Key Parameters of Optical Microscopes：Numerical Aperture (NA)

The numerical aperture (NA) is a dimensionless number that measures the ability of an optical system to collect light. It specifically describes the size of the light cone accepted by the lens.

The numerical aperture is an important indicator of the performance of the objective lens and condenser. Its value directly affects key parameters of the microscope, such as resolution, contrast, and brightness.

Calculation Formula:

NA = n × sin α

Where:

· n is the refractive index of the medium between the lens and the specimen.

· α is half of the aperture angle (the angle formed by a point on the object on the optical axis and the effective diameter of the front lens of the objective).

The numerical aperture is influenced by several factors:

1.Objective Lens Structure and Design: The maximum half-angle of view, design precision, and type of objective lens affect the NA. Generally, a larger maximum half-angle, higher structural precision, and more advanced design result in a larger NA.

2.Medium Refractive Index: A higher refractive index of the medium between the lens and the specimen leads to a larger NA.

3.Light Source Wavelength: A shorter wavelength of the light source typically results in a smaller NA.

4.Optical Characteristics: Optical properties of the objective lens, such as spherical aberration, refraction, and chromatic aberration, also affect the NA. Good optical characteristics ensure a higher NA.

5.Observation Object: The NA is also related to the shape and size of the observed object. When the objective lens can image the entire shape of the object, its minimum NA can reflect the size and shape of the object.