What is objective lensin microscope

Fig. The objective lens consists of a polepiece to determine the magnetic-field of the lens, a yoke to create a magnetic path, and a copper wire coil. The coil is wound in the yoke around the circumference of the optical axis. A magnetic field is generated by the electric current passing through the coil and a magnetic flux is generated in the yoke. Then, a leakage magnetic field is created in vacuum from the polepiece attached to the end of the yoke. The polepiece of the lens is made of a magnetic component and has a gap to create a leakage magnetic field. As is seen in Figure, the polepiece is cylindrically symmetric and an electron beam passes through the center of the polepiece. The leakage magnetic field (B) has a curvature (shown in Figure) and acts as a lens to converge the electron beam. The strength of the lens can be varied by changing the electric current of the coil. A specimen usually set on a side-entry type holder is placed in the magnetic field (lens) produced by the polepiece. The polepiece determines the performance of the lens, that is, the strength and shape of the magnetic field of the lens. Thus, the polepiece is made of a high-quality magnetic material than that of the yoke and is processed by high precision machining. The objective lens is requested to possess a stronger magnetic field than the intermediate lens, etc., because the focal length of the objective lens is shorter than the other lens. To meet this requirement, the objective lens is made of a magnetic material with a large saturation magnetization. The objective lens used for high spatial-resolution observation requires a further-stronger magnetic field to decrease the aberration of the lens. For this purpose, a polepiece with a narrow gap (a short distance between the upper and lower poles) is used. On the other hand, the objective lens used for element analysis uses a polepiece with a little wider gap at a little sacrifice of image resolution because a more space is needed for an EDS detector to place close to the specimen for increasing the detection efficiency or to ensure a large tilt angle of the specimen.

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What is the function of objective lensin microscope

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The "objective lens" is the first-stage lens to form an image using electrons exiting from the specimen. The objective lens is the most important lens in the imaging lens system because the performance of this lens determines the image quality (resolution, contrast, etc). A good objective lens has both a small spherical aberration (Cs) coefficient and a small chromatic aberration (Cc) coefficient. To decrease these coefficients, shortening the distance between the two magnetic poles and decreasing the bore diameter of the polepiece are required. Since the side-entry-type specimen holder is inserted between the two magnetic poles, there is a limitation on shortening the distance. For the top-entry-type specimen holder, an asymmetric polepiece whose upper bore diameter is larger than that of the lower one is used.