Lenses in the microscope

Magnetic lenses for electron beams and glass lenses for light beams are very different in form. Magnetic lenses are focused by changing their power rather than their position as is done for 

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Figure Bl.18.1. Light rays and imaging lenses in the microscope. The illumination system is not included.

Electron diffraction patterns are usually produced with transmission electron microscopes. These instruments are composed of several magnetic lenses. The main lens is the objective lens, which, in addition to forming the first magnified image of the specimen, also produces the first diffraction pattern. This original pattern is then magnified by the other lenses of the microscope so as to produce the final diffraction patterns on the screen or on a camera. 

In practice, however, lenses in the electron microscope have unavoidable aberrations, and a more realistic expression for the resolution is... 

The basic principles that govern electron microscopy are analogous to optical microscopy. Whereas optical microscopes use light and optical lenses to illuminate and magnify the sample, electron microscopes utilize high-energy electrons and electromagnetic lenses. There are two types of lenses in electron microscopes (see... 

Calculate the resolution and the depth of field of the objective lenses of a light microscope, listed below. The refractive index of vacuum is 1, and that of air can be treated as 1. Assume blue light is used in the microscope. 

Figure 6.20 Electron Microscope A schematic diagram of a transmission electron microscope. The sample is placed in the microscope in the path of an electron beam generated by the Field Emission Gun (FEG) between condenser and objective lenses.

If a tube lens is in the fluorescence detection path, the beam configuration may be slightly different than that shown in Fig. 5.90. The microscope may also have additional lenses in the beam path to project an image on a camera, or to increase the light-collection area of direct detection. In any case, there is a simple way to find the image of the microscope lens behind the field lens Turn on the microscope lamp in the transmission beam path, so that the condenser lens fully illuminates the aperture of the microscope lens. The image of the microscope lens can then easily be found by holding a sheet of paper behind the field lens. 

Cold light sources are used when strong illumination is needed, but drastic heat input into the object must be avoided, e.g., during microscopic observations. Usually a halogen lamp is installed with a concave mirror at the back and a system of lenses in the front. Unwanted IR and UV... 

A typical TLM system is illustrated in Fig. 4. The output of the excitation beam is intensity modulated by a light chopper. The excitation beam is expanded by a beam expander to adjust the beam size to the size of the input pupU of an objective lens. Then, the excitation beam is introduced into a microscope through a dichroic mirror and a prism. The probe beam is also expanded by a beam expander. The distance between the two lenses in the beam expander is adjusted to control... 

Almost any material can be observed within the SEM. Scientists routinely look at materials as diverse as contact lenses and engine parts, through to the fat particles in milk and crystals in diesel oil. The common factor between all types of specimens is the need to make them ready for viewing in the microscope. 

Laser interferometers have hugely affected the quality control of lenses in the optical-manufacturing industries. Interferometers are the method of choice to measure curvature and smoothness of lenses used in microscopes, telescopes, and eyeglasses. Companies specializing in optical measurements are available for on- or off-site testing of optical components. Astronomy, in particular, has benefited from the precision testing of polished surfaces made possible by laser interferometers mirrors such as the ones used in the Hubble Space Telescope would not be possible without a laser interferometry testing system. 

Table 3.2 Diffraction limited resolution d for objective lenses in the optical microscope, A — 0.5 /im...

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