TABLE OF CONTENTS The imaging lenses of a TEM produce a magnified image or an electron-diffraction pattern of the specimen on a viewing screen or camera system. The spatial resolution of the image is largely dependent on the quality and design of these lenses, especially on the first imaging lens: the objective.
As in the case of a light-optical microscope, the lens closest to the specimen is called the objective. It is a strong lens, with a small focal length; because of its high excitation current, the objective must be cooled with temperature-controlled water, thereby minimizing image drift that could result from thermal expansion of the specimen stage. Because focusing power depends on lens excitation, the current for the objective lens must be highly stabilized, using negative feedback within its dc power supply. The power supply must be able to deliver substantially different lens currents, in order to retain the same focal length for different electron-accelerating voltages. The TEM also has fine controls that enable the operator to make small fractional adjustments to the objective current, to allow the specimen image to be accurately focused on the viewing screen.
The objective produces a magnified real image of the specimen ( M ? 50 to 100) at a distance v ? 10 cm below the center of the lens. Because of the small value of f, Eq. (2.2) indicates that the object distance u is only slightly greater than the focal length, so the...
