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Resolution in the context of structural biology is the ability to distinguish the presence or absence of atoms or groups of atoms in a biomolecular structure. Usually, the structure originates from methods such as X-ray crystallography , electron crystallography , or cryo-electron microscopy .
Memorial in Jena, Germany to Ernst Karl Abbe, who approximated the diffraction limit of a microscope as = , where d is the resolvable feature size, λ is the wavelength of light, n is the index of refraction of the medium being imaged in, and θ (depicted as α in the inscription) is the half-angle subtended by the optical objective lens (representing the numerical aperture).
Thus, the resolution limit is usually around λ 0 /2 for conventional optical microscopy. [17] This treatment takes into account only the light diffracted into the far-field that propagates without any restrictions. NSOM makes use of evanescent or non propagating fields that exist only near the surface of the object.
The limit of resolution obtainable in a TEM may be described in several ways, and is typically referred to as the information limit of the microscope. One commonly used value [ citation needed ] is a cut-off value of the contrast transfer function , a function that is usually quoted in the frequency domain to define the reproduction of spatial ...
There is a diffraction-limited resolution depending on incident wavelength; in visible range, the resolution of optical microscopy is limited to approximately 0.2 micrometres (see: microscope) and the practical magnification limit to ~1500x. [13] Out-of-focus light from points outside the focal plane reduces image clarity. [14]
Stimulated emission depletion is a simple example of how higher resolution surpassing the diffraction limit is possible, but it has major limitations. STED is a fluorescence microscopy technique which uses a combination of light pulses to induce fluorescence in a small sub-population of fluorescent molecules in a sample.
The diffraction limit is a feature of conventional lenses and microscopes that limits the fineness of their resolution depending on the illumination wavelength and the numerical aperture (NA) of the objective lens. Many lens designs have been proposed that go beyond the diffraction limit in some way, but constraints and obstacles face each of them.
Super-resolution microscopy is a series of techniques in optical microscopy that allow such images to have resolutions higher than those imposed by the diffraction limit, [1] [2] which is due to the diffraction of light. [3]