I tried to research a list of different types, based on the physical principle used to make an image. Of course, one could also classify the microscopes based on their area of application, their cost, their versatility or any other aspect. These classification systems do have a problem: In this case one one type of microscope can be allocated to several groups, and the system becomes “messy”.
Optical Microscopes: These microscopes use visible light (or UV light in the case of fluorescence microscopy) to make an image. The light is refracted with optical lenses. The first microscopes that were invented belong to this category. The price of optical microscopes varies from very cheap to nearly unfordable (for the private person, at least). Optical microscopes can be further subdivided into several categories:
- Compound Microscope: These microscopes are composed of two lens systems, an objective and an ocular (eye piece). The maximum useful magnification of a compound microscope is about 1000x.
- Stereo Microscope (dissecting microscope): These microscopes magnify up to about maximum 100x and supply a 3-dimensional view of the specimen. They are useful for observing opaque objects.
- Confocal Laser scanning microscope: Unlike compound and stereo microscopes, these devices are reserved for research organizations. They are able to scan a sample also in depth. A computer is then able to assemble the data to make a 3D image.
X-ray Microscope: As the name suggests, these microscopes use a beam of x-rays to create an image. Due to the small wavelength, the image resolution is higher than in optical microscopes. The maximum useful magnification is therefore also higher and is between the optical microscopes and electron microscopes. One advantage of x-ray microscopes over electron microscopes is, that it is possible to observe living cells.
Scanning acoustic microscope (SAM): These devices use focused sound waves to generate an image. They are used in materials science to detect small cracks or tensions in materials. SAMs can also be used in biology where they help to uncover tensions, stress and elasticity inside biological structure.
Scanning Helium Ion Microscope (SHIM or HeIM): As the name suggests, these devices use a beam of Helium ions to generate an image. There are several advantages to electron microscopes, one being that the sample is left mostly intact (due to the low energy requirements) and that it provides a high resolution. It is a relatively new technology and the first commercial systems were released in 2007.
Neutron Microscope: These microscopes are still in an experimental stage. They have a high resolution and may offer better contrast than other forms of microscopy.
Electron Microscopes: Modern electron microscopes can magnify up to 2 million times. This is possible, because the wavelength of high energy electrons is very small. At the same time, the high energy electrons are pretty tough on the sample being observed. It may take a long time to completely dehydrate and prepare the specimen. Some biological specimens also need to be coated with a very thin layer of a metal before they can be observed.
- Transmission electron microscopy (TEM): In this case, the electron beam is passed through the sample. The result is a two dimensional image.
- Scanning electron microscopy (SEM): Here the electron beam is projected on the sample. The electrons do not go through the sample but bounce off. This way it is possible to visualize the surface structure of the specimen. The image appears 3 dimensional.
Scanning Probe Microscopes: It is possible to visualize individual atoms with these microscopes. The image of the atom is computer-generated, however. A small tip measures the surface structure of the sample by rastering over the surface. If an atom projects out of the surface, then a higher electrical current will flow through the tip. The amount of current is proportional to the height of the structure. A computer will then assemble the position data of the tip and the current to generate an image.
Conclusion: Microscopes can be classified based on the physical principle that is used to generate an image. Different microscopes visualize different physical characteristics of the sample (eg. elasticity can be visualized with acoustic microscopes). Image contrast, resolution (which determines magnification) and destructiveness of the sample are other relevant parameters.