corrugated lens

A corrugated lens is an optical element with its surface deliberately grooved or stepped in concentric rings, reducing mass while preserving focal power. Instead of grinding a solid lens to its final shape, material is removed in alternating bands, leaving a ridged pattern that resembles a phonograph record or corrugated cardboard when viewed edge-on. This geometry works because light bends at the lens surfaces; removing material from the body does not significantly degrade performance if the working surface itself remains intact.

The physics hinges on Fresnel's principle: a lens can be approximated as a series of thin annular zones, each contributing to the focusing effect. A corrugated lens makes this approximation literal, cutting away the non-essential bulk between zones. Typical designs retain the outer perimeter and central zone at full thickness, while intermediate rings are thinned or removed entirely. The step height and radial spacing are calculated during optical design to match the phase delay of an equivalent solid lens across the wavelength band in use.

These lenses appear commonly in infrared and long-wavelength systems where solid optics become prohibitively heavy or expensive. A large thermal imaging lens, infrared projection optics, or microwave horn-fed reflector may use a corrugated front element to cut weight without a disproportionate cost in performance. They also feature in some high-power laser systems where thermal load and alignment sensitivity demand minimal mass. Visible-light applications are rarer because human-eye systems tolerate small aberrations less, and the manufacturing cost advantage shrinks when solid optics are already small and light.

Manufacture and limits

Corrugated lenses are ground and polished like conventional optics, but the stepped surface demands tighter positional tolerances on each ring. Any radial error in the groove positions or step heights introduces phase errors that accumulate across the aperture, degrading contrast and off-axis image quality. The manufacturing process is more labor-intensive than pressing a simple lens, so the design is chosen only when weight reduction justifies the cost or when material scarcity demands it.

The grooved surface also scatters light at visible wavelengths more than a polished surface does, and dust or scratches lodge in the grooves, making cleaning difficult. In infrared, where surface roughness scatters less and coatings are thicker, these drawbacks are minor. The name "corrugated" refers directly to the fluted appearance; the term is used in optics, antenna design, and some mechanical engineering contexts, always describing a repetitive ridge-and-valley profile imposed to reduce weight or cost without abandoning function.