Variable Pressure or Low Vacuum scanning electron microscopy (LVSEM)
This type of machine is basically like a conventional SEM but has the advantage in low vacuum (LV) mode that the pressure can be adjusted in the sample chamber until the artefact of "electron charging" is removed from images. This charging artefact is the result of electrons from the electron beam building up in a nonconductive sample. The extra electrons then jump from the sample unpredictably, causing lines and streaks on the image. Alternatively the unpredictable electron discharge repels the beam, causing jumps in the image or the appearance of black patches.
This means LVSEM can be used to image the surface of non-conductive samples (no metal needs to be added to the surface of such samples). It is particularly useful for viewing polymers, biological samples, and museum samples that cannot be changed in any way, particulate samples, and geological materials. Imaging uses backscattered electrons (BSE).
Backscattered electron imaging (BSE) of nonconductive, uncoated samples can provide information about composition via the contrast of the image: whiter regions have a higher average atomic number than darker regions.
The LV mode can also be used to freeze-dry samples for viewing. The sample is placed on a conventional SEM mount, plunged into liquid nitrogen and then placed on the SEM machine stage. The chamber is pumped free from air (evacuated) and the sample left for about 10 minutes by which time it is dry. The technique works best on hydrated samples that have some basic structural integrity, such as plant tissue.
It should be noted that while LV mode allows adjustment of the pressure within the sample chamber this is not to the high degree achieved in an Environmental SEM.