Background fluctuation

The two error terms refer to Yp and the regression slope respectively. In contrast to some earlier work, based on less homogeneous data sets and apparently affected by underlying absorption lines, notably in I Zw 18, this result, together with a similar one by Peimbert, Luridiana and Peimbert (2007), gives a primordial helium abundance in excellent agreement with the one predicted theoretically on the basis of the microwave background fluctuations and the lower estimates of deuterium abundance (see Fig. 4.3), a comparatively small value of about 2 for AT/AZ and no... 

In many cases it may be impossible to separate fluctuations in ambient HO from background fluctuations or other sources of noise. The proper uncertainty in the net HO signal, averaged over any chosen time interval, is the standard error of the mean calculated from standard statistical formulas from the net data only. This calculation is independent of the applicability... 

Rose et al. (1979) discuss the concept of threshold, the upper limit of normal background fluctuations. Values above background are considered anomalous. This approach is directly applicable to contamination studies since a contaminated soil is an anomalous soil. The simplest way of identifying threshold concentrations is by collecting samples from apparently uncontaminated areas (eg, those remote from urban or industrial influences). After analysis the geometric means and deviations are calculated. The threshold is then the value lying two or more standard deviations from... 

Rg. 3.11. Peak-to-peak noise level as a basis for detection limit. The background fluctuations represent continuously recorded background signals, with the analyte measurement represented by the peak signal. A detectable analyte signal would be 12 divisions above a line dravra through the average of the baseline fluctuations. 

As in the case of minimum detectable activity (Sec. 2.20), two types of errors are encountered when one tries to identify peaks in a complex energy spectrum. Type I arises when background fluctuations are falsely identified as true peaks. Type II arises when fluctuations in the background obscure true peaks. Criteria are set in the form of confidence limits (see Sec. 2.20 and Ref. 38) that can be used to avoid both types of errors. 

For flame atomic absorption spectrophotometry, the detection limit Is defined as the concentration that produces absorption equivalent to twice the magnitude of the background fluctuation. No mention is made of the blank or blank correction. This definition implies an instrument detection limit rather than a detection limit of a complete analytical procedure. Finally, no mention Is made of the need to determine the variability of responses. 

The analysis of the signal and background limits which follows, while detailed, is not exhaustive. Since thermal detectors respond to radiant power rather than photon arrival rate, the signal and background fluctuation limits derived below do not apply to them. Rather, it is necessary to repeat the derivations in terms of power rather than photon rate. An analysis of the background fluctuation limit for thermal detectors is found elsewhere [2.159]. 

Composite Signal Fluctuation and Background Fluctuation Limits... 

The intersections of three pairs of curves for which the bandwidths of the signal and background fluctuation limits are equal are emphasized in Fig. 2.19. Note that all lie between 1.0 pm and 1.5 pm. To illustrate the composite, that for an area of Icm (applicable to the background fluctuation limit) and a... 

Fig. 2.19. Minimum detectable monochromatic power as a function of wavelength for composite of signal fluctuation limit (SFL) and background fluctuation limit (BFL) for two detector areas and electrical bandwidths. Background temperature is 290 K and field ofview is 2x steradians. Detector long wavelength limit is assumed equal to source wavelength...

Figure 2.20 illustrates the minimum detectable power from a 500 K black body as a function of wavelength, illustrating the composite of the signal fluctuation and background fluctuation limits. The same values of the param-... 

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