numeric aperture

nu·mer·ic ap·er·ture (N.A.),

defined by the formula n sine a, where n is the refractive index of the medium between the object and objective lens and a is the angle between the central and the marginal rays entering the objective.
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With the addition of the Bioptechs Objective Heater system for high numeric aperture applications, the FCS2 has become the prime micro-environmental system for long-term time-lapse studies, such as those Latimer's lab performs.