Concepts - introduction
The most important concept in scanning electron microscopy is the use of electrons. Much logically flows from this, such as that a vacuum is needed to generate the electron beam, that electrons are used for imaging, and that we need to understand the interactions of electron beam with the sample in order to interpret our images. The use of electrons also impacts on image resolution and image colour, and explains the 3D nature of the micrographs (photos). The second most important concept is that we are looking only at the surface of a sample, and penetrating only a small way into the sample with the electron beam. All these notions will be explored in more detail in the list of sections available under this component (Concepts in SEM). Specific terms can be explored at any time by using the Glossary.
Electron images are monochromatic (gray scale), not in colour. Electron wavelengths are much smaller than the visible light we see with our eyes, but the main reason SEM images are not coloured is that they are formed by electrons hitting a detector (see below). Any coloured images you see from a scanning electron microscope have been artificially coloured.
Why is there no colour?
Although electrons may have different wavelengths or energies, they do not have colour as we see it. Colours we see with our eyes correspond to different wavelengths of visible light, with violet being the shortest and red the longest. Secondary electron SEM images — the most common form of images produced by these machines — are effectively intensity maps of electrons collected by a detector. The intensity of brightness on the screen is in proportion to the number of electrons originally produced. SEM images are displayed as monochrome grayscale (or greyscale) digital images in which each pixel carries only intensity information in a shade of gray varying from black at the weakest intensity to white at the strongest. Sometimes these grayscale images can be post-processed to display false colour i.e. colourised grayscale.