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

The Transmission Electron Microscope 85
Projector lens
The purpose of the projector lens is to produce an image or a diffraction
pattern across the entire TEM screen, with an overall diameter of several
centimeters. Because of this requirement, some electrons (such as the solid
single-arrow ray in Fig. 3-14) arrive at the screen at a large distance from the
optic axis, introducing the possibility of image distortion (Chapter 2). To
minimize this effect, the projector is designed to be a strong lens, with a
focal length of a few millimeters. Ideally, the final-image diameter is fixed
(the image should fill the TEM screen) and the projector operates at a fixed
excitation, with a single object distance and magnification v/u � 100.
However, in many TEMs the projector-lens strength can be reduced in order
to give images of relatively low magnification (< 1000) on the viewing
screen. As in the case of light optics, the final-image magnification is the
algebraic product of the magnification factors of each of the imaging lenses.
TEM screen and camera
A phosphor screen is used to convert the electron image to a visible form. It
consists of a metal plate coated with a thin layer of powder that fluoresces
(emits visible light) under electron bombardment. The traditional phosphor
material is zinc sulfide (ZnS) with small amounts of metallic impurity added,
although alternative phosphors are available with improved sensitivity
(electron/photon conversion efficiency). The phosphor is chosen so that light
is emitted in the middle of the spectrum (yellow-green region), to which the
human eye is most sensitive. The TEM screen is used mainly for focusing a
TEM image or diffraction pattern, and for this purpose, light-optical
binoculars are often mounted just outside the viewing window, to provide
some additional magnification. The viewing window is made of special glass
(high lead content) and is of sufficient thickness to absorb the x-rays that are
produced when the electrons deposit their energy at the screen.
To permanently record a TEM image or diffraction pattern, photographic
film can be used. This film has a layer of a silver halide (AgI and/or AgBr)
emulsion, similar to that employed in black-and-white photography; both
electrons and photons produce a subtle chemical change that can be
amplified by immersion in a developer solution. In regions of high image
intensity, the developer reduces the silver halide to metallic silver, which
appears dark. The recorded image is a therefore a photographic negative,
whose contrast is reversed relative to the image seen on the TEM screen. An
optical enlarger can be used to make a positive print on photographic paper,
which also has a silver-halide coating.