Introduction to numerical aperture of microscopes
Numerical aperture, abbreviated as NA, is the main technical parameter of the objective lens and condenser lens. It is an important symbol for judging the performance of the two (especially the objective lens). The numerical values are marked on the housings of the objective lens and condenser respectively.
Numerical aperture (NA) is the product of the refractive index (n) of the medium between the objective front lens and the object being inspected and the sine of half the aperture angle (u). The formula is expressed as follows: NA=nsinu/2
Aperture angle, also known as "lens angle", is the angle formed by the object point on the optical axis of the objective lens and the effective diameter of the front lens of the objective lens. The larger the aperture angle, the greater the light entering the objective lens. It is directly proportional to the effective diameter of the objective lens and inversely proportional to the distance from the focus.
When observing under a microscope, if you want to increase the NA value, the aperture angle cannot be increased. The only way is to increase the refractive index n value of the medium. Based on this principle, water immersion objective lenses and oil immersion objective lenses are produced. Since the refractive index n value of the medium is greater than 1, the NA value can be greater than 1.
The maximum value of numerical aperture is 1.4, which reaches the theoretical and technical limit. At present, bromonaphthalene with high refractive index is used as the medium. The refractive index of bromonaphthalene is 1.66, so the NA value can be greater than 1.4.
It must be pointed out here that in order to give full play to the numerical aperture of the objective lens, the NA value of the condenser should be equal to or slightly larger than the NA value of the objective lens when observing.
Numerical aperture is closely related to other technical parameters. It determines and affects almost all other technical parameters. It is directly proportional to the resolution, directly proportional to the magnification, and inversely proportional to the depth of focus. As the NA value increases, the width of the field of view and the working distance will decrease accordingly.
