Absolute reflection spectrum

Absolute Reflection Spectrum versus Modulated Reflection Spectrum... 

The modulation technique resolves the main technical difficulties of the constant potential spectral measurement at the electrode surface. Basically, in the modulation spectroscopic measurement, only the change associated with the change of the modulated parameter is detected. Thanks to lock-in ampHfication, we can significantly increase the sensitivity of the optical signal detection. It must be noted that what we can obtain is the change of the spectrum with respect to the modulated parameter but not the absolute reflection spectrum. In other words, modulation methods give the difference or differential spectrum. From the modulation spectrum, we cannot obtain explicitly the absolute spectrum at a unique condition unless a perfect reference absolute reflection spectrum is already in our hands. One should be careful in the interpretation of the spectral curves at this point. 

To obtain absolute values, the data have to be multiplied by the reflectance spectrum of the standard. 

A key step in the application of rapid analysis methods is the collection of high quality NIR spectra. To minimize the effect of water in the biomass spectra, each sample was air-dried to less than 10 percent moisture prior to NIR spectroscopic analysis. Spectroscopic techniques were used that enable a high quality, reproducible, and representative NIR reflectance spectrum to be obtained. For each sample, a total of 35 spectra were collected and averaged to compensate for sample heterogeneity. Each calibration sample was subsampled three times. Sample cells were emptied and repacked between subsamples. Instrument reproducibility tests demonstrated that the reproducibility limits of the NIR spectrometer contributed less than 0.1 percent to the absolute prediction errors in the rapid analysis method. 

Dupont trade name of Teflon, or in Grum and Saltzman s original work. Allied Halon G-80) powder, packed to a density of 1.0 g/cm. Grum and Saltzman s work was rapidly followed by papers by Jack Hsia and Vic Weidner (1981) of the National Bureau of Standards (now NIST) that thoroughly characterized packed PTFE. The initial Weidner and Hsia paper reported absolute reflectance data for packed PTFE powder from 200 to 2500 nm (0.2-2.5 /an) and established this material as the standard of choice for diffuse reflectance measurements in the UV-VIS-NIR region of the spectrum. A follow-up study involving a laboratory intercomparison of results showed that even as much as a 20% error in compression of the powder gave excellent results in consistency of reflectance (Fig. 3). 

In some cases, thermal neutrons can also be used to measure the absolute abundances of other elements. Transforming the neutron spectrum into elemental abundances can be quite involved. For example, to determine the titanium abundances in lunar spectra, Elphic et at. (2002) first had to obtain FeO estimates from Clementine spectral reflectances and Th abundances from gamma-ray data, and then estimate the abundances of the rare earth elements gadolinium and samarium from their correlations with thorium. They then estimated the absorption of neutrons by major elements using the FeO data and further absorption effects by gadolinium and samarium, which have particularly large neutron cross-sections. After making these corrections, the residual neutron absorptions were inferred to be due to titanium alone. 

The thickness of thin film layers separated by uniform, parallel interfaces can be determined from optical interference patterns that result. These measurements can be made from about 400 nm out through the visible spectrum and on into the near-infrared (NIR) region. Since film thickness measurements rely not on the absolute magnitude of the reflected light, but on the variation of that signal with wavelength, the choice of units is less important. Typically %R is used, but in some cases raw intensity is also satisfactory. We will treat thickness determinations in more detail in the applications section of this chapter. 

The absolute differences in the CH2 stretching frequencies in the spectra of the coagel phases of the three different systems reflect the differences in packing of the methylene chains adopted at low temperatures. The CH2 scissoring band is doubled in the spectrum of the SDS/0.3M NaCl system below the cmt, suggesting that an orthorhombic subcell is readily adopted. Examination of the S-0 stretching bands of... 

Compared to the PAS spectra of cotton cloth, in the Specular Reflectance spectra, the signal-to-noiae is higher and any noise is low enough to show clearly the spectral features described here. Even though there are differences due to the change in sampling conditions, the spectra can be compared for a one-to-one comparison and identification of absorption bands. Mote however that in the PAS spectrum relative intensities are displayed and that the Specular Reflectance spectra show an absolute response and measurement which leads to a better possibility for quantification and a more rigorous treatment of the data. 

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