Single-beam spectral intensity

A single-beam spectrophotometer, hke the one shown in Figure 1.5(a), presents a variety of problems, because the spectra are affected by spectral and temporal variations in the illumination intensity. The spectral variations are due to the combined effects of the lamp spectrum and the monochromator response, while the temporal variations occur because of lamp stability. 

As for all absorbance-based spectral measurements, the intensity data represented in a raw (single-beam) chemical image are a combination of the spectral response of both the instrument and the sample. In order to remove the instrument response component, it is necessary to ratio the data to a background reference. For reflectance measurements, the background is a separate data cube typically acquired from a uniform. 

The other approach, internal reflection, takes advantage of the fact that if an electromagnetic wave is totally reflected at the interface between an optically thicker and an optically thinner medium (refraction index of the optical thicker medium is higher), a part of the electromagnetic wave penetrates into the optical thinner medium (the so-called evanescent wave) where it may interact with any species present and may be absorbed. The spectral intensity distribution of the reflected light thus differs from that of the incident beam and contains information about species present in the interphase of the optical thinner medium. In practical setups ATR (attenuated total reflection) single crystals are used, onto which the electrode under investigation is deposited. The beam baths of internal and external reflection are visualized in Fig. 1. 

Only the single-beam type of photometric system of an FT-IR spectrometer is commercially available at present. This requires that two measurements must be made the first is for a reference single-beam background spectrum recorded without the sample, and the second for the sample placed in the beam. As these two measurements are made under the same optical conditions, accuracy of spectral intensity can be expected. Care must be taken, however, not to change the conditions of measurements during the time interval between the two measurements. 

For comparison the output power of a high-pressure mercury lamp (Osram HBO 200) also is listed. The reader has to consider, however, that the mercury lamp radiates this power into the unit solid angle (= 60°) distributed over the spectral range from 2000 to 6000.A, whereas the laser intensity is concentrated at a single wavelength and collimated in a beam with a very small divergence between 10 and 10" sterad. 

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