SNR on

Fig. 16. Maximum achievable signal-to-noise ratio (SNR) on read-out of different writable optical data storage systems as a function of the writing energy (laser power) (121). SQS = Organic dye system (WORM) PC = phase change system (TeSeSb) MO = magnetooptical system (GbTbFe). See text.

The expression for Raman signal from Chapter 3 may be combined with Eq. (4.11) to arrive at the dependence of experimental SNR on various sample and measurement variables. SNR is a generally more important indicator of the utility of the measurement than raw signal, since SNR determines the detection limit and overall information content. In addition, SNR may be compared for spectra with quite different intensity units, such as dispersive/CCD and FT-Raman instruments. In the remainder of this chapter, we will derive SNR expressions for several situations, and define a figure of merit for SNR. 

By analogy with Equation 1.2, Sniveley and Koenig showed that the SNR on the baseline of spectra measured with an imaging FT-IR spectrometer is given by ... 

Compared to fluorescence decay measurements, the effieieney of FCS measurements is relatively low. For a given number of photons, N, the SNR of a fluorescence decay measurement is proportional to N, whereas the SNR of FCS is proportional to N itself This somewhat surprising behaviour results from by the fact that two photons are needed to obtain one maerotime eoineidenee in the eorre-lation histogram. The strong dependenee of the SNR on the number of deteeted photons makes detection efficiency an important issue in FCS. 

The strong dependenee of the SNR on the photon number makes it important to reeord as many photons as possible. In prineiple, the count rate of an PCS experiment ean be inereased by inereasing the laser power. Theoretically, a single molecule with a fluoreseenee lifetime of if can perform about l/if absorption-emission cycles per second. For a molecule with a quantum efficiency close to one,... 

As discussed in Section 2.2, the maximum sensitivity in IRRAS is achieved with p-polarized radiation at grazing angles of incidence. With a single reflection, the method can detect CO adsorbed on Ir at 0.002-monolayer (ML) coverage [90]. A SNR on the order of 1000 can be reached, even with a conventional doublebeam dispersive spectrophotometer [91]. This is crucial for studies of catalysis... 

SNR on a metal substrate. However, the use of a highly sensitive IR detector can compensate for most of these inherent sensitivity differences, as will be discussed below, and usually allows high-quality spectra in the monolayer coverage regime to be obtained on almost any type of substrate material. 

A second fundamental question arising with small instfument width is the dependence of the SNR on AAi str- Calculating a numerically and using the result of Equation 2.26, which indicates the inverse proportionality of shot-noise determined and AAinstr, one gets the relationship between Amin and the instrumental width, since Amm is related to min... 

Often the dominant signal frequencies may coincide with that of strong noise. Then frequency filtering does not help to improve the SNR. On the other hand, the horizontal propagation... 

The inverse-fourth-power dependence of SNR on target distance is a severe limitation in ground-based observations, but it can be overcome by constructing very powerful radar systems. 

The two internal standards are added after the dilutions because they are always at the same concentration. The different solutions are analyzed by the method chosen by increasing concentrations to avoid residual effects. The limit of detection depends on each analyte it is estimated from the SNR on the corresponding chromatographic peak. There is no universal convention on the SNR value at which we consider that the limit of detection is reached. This value generally oscillates between 3 and 5, if not 10 for the most demanding analyses. The response function is plotted for each analyte. 

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]. 

Sandborg, M. and G. Alm-Carlsson, Influence of x-ray energy spectrum, contrasting detail and detector on the signal-to-noise ratio (SNR) and detective quantum efficiency (DQE) in projection radiography. Phys. Med. Biol., 1992. 37(6) p. 1245-1263. 

In fig. 2 an ideal profile across a pipe is simulated. The unsharpness of the exposure rounds the edges. To detect these edges normally a differentiation is used. Edges are extrema in the second derivative. But a twofold numerical differentiation reduces the signal to noise ratio (SNR) of experimental data considerably. To avoid this a special filter procedure is used as known from Computerised Tomography (CT) /4/. This filter based on Fast Fourier transforms (1 dimensional FFT s) calculates a function like a second derivative based on the first derivative of the profile P (r) ... 

The filter according equation (1) allows a practical application of a second derivative, because it has only the noise amplification like a first derivative. This is shown in fig. 3 on a experimental data set. The SNR of the true second derivative is too low for correct edge detection, whereas the CT filter gives reliable results. 

Prompted by the success, TOFD measurements were conducted on a fatigue crack in a stainless steel compact tension specimen. Test and system parameters were optimised following the same procedure used for carbon steel specimens. A clear diffracted signal was observed with relatively good SNR and its depth as measured from the time-of-flight measurements matched exactly with the actual depth. 

When used for superresolution, the laser beam is incident on b, which hides the domains in s. During read-out, b is heated and the domains in s are copied to b. The optical system sees only the overlap area between the laser spot and the temperature profile which is lagging behind, so that the effective resolution is increased. Experimentally it is possible to double the linear read-out resolution, so that a four times higher area density of the domains can be achieved when the higher resolution is also exploited across the tracks. At a domain distance of 0.6 pm, corresponding to twice the optical cutoff frequency, a SNR of 42 dB has been reached (82). 

The synthesis and properties of the tin-carbon bond were reviewed in reference (35). Three recent volumes of Gmelin, written by H. Schumann and I. Schumann, comprehensively cover the literature, up to the end of 1973, on tetraalkyltin compounds R Sn (36), RaSnR (37), and RtSnR, RjSnR R", and RR SnR"R " (38), and are invaluable sources of reference. 

The reaction of bromine with optically active sec-butyltin compounds Bu SnRs, to give sec-butyl bromide, can give retention or inversion in the sec-butyl group, depending on the natime of the group R... 

Figure 15.2(b) A schematic presentation of possible basal ganglia circuitry in Parkinson s disease. In PD there is little or no inhibitory nigrostriatal input to the striatum so the Ind Path is active and GPext is inhibited. This will then have less depressant effect on the SThN which will be free to drive the GPint (and SNr) and so reduce cortico-thalamic traffic and produce akinesia. See text for detail. Pathway activity — low — normal — high... 

Figure 15.9 Peptide modulation of striatal input to the globus pollidus. Enkephalin released from axon terminals of neurons of the indirect pathway (see Fig. 15.2 for details) is thought to inhibit GABA release from the same terminals so that feedback (auto) inhibition is reduced. This will free the neurons to inhibit the subthalamic nucleus (SThN) and its drive to GPint and SNr which in turn will have less inhibitory effect on cortico-thalamic traffic and possibly reduce akinesia. Dynorphin released from terminals of neurons of the direct pathway may also reduce glutamate release and excitation in the internal globus pallidus and further depress its inhibition of the cortico-thalamic pathway. High concentrations of these peptides may, however, result in dyskinesias. (See Henry and Brotchie 1996 and Maneuf et al. 1995)...

Pakalapati SNR, BN Popov, RE White (1996) Anodic oxidation of ethylenediaminetetraacetic acid on platinum electrode in alkaline medium. J Electrochem Soc 143 1636-1643. 

The fact that there is a potential difference between points close to the surfaces of two conductors in contact implies that the excess charge densities on their exposed snrfaces are different. This also implies that when two condnctors come in contact, there will be a redistribution of free electrons not only at the actnal inner contact snr-face (which gives rise to the Galvani potential) bnt also at their exposed surfaces. 

The catalytic activity of an electrode is determined not only by the natnre of the electrode metal (its bulk properties) but also by the composition and stmcture of the snr-face on which the electrochemical reaction takes place. These parameters, in tnm, depend on factors such as the method of electrode preparation, the methods of snr-face pretreatment, conditions of storage, and others, all having little effect on the bulk properties. 

Solid-state detectors based on silicon- or germanium-diodes possess better resolution than gas counters, particularly when cooled with liquid nitrogen, but they allow only very low count rates. PIN diodes have also recently become available and have been developed for the instruments used in the examination of Martian soils (Sects. 3.3 and 8.3). A very recent development is the so-called silicon-drift detector (SDD), which has very high energy resolution (up to ca. 130 eV) and large sensitive detection area (up to ca. 1 cm ). The SNR is improved by an order of magnitude compared to Si-PIN detectors. Silicon drift detectors may also be used in X-ray florescence spectroscopy, even in direct combination with Mossbauer spectroscopy (see Sects. 3.3 and 8.3). 

Interestingly, the curve shape of SNR( ) does not depend explicitly on the concentration of Mossbauer nuclei in the sample. The function has a maximum at f = 2 (see Fig. 3.13), as one can easily verify from the root of the first derivative. 

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