numerical aperture

noun

Microscopy.

1. a measure of the resolving power of a microscope, equal to the index of refraction of the medium in which the object is placed multiplied by the sine of the angle made with the axis by the most oblique ray entering the instrument, the resolving power increasing as the product increases. N.A.

Word History and Origins

Origin of numerical aperture¹

First recorded in 1875–80

Example Sentences

Although better sectioning is possible by using a higher illumination numerical aperture, this creates a shorter depth of focus that reduces the system's usable field of view.

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Such imaging is “fabulous for very tiny tissue volumes”, McConnell says, but it cannot be applied to large specimens — such as late-stage mouse embryos — “because of the low numerical aperture of a low-magnification lens” that would be needed for such large samples.

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If we let u represent one half the angular aperture of an objective, represented in the diagram by D'BN in the case of the immersion, and n the index of refraction of the medium interposed between the cover-glass and the objective we have in the formula n sin. u a mathematical expression of the optic power of the various systems of lenses, or in other words for what is designated the numerical aperture.

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It is generally assumed that a good objective requires an illuminating cone equivalent to two-thirds of its numerical aperture.

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