Depth of field

Equation 1.7 indicates that a large depth of field and high resolution cannot be obtained simultaneously thus, a larger Df means a larger R and worse resolution. We may reduce angle ato obtain a better depth of field only at the expense of resolution. For a light microscope, a is around 45° and the depth of field is about the same as its resolution. 

We should not confuse depth of field with depth of focus. Depth of focus refers to the range of image plane positions at which the image can be viewed without appearing out of focus for a fixed position of the object. In other words, it is the range of screen positions in which and images can be projected in focus. The depth of focus is M2 times larger than the depth of field. 

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Optical Techniques. The most important tool in a museum laboratory is the low power stereomicroscope. This instmment, usually used at magnifications of 3—50 x, has enough depth of field to be useful for the study of surface phenomena on many types of objects without the need for removal and preparation of a sample. The information thus obtained can relate to toohnarks and manufacturing techniques, wear patterns, the stmcture of corrosion, artificial patination techniques, the stmcture of paint layers, or previous restorations. Any art object coming into a museum laboratory is examined by this microscope (see Microscopy Surface and interface analysis). 

The classical polarizing light microscope as developed 150 years ago is still the most versatile, least expensive analytical instrument in the hands of an experienced microscopist. Its limitations in terms of resolving power, depth of field, and contrast have been reduced in the last decade, in which we have witnessed a revolution in its evolution. Video microscopy has increased contrast electronically, and thereby revealed structures never before seen. With computer enhancement, unheard of resolutions are possible. There are daily developments in the X-ray, holographic, acoustic, confocal laser scanning, and scanning tunneling micro-... 

Figure 5 Micrographs of a machine screw illustrating the great depth of field of the SEM (a) optical micrograph of the very tip of the screw (b) and (c) the same area in the SEM and a second image taken at an angle (the latter shows the depth of field quite clearly) (d) lower magnification image.

The various techniques which may be used to provide optimum conditions for the examination of specimens have been described [202—205]. If the sample is opaque, then microscopic investigation is limited to the surface. The depths of penetration for the study of transparent crystals are controlled by the limited depth of field of the optical microscope at high magnifications. This limitation can sometimes be overcome by cleavage of the crystal at an appropriate value of a and examination of the surfaces exposed [120],... 

The development and the recent increase in availability of the scanning electron microscope with its considerable depth of field and reduced beam intensity has widened the range of samples which can be examined... 

The second requirement is related to chemical imaging applications. Very flat samples are required to avoid problems of depth of field. Even if this is common to every imaging technique, it is in this case coupled with surface pollution problems. Sample preparation must then lead to flat surfaces without surface pollution. For cultural heritage samples,... 

For very high quality microscopes (NA = 1.4), R is on the order of 0.2 /urn that is, particles separated by less than this distance cannot be distinguished from each other. Magnification is the product of the eyepiece and objective numbers maximum magnification is 1000 times the numerical aperture. Depth of field is also related to the aperture, decreasing when the latter increases. A large depth of field is useful when evaluating particles of many different... 

LEED requires a coherence length of ordered domains on the order of at least 100 A to be useful (36). SEM images are typically limited to resolution of 50 A, but are useful due to their depth of field and wide field of view (37). High resolution TEM can yield images of solids to a resolution of 1-2 A, but requires extensive sample preparation and also requires that the surface atoms be in registry with the remainder of the bulk sample under study (14,38). Clearly, each technique provides valuable information which complements the others, and ideally a combination of the probes would be used to obtain a complete characterization of the surface under study. 

For samples thicker than the depth of field, the images are blurred by out-of-focus fluorescence. Corrections using a computer are possible, but other techniques are generally preferred such as confocal microscopy and two-photon excitation microscopy. It is possible to overcome the optical diffraction limit in near-field scanning optical microscopy (NSOM). 

Preference for savanna-like parks displaying depth of field and a rich diversity of plant life (phytophilia) may be an evolved, innate preference for an environment of evolutionary... 

The ideal deposition process would leave a perfectly flat surface. That does not happen, so a planarization process is required to maintain depth of field requirements. For a dielectric planarization process, the ideal planarization process would remove only material in the up areas and remove no material in the down areas. Metal CMP involves the removal of metal overburden, leaving filled plugs or vias (single damascene) or filled vias and inlaid metal lines (dual damascene) with no removal of metal in the inlaid region and no removal of dielectric. 

Depth of field depends on substrate reflectivity, the degree of partial coherence and the minimum feature size (5). In practice, however, the classical depth of field for the incoherent case (X-f2(N.A.)2) gives a reasonable approximation. Two layer resist processes in which the image is formed in a thin, flat, resist layer on top of a much thicker planarizing layer, alleviate the need for a large depth of field and make it easier to form high resolution, high aspect ratio, resist patterns (6,7). Satisfactory results can be obtained at contrast levels as low as 40%. 

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