3.3: Condenser-Lens System

3.3 Condenser-Lens System

The TEM may be required to produce a highly magnified ( e.g, M = 10 ⁵) image of a specimen on a fluorescent screen, of diameter typically 15 cm. To ensure that the screen image is not too dim, most of the electrons that pass through the specimen should fall within this diameter, which is equivalent to a diameter of (15 cm)/ M = 1.5 m at the specimen. For viewing larger areas of specimen, however, the final-image magnification might need to be as low as 2000, requiring an illumination diameter of 75 m at the specimen. In order to achieve the required flexibility, the condenser-lens system must contain at least two electron lenses.

The first condenser (C1) lens is a strong magnetic lens, with a focal length f that may be as small as 2 mm. Using the virtual electron source (diameter d _s) as its object, C1 produces a real image of diameter d _l. Because the lens is located 20 cm or more below the object, the object distance u ? 20 cm >> f and so the image distance v ? f according to Eq. (2.2). The magnification factor of C1 is therefore M = v / u ? f / u ? (2 mm) / (200 mm) = 1/100, corresponding to demagnification by a factor of a hundred. For a W-filament electron source, d