Focal length of objective lens
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This book (Practical Electron Microscopy and Database) is a reference for TEM and SEM students, operators, engineers, technicians, managers, and researchers.




Except for the optical properties mentioned above (e.g. spherical aberration and chromatic aberration coefficients), the other main properties of the objective lens are focal length and minimum step of defocus. An example of focal length of objective lens in TEM system is ~ 3 mm. The schematic diagram in Figure 4927 shows focal length of objective lens (f) in TEMs. In general, a shorter focal length of objective lens provides smaller spherical aberration and higher spatial resolution, while a longer focal length gives higher image contrast.

Collection angle versus objective aperture

Figure 4927. Schematic diagram showing focal length of objective lens (f).

Glaser’s famous bell-shaped model suggested that in order to attain the minimum focal length and the minimum aberrations of a magnetic lens used in a TEM the specimen should be put at the middle of the gap of a pair of symmetrical polepieces [1], e.g. called objective polepiece. In reality, in TEM the spherical aberration coefficient of a round lens is always positive and on the order of its focal length. However, in modern TEMs, the focal length of the objective lens is almost constant because the objective lens works under conditions of constant lens excitation.

Note that the focal length at low accelerating voltages is much smaller than that at high accelerating voltages.

The diffraction effect of the objective aperture is important for HRTEM imaging. The radius of the Airy disk produced by the objective aperture is given by,
         rAiry ~ 1.2λf/D ---------------- [4927]
         f -- The focal length of the lens;
         D -- The aperture diameter.

In the SEMs with symmetric immersion lens, a specimen smaller than <5 mm is placed inside the lens gap, reducing significantly the focal length to the range of 2 - 5 mm.

For TEMs with top-entry specimen stages, if the focal length is kept really short to give the highest spatial resolution, then such designs are typically unable to be tilted more then a few degrees without blocking the beam path or interfering with the objective lens. Moreover, incorporation of the tilting mechanisms requires a larger upper pole piece bore which degrades the optical properties of the objective lens.

The magnification in TEM can be adjusted in a wide range (e.g. from x50 to x20, 000, 000) by changing the strength of the magnetic field in the magnetic lens and thus changing the focal length.





[1] Cosslett, V. E. 1991. Fifty years of instrumental development of the electron microscope, in Advances in Optical and Electron Microscopy, Barer, R. and Cosslett, V. E. Eds., Academic Press, London, 215–267.




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