Five parameters of microscope objective lens
Numerical Aperture (NA)
The sine of the half-vertex angle of the largest cone that meridian rays can enter or leave the fiber core (optical system or hanging optical device) is multiplied by the refractive index of the medium where the cone top is located. The numerical aperture is used to judge the performance of the objective lens (resolution, focal depth, brightness). etc.) important index.
Numerical aperture is also called lens opening ratio, abbreviated as NA. It is the product of the refractive index (n) of the medium between the object and the objective lens and the sine value of half the objective lens aperture angle (θ\2). Its size is determined by the following formula: NA=n×sinθ/2. Beijing Zhongxian Hengye Instrument Co., Ltd.
The abbreviation of numerical aperture is NA (the abbreviation of numerical aperture of Zeiss microscope is CF). Numerical aperture is the main technical parameter of the objective lens and condenser lens. It is used to judge the performance of the two (especially for the objective lens) (that is, the ability to eliminate positional chromatic aberration. Zeiss' numerical aperture The hole is an important symbol representing the ability to eliminate positional chromatic aberration and magnification chromatic aberration). The numerical values are marked on the housings of the objective lens and condenser respectively.
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. One way is to increase the dielectric ratio n value. Based on this principle, water immersion objective lenses and oil immersion objective lenses have been 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.
Relationship with other parameters:
Numerical aperture is an important parameter of a microscope objective lens and determines the resolution of the objective lens. It is directly related to the magnification, working distance and depth of field of the objective lens.
Generally speaking, it is directly proportional to the resolution, directly proportional to the magnification, and the focal depth is inversely proportional to the square of the numerical aperture. As the NA value increases, the field of view width and working distance will decrease accordingly.
Common misunderstandings:
Numerical aperture is proportional to resolution, but this does not mean that when selecting an objective lens, you must choose the largest numerical aperture (NA), because the objective lens also has many other important parameters, such as fluorescence transmittance and working distance. Wait, it's best to choose based on your own experiments.
