Lens illumination systems

As the beam travels down the column, a number of electromagnetic lenses are used to guide the beam to the sample [44], The condenser lenses are part of the illumination system and are used to deliver electrons from the electron gun crossover to the sample. The condenser lenses determine the beam current reaching the sample. The objective, or final, lens determines the final spot size of the beam. A set of scanning coils are also present in the instrument column to scan the beam in a raster pattern over an area of the sample. At each point, data is collected and the points are combined to form the image. More detail on the data collection is given in the image formation section. 

The illumination system comprises a series of relay optics and uniformizing optics that project the radiation from the source through the photomask to the entrance pupil of the projection optics of the lithographic lens, where it forms an image of the illumination source (see Fig. 13.8). This type of illumination system is referred to as Kohler illumination. The fraction of the pupil filled by the illuminator source s image determines the degree of coherence in the projection... 

The basic parts of Raman spectrometer are (i) excitation source, a laser beam, (ii) sample illumination system (usually an intense, polarized and coherent laser beam in the UV, visible, or near-infrared range) and light collection optics (collected with a lens and is sent through interference filter or spectrophotometer), (iii) wave length selector i.e. filter spectrophotometer and (iv) detector system. 

Image on the Entrance Slit. The third illumination system produces an image on the entrance slit with the aid of one lens the structure of the source is imaged on the entrance collimator and the detector. This allows spatially resolved line intensity measurements when a detector with two-dimensional resolution, such as a photographic emulsion or an array detector (see Section 21.3.2.2), is employed. This type of imaging is often used for diagnostic studies. 

There is an aperture associated with each condenser lens, and the apertures and lenses control the area illuminated and the angular divergence of the illumination. The details of the illumination system are described in Section 3.1.5. In both optical and electron microscopes the resolution and contrast of the image may be degraded if the illumination is not properly adjusted. 

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