Compositions and Classifications of Metallographic Microscopes
Numerical aperture, abbreviated as NA, is the main technical parameter of objective lenses and focusing lenses, and is an important indicator for judging the performance of both. The magnitude of its value is marked on the casing of the objective lens and the condenser lens, respectively.
Numerical aperture (NA) is the product of the refractive index (h) and half of the aperture angle (u) of the medium between the front lens of the objective lens and the object being inspected. The formula is as follows: NA=hsinu/2
Aperture angle, also known as "mirror angle", is the angle formed between 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 brighter the light entering the objective lens, which is proportional to the effective diameter of the objective lens and inversely proportional to the distance from the focal point.
When observing under a microscope, if you want to increase the NA value, the aperture angle cannot be increased. The solution is to increase the refractive index h value of the medium. Based on this principle, water immersion and oil immersion lenses have been developed, as the refractive index h of the medium is greater than one, the NA value can be greater than one.
The maximum numerical aperture value is 1.4, which has reached its limit both theoretically and technically. At present, bromonaphthalene with high refractive index is used as the medium, and its refractive index is 1.66, so the NA value can be greater than 1.4.
The numerical aperture is closely related to other technical parameters, and it almost determines and affects other technical parameters. It is proportional to resolution, proportional to magnification, and inversely proportional to depth of focus. As the NA value increases, the field of view width and working distance will correspondingly decrease.
