflector large enough to concentrate sunlight would be heavy and expensive, but cutting its surface into concentric grooved rings and flattening them gives a reflector that focuses nearly as well at a fraction of the depth, which is how some solar cookers and lightweight searchlights are built. Sheets of this kind are also used in theatre and studio lighting to soften and spread a beam. The trade-off is the same, with light scattering at every step, so the devices concentrate energy effectively and would form a poor image.

The design is far more common today than in its original application, since lighthouses have largely gone over to small powerful lamps. Vehicle headlights and rear lamps use moulded plastic versions to shape the beam. Overhead projectors used a large one under the writing surface. Traffic lights use them to widen the visible angle. Camera focusing screens carry a fine one. Rear window panels on some vehicles act as a wide-angle viewer. Solar concentrators use large plastic ones to focus sunlight onto a small cell. Cheap plastic sheets sold as page magnifiers are the same thing. Mass moulding in plastic is what made all of this possible, since the fine ring structure is easy to mould and difficult to grind.

Only the surface angles of a lens bend light, so cutting the surface into concentric rings and collapsing them into one plane keeps the optics and discards almost all the glass. Fresnel developed it for lighthouses in the 1820s, where a solid lens of the needed aperture could not be cast or lifted. The ring edges scatter light, so the design gathers and directs well and images badly.