.The resolution of an optical imaging system — a microscope, telescope, or camera — can be limited by factors such as imperfections in the lenses or misalignment. However, there is a fundamental maximum to the resolution of any optical system which is due to diffraction. An optical system with the ability to produce images with angular resolution as good as the instrument's theoretical limit is said to be diffraction limited.[1]
The resolution of a given instrument is proportional to the size of its objective, and inversely proportional to the wavelength of the light being observed. For telescope with circular apertures, the size of the smallest feature in an image that is diffraction limited is the size of the Airy disc.
In astronomy, a diffraction-limited observation is one that is limited only by the optical power of the instrument used. However, most observations from earth are seeing-limited due to atmospheric effects. Optical telescopes on the Earth work at a much lower resolution than the diffraction limit because of the distortion introduced by the passage of light through several kilometres of turbulent atmosphere. Some advanced observatories have recently started using adaptive optics technology, resulting in greater image resolution for faint targets, but it is still difficult to reach the diffraction limit using adaptive optics.
Radiotelescopes are frequently diffraction-limited, because the wavelengths they use (from millimeters to meters) are so long that the atmospheric distortion is negligible. Space-based telescopes (such as the HST, or a number of non-optical telescopes) always work at their diffraction limit, if their design is free of optical aberration.
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Obtaining higher resolution
There are techniques for producing images that appear to have higher resolution than allowed by simple use of diffraction-limited optics.[2] Although these techniques improve some aspect of resolution, they generally come at an enormous increase in cost and complexity. Usually the technique is only appropriate for a small subset of imaging problems or are just badly named. An example is finding the center of sparsely-distributed blobs on a two-dimensional surface. Although the location is located exactly, the resolution is hardly improved. For those tracking protein motion in a membrane or examining the motion of fluorescently-labeled myosin on a slide, the improvement in spatial resolution is useful. But the image itself does not have more resolution, so it is a trick that applies in certain situations.
Techniques such as speckle imaging can be used to obtain diffraction-limited images of bright objects.
For a given numerical aperture (NA), the resolution of microscopy for flat objects under coherent illumination can be improved using interferometric microscopy. Using the partial images from a holographic recording of the distribution of the complex optical field, the large aperture image can be reconstructed numerically.[3]
Also, through the use of metamaterials, a superlens may be constructed and the diffraction limit is no longer the limit/constraint.
Other waves
The same equations apply to other wave based sensors, such as radar and the human ears.[4]
References
- ^ Born, Max; Emil Wolf (1997). Principles of Optics. Cambridge University Press. ISBN 0521639212.
- ^ Niek van Hulst (2009). "Many photons get more out of diffraction". Optics & Photonics Focus 4 (1). http://www.opfocus.org/index.php?topic=story&v=4&s=1.
- ^ Y.Kuznetsova; A.Neumann, S.R.Brueck (2007). "Imaging interferometric microscopy–approaching the linear systems limits of optical resolution". Optics Express 15: 6651–6663. doi:. http://www.opticsexpress.org/abstract.cfm?id=134719.
- ^ How We Localize Sound
External links
- Puts, Erwin (September 2003). "Chapter 3: 180 mm and 280 mm lenses" (PDF). Leica R-Lenses. Leica Camera. http://en.leica-camera.com/assets/file/download.php?filename=file_1864.pdf. Describes the Leica APO-Telyt-R 280mm f/4, a diffraction-limited photographic lens.
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