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Diffraction-limited system
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===Extending numerical aperture=== The effective resolution of a microscope can be improved by illuminating from the side. In conventional microscopes such as bright-field or [[Differential_interference_contrast_microscopy|differential interference contrast]], this is achieved by using a condenser. Under spatially incoherent conditions, the image is understood as a composite of images illuminated from each point on the condenser, each of which covers a different portion of the object's spatial frequencies.<ref>{{cite journal |first=Norbert |last=Streibl |title=Three-dimensional imaging by a microscope |journal=Journal of the Optical Society of America A |volume=2 |issue=2 |date=February 1985 |pages=121–127 |doi=10.1364/JOSAA.2.000121 |bibcode=1985JOSAA...2..121S}}</ref> This effectively improves the resolution by, at most, a factor of two. Simultaneously illuminating from all angles (fully open condenser) drives down interferometric contrast. In conventional microscopes, the maximum resolution (fully open condenser, at N = 1) is rarely used. Further, under partially coherent conditions, the recorded image is often non-linear with object's scattering potential—especially when looking at non-self-luminous (non-fluorescent) objects.<ref>{{cite journal |first1=C.J.R. |last1=Sheppard |author-link1=Colin Sheppard |first2=X.Q. |last2=Mao |title=Three-dimensional imaging in a microscope |journal=Journal of the Optical Society of America A |volume=6 |issue=9 |date=September 1989 |pages=1260–1269 |doi=10.1364/JOSAA.6.001260 |bibcode=1989JOSAA...6.1260S }}</ref> To boost contrast, and sometimes to linearize the system, unconventional microscopes (with [[Structured light|structured illumination]]) synthesize the condenser illumination by acquiring a sequence of images with known illumination parameters. Typically, these images are composited to form a single image with data covering a larger portion of the object's spatial frequencies when compared to using a fully closed condenser (which is also rarely used). Another technique, [[4Pi microscope|4Pi microscopy]], uses two opposing objectives to double the effective numerical aperture, effectively halving the diffraction limit, by collecting the forward and backward scattered light. When imaging a transparent sample, with a combination of incoherent or structured illumination, as well as collecting both forward, and backward scattered light it is possible to image the complete [[Ewald's sphere|scattering sphere]]. Unlike methods relying on [[Super-resolution microscopy#Localization microscopy|localization]], such systems are still limited by the diffraction limit of the illumination (condenser) and collection optics (objective), although in practice they can provide substantial resolution improvements compared to conventional methods.
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