In the TEM, the condenser lenses focus (condense) the beam to a large spot which passes through the thin sample, forming an image in the objective lens which is then magnified by subsequent lenses (projector, intermediate and/or diffraction) and ultimately viewed on the view screen/digital camera.
In the SEM, the condenser lenses focus (condense) the beam to a very small spot which is then scanned over the surface of the sample a dot at a time, in a rectangular pattern called a raster. At each dot, the detector counts the number of electrons 'reflected' from the surface and displays that number as a brightness or intensity on the viewscreen. Each dot of the raster pattern has a matching dot in the raster pattern on the view screen
SEM configuration
Faraday cage-- highly positively charged cage around scintillator that attracts low energy SEs into scintillatorScintillator-- Yttrium-aluminum-garnet crystal that converts electrons hitting it into photons
Light tube-- A fiberoptic tube that transmits the photons from inside the microscope to outside the scope
Photomultiplier tube-- an amplifier that amplifies the photons created by the scintillator
View CRT-- the monitor the numbers of electrons counted by the PMTube at each raster spot is displayed. The more photons counted, the brighter the spot.
Images in the SEM are created mainly by the interaction of the primary electron beam and the surface of the sample. As the accelerating voltage increases, or if the composition of the sample includes only lighter elements, then the size of the interaction volume, the Monte Carlo configuration, increases in size and depth. Conversely, if the sample is composed of heavier elements or the accelerating voltage is decreased, then the interaction volume decreases in size. If the kV is kept low and the penetration volume is small, then secondary electrons come from very close to the surface and carry more surface information that electrons that come from deeper in the sample.
As the electrons come from a larger area of the Monte Carlo configuration, then the effective size of the spot from which secondary electrons are collected becomes larger. Since resolution in the SEM comes from the size of the electron spot, then resolution decreases as the electrons are collected from deeper in the sample. For example, the backscatter images have lower resolution than do secondary electron images.
We have also discussed the fact that increasing the working distance (the distance from the lens to the sample) decreases the effective cone of illumination on the sample. As the angle of the cone gets smaller, the depth of field gets larger. The same effect is caused by using a smaller aperture in the objective lens. This too causes the cone of illumination to decrease in size, resulting in a greater depth of field..