Contrast visibility

Contrast C is the contrast of the luminances between the object Lo and its surroundings Ls C = (Lo - Ls)/Ls. With increasing contrast, visibility increases Adaptation luminance L,d, is the luminance to which the eye adapts its sensitivity. It corresponds to the luminance of the field of vision. With increasing adaptation luminance, visibility increases. 

MeV per photon. In contrast, visible light has energies measured in eV per photon. 

Frequency Doubling. As the name implies, in frequency doubling a substance doubles the frequency of the incident laser radiation. This effect is important in telecommunications and optical data storage. For example, in telecommunications the most efficient way to transmit data is by using infrared radiation, e g., 1200 nm radiation from an indium phosphide laser [60], Detection of infrared radiation is inefficient. In contrast, visible radiation is much easier to detect but is an inefficient transmitter of data. Consequently, an important application of nonlinear optical (NLO) materials is to convert infrared radiation into visible and thus enable easier detection of the signal. 

Sophisticated correction techniques are needed to obtain C-arm CT results with good low-contrast visibility. They are explained below. We start with overexposure correction, then turn to scatter correction, and next look into beam-hardening correction. After that, we briefly visit truncation correction and finish up with ring correction. 

Tectonic features, faults, fractures N D shadows cast, IR, therm, contrasts Visible when topographic feature... 

Other logs employed to determine N/G ratio include the spontaneous potential (SP) log and the microlog, which differentiate permeable from non-permeable intervals. The N/ G ratio can also be measured directly on cores if there is visible contrast between the reservoir and non-reservoir sections, or from permeability measurements on core samples, providing sample coverage is sufficient. 

Additional information can be obtained, if one calculates the smallest thickness difference Ad of sf eel - for instance the depth of a crack - which can be discerned on a radiograph whose granularity is just as high as the limiting value a, of the respective class of the standard EN 584-1. For this estimation the well known relation for the (optical) density difference AD (visible contrast) which results from a difference of thickness Ad in steel is used ... 

The visibility level VL of special viewing tasks may be estimated from the the correlations of the quantitative visual recognition. For an adaptation luminance of 10 cd/m2 and given contrasts (C = 0.5 1.0 2.0), VL is described in Fig. 1 in dependance on the object dimension 3 in angular minutes. An object dimension of T represents a circle with a diameter of 0 12 mm in a distance of 0 4 m. 

As already discussed, transparent specimens are generally only weakly visible by their outlines and flat areas caimot be distinguished from the surroundings due to lack of contrast. In addition to the phase contrast teclmiques, light interference can be used to obtain contrast [8, 9]. 

In contrast to the ionization of C q after vibrational excitation, typical multiphoton ionization proceeds via the excitation of higher electronic levels. In principle, multiphoton ionization can either be used to generate ions and to study their reactions, or as a sensitive detection technique for atoms, molecules, and radicals in reaction kinetics. The second application is more common. In most cases of excitation with visible or UV laser radiation, a few photons are enough to reach or exceed the ionization limit. A particularly important teclmique is resonantly enlianced multiphoton ionization (REMPI), which exploits the resonance of monocluomatic laser radiation with one or several intennediate levels (in one-photon or in multiphoton processes). The mechanisms are distinguished according to the number of photons leading to the resonant intennediate levels and to tire final level, as illustrated in figure B2.5.16. Several lasers of different frequencies may be combined. 

Another interesting physical feature relates to the cliromophoric character of fullerenes. Based on the symmetry prohibitions, solutions of [60]fullerene absorb predominantly in the UV region, with distinct maxima at 220, 260 and 330 nm. In contrast to extinction coefficients on the order of 10 cm at these wavelengths, the visible region shows only relatively weak transitions (X at 536 nm s =710 cm ) [142]. 

The space filling model developed by Corey, Pauling, and Koltun is also known as the CPK model, or scale model [197], It shows the relative volume (size) of different elements or of different parts of a molecule (Figure 2-123d). The model is based on spheres that represent the "electron cloud . These atomic spheres can be determined from the van der Waals radii (see Section 2.10.1), which indicate the most stable distance between two atoms (non-bonded nuclei). Since the spheres are all drawn to the same scale, the relative size of the overlapping electron clouds of the atoms becomes evident. The connectivities between atoms, the bonds, are not visualized because they are located beneath the atom spheres and are not visible in a non-transparent display (see Section 2.10). In contrast to other models, the CPK model makes it possible to visualize a first impression of the extent of a molecule. 

Fig. 7. Graphical representations of (a) the Highest Occupied Molecular Orbital (HOMO) and (b) the Lowest Unoccupied Molecular Orbital (LUMO) for ranitidine. It is possible, in the ordinarily visible color-coded data not shown here, to distinguish the strong localization (a) of the HOMO to the sulfur atom and adjacent nitroethyleneamine fragment and the contrasting localization (b) of the LUMO to the nitroethylenearnine fragment. Neither the LUMO not HOMO appear to have contributions from the dimethylaminomethyl-suhstitiited furan.

Another parameter of relevance to some device appHcations is the absorption characteristics of the films. Because the k quantum is no longer vaUd for amorphous semiconductors, i -Si H exhibits a direct band gap (- 1.70 eV) in contrast to the indirect band gap nature in crystalline Si. Therefore, i -Si H possesses a high absorption coefficient such that to fully absorb the visible portion of the sun s spectmm only 1 p.m is required in comparison with >100 fim for crystalline Si Further improvements in the material are expected to result from a better understanding of the relationship between the processing conditions and the specific chemical reactions taking place in the plasma and at the surfaces which promote film growth. 

In order to see an object, an observer must be able to detect the contrast between the object and its surroundings. If this contrast decreases, it is more difficult to observe the object. In the atmosphere, visibility can decrease for a number of reasons. For example, we may be farther away from the object (e.g., an airplane can move away from us) the sun s angle may change with the time of day and if air pollution increases, the contrast may decrease, reducing our ability to see the object. 

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