Signal image quality

The detectability of critical defects with CT depends on the final image quality and the skill of the operator, see figure 2. The basic concepts of image quality are resolution, contrast, and noise. Image quality are generally described by the signal-to-noise ratio SNR), the modulation transfer function (MTF) and the noise power spectrum (NFS). SNR is the quotient of a signal and its variance, MTF describes the contrast as a function of spatial frequency and NFS in turn describes the noise power at various spatial frequencies [1, 3]. 

Contrast Sensitivity S For this purpose a test piece is used whose least material thickness to be resolved in the video signal corresponds to the contrast sensitivity. The test piece can be a step/hole image quality indicator as per EN 462-2. 

The most difficult materials to study by NMR microscopy are those with short T2 or T2 relaxation times and/or with low concentrations of the nudear spins, which normally result in poor NMR signal intensities. One possibility for improving the image quality is to adapt the shape and size of the rf coils to the size of the objects in order to achieve the best possible filling factor and therefore the best sensitivity [1]. In addition, methods with short echo or detection times have been developed, such... 

The information rate (the number of bits embedded in an image) the image quality, and the error rate are controlled as a trade-off by the number (N) of watermark coefficients pairs, the total energy (E) of the watermark signal and by the ECC expansion ratio (n/k). For reliable communication the error rate should be kept negligible and hence ... 

Besides space charge, image quality is limited by the combination of the number of pixels of the CCD camera and the size of a single ion event. The PSD usually employs two micro-channel plates (MCPs) mounted in a chevron configuration and backed by a phosphor screen. A product ion causes an electron cascade through the channel plates and onto the phosphor, which emits a pulse of diffuse light that must be focused properly onto the CCD array with a camera lens or fiber optic taper. In this process one product ion results in a signal on the CCD that is usually several pixels... 

The dyes must also have appropriate redox properties to function properly as sensitizers. If the dyes are oxidized too readily in the ground state, silver atoms can be prematurely and unselectively reduced, and this causes photographic fog. Fog is the unwanted, indiscriminate reduction of silver. Photographic fog causes a loss of signal-to-noise ratio and degrades image quality. The redox potentials of sensitizing dyes have also been extensively measured and correlated to performance [15],... 

High-speed computers and the ability to digitize electron signals at video rates mean that, in spite of poor initial image quality in dealing with direct observation of frozen hydrated samples, several relatively noisy im-... 

This mode is often simply called the non-contact mode. This mode can provide true atomic resolution and image quality comparable to an STM. The cantilever is excited by the piezoactuator to oscillate at or near its resonant frequency. The vibrating tip is brought near a sample surface, but not touching the surface, to sense the weak attractive force between tip and sample instead of strong repulsive force in the contact mode. As illustrated in Figure 5.13, the interactions between the tip and the sample shift the oscillation frequency. The frequency shift signals are used to control the tip-sample distance. The interaction forces detected in the non-contact mode provide excellent vertical resolution. However, this mode cannot be operated in a liquid environment. Even in air, liquid attached on the tip can cause failure in operation. 

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