How to Evaluate the Quality of Lens Groups Used in Microscopes
Hollow metal cylinders of a microscope where several axes lie on the same straight line (i.e., the optical axis), or metal diaphragms with a central round hole arranged in parallel in Olympus microscopes, are electrodes with an axially symmetric structure. When a certain voltage is applied to them, an electrostatic field with axial symmetry is generated. This electric field can focus and image electrons in the microscope, so it is called an electrostatic microscope. There are many different types of electrostatic lenses.
Based on the potential distribution along the symmetry axis after voltage application, Olympus microscopes can be roughly divided into four categories:
Unipotential lens: The axial potential on both sides of the microscope is constant and equal in value.
Immersion lens: The axial potential on both sides of the microscope is constant but different in value.
Single-aperture lens (also called hole lens): The lens effect is concentrated near the circular hole of the diaphragm. The axial electric field intensity on both sides of the microscope is constant. A single circular diaphragm is of little use on its own, but it can often serve as a component of some complex lenses.
Immersion objective: The object emitting electrons is directly immersed in the electric field of this type of microscope, while the axial potential on the other side (image side) of the Nikon lens is constant.
Except for Category 4, the other three types of electrostatic lenses can be regarded as microscope lenses, from which the relationships between their object-side focal length and image-side focal length can be derived.The focal length formulas for Category 1 and 2 lenses are as follows:
It can be easily proved that the focal lengths in unipotential lenses and immersion lenses are always positive, which means these Nikon microscopes are always converging. A single-aperture lens, however, can be a diverging lens. The performance of an immersion objective must be analyzed according to specific conditions.
A typical example is the electron gun used in Nikon microscopes and other electron beam tubes. Its function is to form an electron beam with a certain cross-sectional shape and current intensity.
