numerical aperture

noun
nu·​mer·​i·​cal aperture | \ n(y)u̇-ˈmer-i-kəl- How to pronounce numerical aperture (audio) \

Medical Definition of numerical aperture

: a number that indicates the resolving power of a microscope objective and is equal to the product of the index of refraction of the medium (as immersion oil) in front of the objective and the sine of half the angle formed by two rays emanating from the object upon which the microscope is focused and entering the objective at either end of a diameter of the objective

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