Zone plate

A zone plate is a device used to focus light or other things exhibiting wave character.

[1] Unlike lenses or curved mirrors, zone plates use diffraction instead of refraction or reflection.

The zone plate's focusing ability is an extension of the Arago spot phenomenon caused by diffraction from an opaque disc.

The zones can be spaced so that the diffracted light constructively interferes at the desired focus, creating an image there.

To get constructive interference at the focus, the zones should switch from opaque to transparent at radii where[3]

Because of this, the smallest size object you can image, Δl, is limited by how small you can reliably make your zones.

Although these contain less energy (counts of the spot) than the principal focus (because it is wider), they have the same maximum intensity (counts/m2).

However, if the zone plate is constructed so that the opacity varies in a gradual, sinusoidal manner, the resulting diffraction causes only a single focal point to be formed.

This type of zone plate pattern is the equivalent of a transmission hologram of a converging lens.

This reference phase can be chosen to optimize secondary properties such as side lobes.

[1] There are many wavelengths of light outside of the visible area of the electromagnetic spectrum where traditional lens materials like glass are not transparent, and so lenses are more difficult to manufacture.

X-rays, for example, are only weakly refracted by glass or other materials, and so require a different technique for focusing.

Zone plates eliminate the need for finding transparent, refractive, easy-to-manufacture materials for every region of the spectrum.

[1] Zone plates are also used in photography in place of a lens or pinhole for a glowing, soft-focus image.

The result is that the effective f-number of a zone plate is lower than for the corresponding pinhole and the exposure time can be decreased.

Common f-numbers for a pinhole camera range from f/150 to f/200 or higher, whereas zone plates are frequently f/40 and lower.

This makes hand held shots feasible at the higher ISO settings available with newer DSLR cameras.

Zone plates have been proposed as a cheap alternative to more expensive optical sights or targeting lasers.

[5] Zone plates may be used as imaging lenses with a single focus as long as the type of grating used is sinusoidal in nature.

A specifically designed Fresnel zone plate with blazed phase structures is sometimes called a kinoform.

It also allows an appropriately patterned Fresnel reflector to be mounted flush to the side of a building, avoiding the wind loading that a paraboloid would be subject to.