Focal surfaces

Plate scale, mechanical size of focal surface (size of instruments)... 

Emission spectrography includes an excitation source (in this instance a direct-current arc), an optical unit using a dispersion system to provide monochromatic images of the input slit on its focal surface, and a detection system (in this instance, a photographic emulsion). 

Different types of system exist, with linear or curved optics, and using prisms or diffraction gratings. A spectrograph fitted with a prism is equipped with a collimator and lens so as to obtain parallel illumination (see Fig. 3.3). The focal surface is almost planar. 

To see this we need to make an approximation. The approximation hinges on the geometry of "focal surfaces" to an interface [4]. These are the two surfaces traced out by the centres of cur ature (the foci) on an interface. The centres of curvature of a h3 erbolic interface lie on both sides of the surface, so that the focal surfaces are on both sides of the surface (Fig. 4.3). 

Figure 4.3. (Left ) Two-dimensional view of a focal surface to an interface. This focal surface describes the location of centres of curvature of the interface. (Right ) Three-dimensional view of the focal surfaces Fi and F2 to a hyperbolic surface, S. (Adapted from [5].)...

Figure 4.4(b) Three parallel surfaces, all hyperbolic. The focal surfaces to homogeneous hypierbolic surfaces degenerate to two curves (AB and CD) on either side of the surface. 

Further, if the interface is h3rperbolic, one of the focal surfaces to the interface must lie at a distance equal to the chain length (1) from the interface, where the head-groups are located. (This condition ensures that no gaps between chains or chain overlap occur.) Consequently, the cross-sectional area for this parallel surface must vanish ... 

Even in the absence of other aberrations, the image surface of a lens is not truly a plane but rather a curved surface. The aberration is known dis field curvature, and the focal surface is called the Petzval surface. The Petzval surface curves toward a thin, positive lens. Its radius of curvature is the reciprocal of the Petzval sum where the... 

To decrease the contact surface (to assimilate it to a point) by the focalization of the magnetic field. 

Fig. 2 shows the CFRP-sandwich specimen and the transducer mounted on the scanner. Fig. 23 presents a C-scan of the specimen as first interesting result. Only the defects visible from the outside are indicated. The distance between transducer and specimen was smaller than the focal length, so that the angle of incidence at the edge of the sound beam converts the longitudinal waves to Rayleigh-waves in the specimen. These waves provide a very sharp image of the surface. This method opens the possibility for a non-contact acoustic microscope. 

Figure Bl.6.1 Equipotential surfaces have the shape of lenses in tlie field between two cylinders biased at different voltages. The focusing properties of the electron optical lens are specified by focal points located at focal lengthsandy2, measured relative to the principal planes, The two principal rays emanating...

From their focal point to the earth s surface seismic w-aves travel through the earth s crust and the soil. The stratification of soil, i.e. the earth s layers above the crust, plays an important role, as the intensity and frequencies of an earthquake, as felt on the earth s surface, will depend upon the type of soil strata. 

Greater depth of soil when there is a deep layer of soil, the intensity of the earthquake will reduce. The greater the distance from the focal point, the smaller will be ground movements. In such cases it is seen that the settlement of the soil below the structure may be negligible as it would have already settled by the time the shock reached the surface, and hence damage to the structure would be reduced. 

Because the laser beam is focused on the sample surface the laser power is dissipated in a very smaU area which may cause sample heating if the sample is absorbing and may cause break-down if the sample is susceptible to photodecomposition. This problem sometimes may be avoided simply by using the minimum laser power needed to observe the spectrum. If that fails, the sample can be mounted on a motor shaft and spun so that the power is dissipated over a larger area. Spinners must be adjusted carefully to avoid defocusing the laser or shifting the focal spot off the optic axis of the monochromator system. 

Natural crystals, synthetic crystals, and glasses often contain small bubbles that preserve samples of the fluid from which the crystals grew or of the atmosphere over the glass melt. Using a long focal length lens, the laser beam can be focused into inclusions at some depth below the crystal or glass surface. The Raman spectra then permit the identification of molecular species dissolved in the aqueous solutions or of components in the gas bubbles. ... 

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