What Is the Difference Between an Image Produced From a TEM and From a SEM?

TEM and SEM

• In a TEM, a thin slice of a sample sits in a special vacuum chamber. A beam of electrons passes through the sample and becomes magnified by magnetic fields acting as lenses. The electrons produce a glow on a phosphor screen, which scientists view directly or store as image data files. The scanning electron microscope uses a thin electron beam to scan the surface of an object. Electrons and other types of radiation scatter off the object̵7;s surface and are picked up by detectors. The detectors convert the scattered radiation into an image. In a scanning electron microscope, electrons do not pass through the sample under study.

Object Size

• Only very thin samples work for a TEM; a sample that is more than 1,000 angstroms or 100 nanometers thick blocks electrons and spoils the image Technicians prepare the samples by carefully slicing thin sections of larger objects. A SEM, on the other hand, images the surface of samples as large as the microscope̵7;s chamber allows; up to several centimeters.

Magnification

• The main advantage of an electron microscope is its magnifying power. An optical microscope̵7;s magnification tops out at about 1,000 times, beyond which point the relatively large wavelengths of light lose their clarity. A TEM̵7;s direct imaging method allows greater magnifying power than a SEM, though both microscopes are much more powerful than optical microscopes. A TEM is capable of magnifying images up to 500,000 times, whereas the SEM magnifies up to 200,000 times.

Depth of Field

• The depth of field of a microscope image has to do with how much of the sample stays in focus. A SEM produces images with better depth of field than a TEM. SEM images are famous for their nearly three-dimensional appearance. Features at various points of an object stay clear and well-defined. A TEM, by contrast, produces a more two-dimensional image with little depth.

Resolution

• A microscope's resolution is its ability to clearly show small details. Depth of field and resolution are trade-offs; though the SEM has better depth of field, its resolution is not as good as a TEM̵7;s. The surface electron scattering that produces the SEM̵7;s image limits its resolution. Specialized high-resolution versions of the TEM, called the HRTEM, can resolve individual atoms in some samples.