Azone plate is a device used tofocus light or other things exhibiting wave character.^[1] Unlikelenses orcurved mirrors, zone plates usediffraction instead ofrefraction orreflection. Based on analysis by French physicistAugustin-Jean Fresnel, they are sometimes calledFresnel zone plates in his honor. The zone plate's focusing ability is an extension of theArago spot phenomenon caused by diffraction from an opaque disc.^[2]

A zone plate consists of a set of concentric rings, known asFresnel zones, which alternate between beingopaque andtransparent. Light hitting the zone plate will diffract around the opaque zones. The zones can be spaced so that the diffracted light constructivelyinterferes at the desired focus, creating animage there.

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

$${\displaystyle r_n=\sqrt{n\lambda f+\frac{1}{4}{n}^2{\lambda }^2}}$$

wheren is aninteger, λ is thewavelength of the light the zone plate is meant to focus andf is the distance from the center of the zone plate to the primary focus. When the zone plate is small compared to the focal length, this can be approximated as$${\displaystyle r_n\simeq \sqrt{n\lambda f}}$$

For plates with many zones, you can calculate the distance to the focus if you only know the radius of the outermost zone,r_N, and its width, Δr_N:$${\displaystyle f=\frac{2r_N\mathrm{\Delta }{r}_N}{\lambda }}$$

In the long focal length limit, the area of each zone is equal, because the width of the zones must decrease farther from the center. The maximum possibleresolution of a zone plate depends on the smallest zone width,$${\displaystyle \frac{\mathrm{\Delta }l}{\mathrm{\Delta }{r}_N}\approx 1.22}$$

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

Zone plates are frequently manufactured usinglithography. As lithography technology improves and the size of features that can be manufactured decreases, the possible resolution of zone plates manufactured with this technique can improve.

Unlike a standard lens, a binary zone plate produces intensity maxima along the axis of the plate at odd fractions of the primary focus(f/3,f/5,f/7, etc.). 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). At even fractions of the primary focus (f/2,f/4,f/6, etc.), the intensity on-axis is zero.^[4]

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 atransmission hologram of a converging lens.

For a smooth zone plate, the opacity (or transparency) at a point can be given by:$${\displaystyle \frac{1\pm \cos \left(k{r}^2\right)}{2}}$$

where${\displaystyle r}$ is the distance from the plate center, and${\displaystyle k}$ determines the plate's scale.^[5]

Binary zone plates use almost the same formula, however they depend only on the sign:$${\displaystyle \frac{1\pm \mathrm{sgn}\left(\cos \left(k{r}^2\right)\right)}{2}}$$

It does not matter to the constructive interference what the absolute phase is, but only that it is the same from each ring. So an arbitrary length can be added to all the paths$${\displaystyle r_n=\sqrt{(n+\alpha )\lambda f+\frac{1}{4}(n+\alpha {)}^2{\lambda }^2}}$$

This reference phase can be chosen to optimize secondary properties such as side lobes.^[1]

There are manywavelengths of light outside of the visible area of theelectromagnetic spectrum where traditionallens materials like glass are nottransparent, and so lenses are more difficult to manufacture. Likewise, there are many wavelengths for which there are no materials with arefractive index significantly differing from one.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 thespectrum. The same zone plate will focus light of many wavelengths to different foci, which means they can also be used to filter out unwanted wavelengths while focusing the light of interest.

Other waves such as sound waves and, due toquantum mechanics, matter waves can be focused in the same way. Wave plates have been used to focus beams of neutrons and helium atoms.^[1]

Zone plates are also used inphotography in place of alens orpinhole for a glowing, soft-focus image. One advantage over pinholes (aside from the unique, fuzzy look achieved with zone plates) is that the transparent area is larger than that of a comparable pinhole. The result is that the effectivef-number of a zone plate is lower than for the corresponding pinhole and theexposure time can be decreased. Commonf-numbers for a pinhole camera range fromf/150 tof/200 or higher, whereas zone plates are frequentlyf/40 and lower. This makes hand held shots feasible at the higher ISO settings available with newerDSLR cameras.

Zone plates have been proposed as a cheap alternative to more expensive optical sights or targeting lasers.^[6]

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 akinoform.^[7]

A zone plate used as a reflector will allow radio waves to be focused as if by a parabolic reflector. This allows the reflector to be flat, and so easier to make. 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.

A bitmap representation of a zone plate image may be used for testing various image processing algorithms, such as:

• Image interpolation and image resampling;^[8]
• Image filtering.^[9]

An open-source zone-plate image generator is available.^[10]

• Arago spot
• Diffraction grating
• Fresnel imager
• Fresnel lens
• Fresnel number
• Fresnel zone antenna
• Microform
• Photon sieve

• Magnetic Soft X-ray microscopy
• Making a photographic zone plateArchived 2020-02-10 at theWayback Machine
• Whiz Kid Technomagic Zone Plate DesignerArchived 2023-04-08 at theWayback Machine
• Examples of zone plate photographs
• "Telescope could focus light without a mirror or lens".New Scientist. 1 May 2008.
• Arndt Last."X-ray zone plates". Retrieved2019-11-21.

• Optical components
• Photography equipment

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• Short description matches Wikidata
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