Objective lenses image formation

Figure 5.17. Illustrating the principle of STEM image formation using two pairs of scan coils between the second condenser lens and the upper objective polepiece. (Reproduced by permission of Williams and...

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. 

Ld.2 image formation. The lens is the most important part of a projection system. It is a device constructed of transparent material and whose whose function is to collect the light emitted from an object (such as a mask) and focus or image this light at the wafer plane (25). Figure 11 illustrates a simple lens that images an object (O) onto the wafer plane (I)... 

Figure 11. Image formation by a refractive lens system showing the object O in the mask or object plane imaged to I in the wafer or image plane.

The vision of the shapes of objects requires the formation of an image on a photosensitive surface. In higher animals the optics of the eye make use of a lens (rather like a photographic camera), but there are some lower species which use a simple pinhole aperture (as was used, incidentally, in the early days of photography). 

Real image formation for a biconvex lens (positive, converging) with refractive index n>1. As long as u>f, a real image forms, upside down, on the opposite side of the lens from the object. 

Chapter 1 is concerned with the fundamental principles of image formation by a lens. These principles were first formulated by Ernst Abbe in 1873 and are basic to the chapters that follow. According to the Abbe theory, the image of an illuminated object is the result of a twofold diffraction process. First, the Fraunhofer diffraction pattern of the object is formed in the back focal plane of the lens. Second, the light waves travel... 

Figure 14.1. Schematic diagram showing the principle of image formation and diffraction in the transmission electron microscope. The incident beam/o illuminates the specimen. Scattered and unscattered electrons are collected by the objective lens and foeused back to form first an electron diffraction pattern and then an image. For a 2D or 3D crystal, the electron-diffraetion pattern would show a lattice of spots, eaeh of whose intensity is a small fraetion of that of the incident beam. In praetiee, an in-focus image has no eontrast, so images are recorded with the objeetive lens slightly defocused to take advantage of the out-of-focus phase-contrast mechanism.

We will now consider how to simulate this method of image formation in the X-ray diffraction experiment where we have to use a mathematical replacement for the objective lens. The studies by Porter are of great importance because they show how the Bragg reflections give the amplitude components of a Fourier series representing the electron density in the crystal (the electron-density map). In effect, Fourier analysis takes place in the diffraction experiment, so that the scattering of X rays by the electron density in the crystal produces Bragg reflections, each with a different amplitude F hkl) and relative phase Qhkl-... 

Figure 6.22 The formation of images in an electron microscope (schematic) (a) to view a diffraction pattern, the intermediate lens is focussed upon the back focal plane of the objective lens (b) to view an image, the intermediate lens is focussed upon the image plane of the objective lens. The way in which the diffracted beams recombine to form the image can be seen by simply varying the focal length of the intermediate lens continuously...

As explained previously, most SEMs employ a "pinhole" objective lens and single SE detector. In this arrangement, there is no discrimination between the various types of SEs detected, and all contribute towards image formation. 

Figure 13. The laser light is focused via the scanner (b) through the tube lens (c) and the objective (d). and illuminates a small spot in the specimen (e). Emitted light emanating from the focal plane and the planes above and below (dotted and dashed lines) is directed via the scanner to the dichroic beam splitter (a) where it is decoupled and directed onto a photomultiplier (i). A pinhole (h) in front of the photomultiplier is positioned at the crossover of the light beams emerging from the focal point. This plane corresponds to the intermediate image of the Kohler illumination described in Section 29.1.3. Light emanating from above and below the focal point has its crossover behind and before the pinhole plane so that the pinhole acts as a spatial Filter. Numerous papers elucidate the basic aspects of confocal image formation [59] - [64].

Figure 41. Image formation in an ideal microscope a) Specimen b) Objective lens c) Back-focal plane, objective aperture d) Image plane...

Fig. 3.1 Image formation by the objective lens. Rays leaving the specimen in a given direction meet at a point in the back focal plane of the lens. The aperture there limits the angular acceptance of the imaging system to a. The intermediate aperture selects the specimen area which contributes to the image.

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