Source intensity

The application of IP can facilitate or even make feasible some experimental techniques in NR, where the neutron source intensity poses a problem for imaging with radiographic films. 

Finally, values of sx are directly proportional to transmittance for indeterminate errors due to fluctuations in source intensity and for uncertainty in positioning the sample cell within the spectrometer. The latter is of particular importance since the optical properties of any sample cell are not uniform. As a result, repositioning the sample cell may lead to a change in the intensity of transmitted radiation. As shown by curve C in Figure 10.35, the effect of this source of indeterminate error is only important at low absorbances. This source of indeterminate errors is usually the limiting factor for high-quality UV/Vis spectrophotometers when the absorbance is relatively small. 

T he measurement principle, based on the ratio of the intensities, is insensitive to fluctuations in the source intensity. Changes associated with wavelength dependence can cause error in the measurement. 

This procedure has the great advantage that some potential sources of error are eliminated. The measurements do not depend upon accurate wavelength positions as they are made with respect to the spectrum itself and any cell errors are avoided by using the same fixed-path-length cell. Also, by employing this measured ratio any variations in the source intensity, the instrument optics or sensitivity are eliminated. 

We are now ready to derive an expression for the intensity pattern observed with the Young s interferometer. The correlation term is replaced by the complex coherence factor transported to the interferometer from the source, and which contains the baseline B = xi — X2. Exactly this term quantifies the contrast of the interference fringes. Upon closer inspection it becomes apparent that the complex coherence factor contains the two-dimensional Fourier transform of the apparent source distribution I(1 ) taken at a spatial frequency s = B/A (with units line pairs per radian ). The notion that the fringe contrast in an interferometer is determined by the Fourier transform of the source intensity distribution is the essence of the theorem of van Cittert - Zemike. 

Er is becoming very small. Here, however, the effect will be masked by other effects contained in the data, such as the effect of small changes in source intensity, external interference or, in the case of FTIR, interferometer misalignment, or any of several other effects that change the actual values of reference and sample energy at the limits of the spectral range. 

Optical methods are especially useful for the selective detection of CO and C02 concentrations. In low-priced sensors, a simple miniature light bulb is used as IR-source. The radiation emitted enters an absorption chamber, through which the flue gas is pumped. An added interference filter lets only the absorption spectra of the target gas pass. The IR detector determines the reduction of the light intensity, which is then transformed into an electrical signal. The correlation between the source intensity and the received intensity is given in the Lambert-Beer equation. 

Here, I is the received radiation intensity, I0 is the source intensity, a is the absorption coefficient, c stands for the concentration of the gas to be measured and the length of the radiation pathway filled with gas is called l. Fig. 3.16 shows an optical sensor for detecting C02. 

One other operational detail merits brief mention before applications to surface spectroscopy are considered. Infrared sources decline markedly in intensity at longer wavelengths and therefore PA spectra must be source intensity normalized before peak heights can be ascribed any quantitative significance. It has sometimes been mistakenly supposed that the PA spectrum of graphite could be used to normalize infrared PA spectra. 

Depending on the source of the graphite, one obtains distinctly different IR/PA spectra (frequently caused by adsorbed species) and the response of the DTGS detector of an IR spectrometer turns out to be a more accurate measure of variable source intensity (12). A normalization technique (13) requiring measurement of the spectrum at two different mirror velocities and corrected by black body spectra taken at the same two velocities appears to be the best normalization method reported thus far. 

It is, however, pertinent to mention here that the application of both emperical ratio method and baseline method help in eliminating to a great extent the errors caused due to changes in source intensity and adjustment of the optical system. 

The ratiometric measurements are preferable because the ratio of the fluorescence intensities at two wavelengths is in fact independent of the total concentration of the dye, photobleaching, fluctuations of the source intensity, sensitivity of the instrument, etc. The characteristics of some fluorescent pH indicators allowing ratiometric measurements are given in Table 10.1. 

The intensity of the combined reflection is proportional to the area of overlap of the rocking ciuves in the duMond diagram if the source intensity is uniform across the relevant wavelength region. 

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