Numerical Aperture
Numerical Aperture is, in the case of microscope, a very important factor that determines spatial resolution.
NA is expressed by the tangent of the angle θthat is formulated by the radius of the pupil at the object and the absolute refractive index in the medium space as follow,
N. A. = n・sinθ
n: refractive index of medium between sample and objective lens
θ: angle between optical axis and the most exterior light beam incident to objective lens
To realize the high resolution on plane surface and shallow focal depth, that are the candid features of the confocal microscope, NA needs to be as large as possible.
For this to realize, either making θbig by using short focus length objective lens or making n larger by using immersion method. |
