Light reflection/transmission instrumentation

Emission. The sample acts as its own light source. The emission spectrum will depend on sample temperature, transmittance and reflectance. Most instruments offer an external port for use in emission experiments. There is normally little point in measuring an emission rather than a transmission spectrum, as considerable reconfiguration of the spectrometer is required and the noise in the spectrum is inherently greater. However, certain measurements may demand such an approach. 

A few companies offer devices that can continuously measure solids content of feed flows. Various principles have been used, including the coriolis effect and the use of a radioactive source. The method using a radioactive source has proved reliable, but there is resistance to using it where a watercourse is involved, and moreover there are stringent regulations with regard to the disposal of the instrument once it is at the end of its useful life. Nevertheless the suppliers naturally offer a comprehensive service. Light reflection or transmission is another method that is in use. 

However, these analogues are actually hypothetical. The reason for this is that it is nearly impossible to obtain optical measurement components, such as the source and the detector, whose response to light across the visible spectrum is flat (or nearly so). However, this is not an impossible task and we find that an excellent match can be obtained to the transmission functions of 7.8.21., i.e.-those of the Standard Observer. This is typical for commercially available instruments. Now, we have an instrument, called a Colorimeter, capable of measuring reflective color. 

Infrared spectra of zeolitic samples can be measured in several different modes. These include transmission, diffuse reflectance, attenuated total internal reflection (ATR) and emission. Transmission and diffuse reflectance are by far the most widely used of these techniques. In the transmission mode, the sample is placed directly in the infrared beam of the instrument and the light passing through or transmitted is measured by the detector. This transmitted signal (T) is ratioed to the open beam (no sample) signal (To) to get the transmission spectrum of the sample. The transmission spectrum is converted to an absorbance spectrum ... 

When light is directed onto a sample it may either be transmitted or reflected. Hence, one can obtain the spectra by either transmission or reflection. Since some of the light is absorbed and the remainder is reflected, study of the diffuse reflected light can be used to measure the amount absorbed. However, the low efficiency of this diffuse reflectance process makes it extremely difficult to measure 120) and it was speculated that infrared diffuse reflection measurements would be futile 120). Initially, an integrating sphere was used to capture all of the reflected light121) but more recently improved diffuse reflectance cells have been designed which allow the measurement of diffuse reflectance spectra using FT-IR instrumentation 122). 

Numerous tests using strips that change colour when dipped into a medium represent current applications of colorimetry. However, the result obtained by visual examination of the reflected light is related to reflectometry more than to transmission colorimetry (see Fig. 11.18). These selective tests, which are ready to use and do not require an instrument, are complementary to established methods. Because they yield immediate results, they are useful for all sorts of semiquantitative analyses. 

Most present-day fiber-optic sensors use linear diode arrays combined with optical gratings and measure the absorption, transmission, fluorescence, and reflection in UV, visible, and NIR regions (see Table 3.1). Light travels to the sampling probe via one fiber-optic cable and returns to the instrument via a second. Laser excitation permits long-distance transmission of excitation radiation to get a useful signal from the sample. 

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