Physical Principles of Electron Microscopy: An Introduction to TEM ...

TEM Specimens and Images 105
as seen in Fig. 4-10. However, the shadows appear dark in a photographic
negative or if the contrast is reversed in an electronically recorded image.
The result is a realistic three-dimensional appearance, similar to that of
oblique illumination of a rough surface by light.
Besides increasing contrast, shadowing allows the height h of a protruding
surface feature to be estimated by measurement of its shadow length. As
shown in Fig. 4-9, the shadow length L is given by:
h = L tan� (4.19)
Therefore, h can be measured if the shadowing has been carried out at a
known angle of incidence �. Shadowing has also been used to ensure the
visibility of small objects such as virus particles or DNA molecules,
mounted on a thin-carbon support film.
C replica
�
Pt layer
Figure 4-9. Shadowing of a surface replica by platinum atoms deposited at an angle � relative
to the surface.
Figure 4-10. Bright-field image (M � 30,000) of a platinum-shadowed carbon replica of PbSe
crystallites, as it appears on the TEM screen. Compare the contrast with that of Fig. 4-8.
h
L
�