How to Judge the Quality of Lens Assemblies Used in Microscopes
Hollow metal cylinders whose axes coincide with the optical axis of the microscope, or metal diaphragms with a central circular hole arranged in parallel (as used in certain microscopes), constitute electrodes with an axisymmetric structure. When a certain voltage is applied to these electrodes, an axisymmetric static electric field is generated. This electrostatic field can focus electrons to form images, hence the name electrostatic electron microscope. There are many types of electrostatic lenses. Classified by the potential distribution generated on the symmetry axis after voltage application, they can be roughly divided into four categories:
Unipotential lens: The potentials on the axis on both the left and right sides of the lens are constant and equal in magnitude. Immersion lens: The potentials on the axis on both sides are constant but of different magnitudes. Single-aperture lens (also known as diaphragm lens): Its optical effect is concentrated near the central hole of the diaphragm. The electric field strength on the axis on either side of the lens is constant. A standalone aperture diaphragm has limited practical use, yet it often serves as a component of more complex lens systems.
Immersion objective lens: The object from which electrons are emitted is immersed directly within the electric field of this lens, while the axial potential on the opposite (image-side) side of the lens remains constant. Except for the fourth type, the other three kinds of electrostatic lenses can be treated as microscope lenses, from which their object-side and image-side focal lengths can be derived. The focal length formulas apply to the first two lens types.
It can be readily proven that the focal lengths of unipotential lenses and immersion lenses are always positive, which means these lenses are always converging. Single-aperture lenses, by contrast, may act as diverging lenses. The performance of immersion objective lenses must be analyzed case by case. A typical application is the electron gun used in electron microscopes and other electron beam instruments, which functions to form an electron beam with a specified cross‑sectional profile and current intensity.
