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What are the functions of three apertures? (a) the condenser aperture (b) the objective aperture (c)...

What are the functions of three apertures? (a) the condenser aperture (b) the objective aperture (c) the selected area aperture.

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(A)Condenser Aperture: The condenser aperture controls the fraction of the beam which is allowed to hit the specimen. It therefore helps to control the intensity of illumination.

(B)Objective aperture: The objective aperture is used for accepting a transmitted wave or one of diffracted waves to obtain a bright-field image or a dark-field image. The objective aperture is inserted into the back focal plane of the objective lens.In other way the function of objective aperture is to select those electrons which will contribute to the image, and thereby affect the contrast of the image.

(C)Selected area aperture:An aperture for selecting a specimen area in the selected area diffraction (SAD) mode. The aperture is inserted into the image plane of the objective lens (or object plane of the intermediate lens).

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