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A zone plate consists of a set of radially symmetric rings, known as Fresnel zones, which alternate between opaque and transparent. Light hitting the zone plate will diffract around the opaque zones. The zones can be spaced so that the diffracted light constructively interferes at the desired focus, creating an image there. Zone plates produce equivalent diffraction patterns no matter whether the central disk is opaque or transparent.
To get constructive interference at the focus, the zones should switch from opaque to transparent at radii where
where n is an integer, λ is the wavelength of the light the zone plate is meant to focus and f is the distance from the center of the zone plate to the focus.
For plates with many zones, you can calculate the distance to the focus if you only know the radius of the outermost zone and its width,
In order to get complete constructive interference at the focus, the amplitude of the diffracted light waves from each zone in the zone plate must be the same. This means that for an evenly illuminated zone plate, the area of each zone is equal.
With more zones, in order for the area to stay constant, the width of each zone must decrease. The maximum possible resolution of a zone plate is given by the Rayleigh criterion as a function of the smallest zone width, 1.22 Δ rN. Therefore, the resolution is limited by how small you can make your zones reliably.
Zone plates can be manufactured using lithographyPhotolithography is a process used in semiconductor device fabrication to transfer a pattern from a photomask (also called reticle) to the surface of a wafer or substrate. It bears a similarity to the conventional lithography used in printing. Lithography. As lithography technology improves and the size of features that can be manufactured decreases, the possible resolution of zone plates manufactured this way can improve.
Unlike a standard lens, a zone plate produces subsidiary intensity maxima along the axis of the plate at odd fractions of the focal length (f/3, f/5, f/7, etc.), though these are less intense than the principal focus.
There are many wavelengths of light outside of the visible area of the spectrumThe noun spectrum (plural: spectra has a variety of meanings. Ghosts Originally a spectrum was what is now called a spectre, i. a phantom or apparition. Spectral evidence is testimony about what was done by spectres of persons not present physically, or h where traditional lensA lens is: a part of the eye an optical device that may be used in a camera or in a telescope; see lens (optics). the genus of the lentil a science-fictional alien device in E. Smith's Lensman series, which gives its bearer telepathic and other abilities. materials like glass are not transparentIn optics, transparency is the property of being transparent or allowing light to pass. The opposite property is opacity . Though transparency usually refers to visible light in common usage, it can actually refer to any type of radiation. For example, fl, and so lenses are more difficult to manufacture for light such as X-rayIn the NATO phonetic alphabet, X-ray represents the letter X. Rontgen An X-ray is a form of electromagnetic radiation with a wavelength approximately in the range of 5 pm to 10 nanometers (corresponding to frequencies in the range 30 PHz to 60 EHz). X-rays. Zone plates solve this problem by eliminating the need for finding transparent, refractive, easy-to-manufacture materials for every region of the spectrum.