Obviously, there are advantages to imaging using electrons instead of visible light. However, electrons have some limitations of their own. First of all, we can't see electrons, secondly, electrons will not freely travel through air - there are enough molecules in air to easily absorb an electron beam. Therefore, the electron source, lenses, and sample must all be under a vacuum. Another limitation is that since electrons are electrically charged, the sample needs to be conductive enough to dissipate this charge.

To image using electrons before the invention of the SEM, the Transmission Electron Microscope (TEM) was developed.

The Transmission Electron Microscope takes electrons from a source and through condenser and objective electromagentic lenses, focuses the beam on an area of the sample. If the sample is thin enough for the electrons to travel through, the projector lens will project an interference pattern, (or the image) onto a phosphorus screen below.

A TEM can have extremely high resolution. There are research instruments which can see atoms. However, a TEM has some limitations. First of all, because the electron beam has to travel through the sample, lengthy sample preparation is usually required to make the sample thin enough. Plus, since the beam is travelling though the sample, the sample bulk and not the surface is being imaged.

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