Transmission Electron Microscope. Developed earlier than the scanning electron microscope (SEM) and differs in a number of fundamental ways. First, while the SEM uses an electron beam to excite secondary electrons from the sample's surface, the TEM sends electrons directly through the sample. Second, while the SEM uses the information gained from the detected secondary electrons to produce an image on a computer monitor, the image viewed using a TEM is in essence the sample's shadow. Electrons that pass through the sample brighten the fluorescent viewing screen, while the screen remains darker beneath parts of the sample which do not yield as easily to electrons.