How to Judge the Quality of the Lens Assemblies Used in Microscopes?
A hollow metal cylinder with several axes on the same straight line (i.e. optical axis) of a microscope, or a metal film with a circular hole in the center arranged in parallel with an Olympus microscope, is an electrode with an axisymmetric structure. When a certain voltage is applied to them, an axisymmetric static electric field can be generated. This type of electric field can focus the microscope electrons for imaging, hence it is called an electrostatic microscope. There are various types of electrostatic lenses. If characterized by the potential distribution generated on the axis of symmetry after applying voltage, Olympus can roughly divide microscopes into four categories:
The Olympus potential on the left and right axes of a single potential lens microscope is constant and equal in value; the potential on the left and right axes of an immersion lens microscope is constant but unequal in value. The effect of a single aperture lens (also known as a diaphragm lens) is concentrated near the circular aperture of the diaphragm. The axial electric field strength of the two microscopes is constant. A single circular aperture membrane is not very useful, but it can often be used 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 (i.e. the image side) of the Nikon lens is constant. Except for the fourth category, the other three types of electrically operated lenses can be regarded as microscopes, and their object side focal lengths and image side focal lengths can be derived accordingly. The focal length formulas for the first and second types of lenses are:
It is not difficult to prove that the focal length in single potential lenses and immersion lenses is always positive, which means that these Nikon microscopes are always convergent. A single aperture lens can form a diverging lens, and the performance of an immersion objective needs to be analyzed based on specific circumstances. A typical example of a drum 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.
