Biomedical Engineering Reference
In-Depth Information
Magnifi cation, brightness and contrast in the SEM
The scan coils are the heart of the modern SEM that allow the instrument
to produce the highly detailed, life-like images that we have become accus-
tomed to seeing in the scientifi c literature. The scan coils are found in the
vicinity of the objective lens which is closest to the specimen surface. They
selectively divert the electron beam, scanning it across the specimen's sur-
face in a vertical pattern of horizontal lines. While the specimen is being
scanned, the viewing or visual cathode ray tube (CRT) is being scanned by
the same horizontal and vertical scan signals. The signals produced by the
beam-specimen interactions at each specimen point are detected by the var-
ious detectors positioned around the specimen chamber of the instrument.
The signals are amplifi ed and used to vary the intensity of each portion of
the image on the CRT in a one-to-one correspondence between a point on
the specimen and a point on the CRT. The control of the signal amplifi cation
results in the ability to manipulate both the brightness and contrast seen in
the fi nal viewed image. The dimensions of the scan on the CRT are fi xed
while the size of the scan on the specimen can be varied. The manipulation
of this relationship results in magnifi cation in the SEM. Magnifi cation is
the ratio between the dimensions of the CRT scan and dimensions of the
specimen scan. Magnifi cation can be increased or decreased by increasing
or decreasing the size of the specimen scan: a smaller raster pattern on the
specimen surface results in increased magnifi cation.
Resolution in the SEM
The resolution obtained using the SEM is dependent on the accelerating
voltage or KV and the probe current/diameter. Theoretically, the higher the
accelerating voltage, the smaller the diameter of the electron probe. Finer
surface structure images can be generally be obtained with lower KV. At
higher KV, >10 KV, the penetration teardrop becomes larger, resulting in
unnecessary signal being generated within the sample. These signals reduce
the image contrast and obscure fi ne surface structures. To obtain images with
the optimum surface structural detail for low concentration samples (bio-
logicals and biomaterials), observe the specimens using low KV (0.5-7KV).
The smaller the electron probe diameter impinging upon the specimen
surface, the higher the magnifi cation and resolution that can be attained.
The image smoothness which is related to the signal-to-noise ratio depends
on probe current: as the probe current is reduced, the probe diameter is
reduced, but the signal-to-noise ratio is also reduced, resulting in less image
smoothness. Overall, resolution increases with the decrease in probe diam-
eter. To get optimum image quality it is necessary to select a probe current
suited for the magnifi cation, experimental conditions and specimen used
during a particular study.
