Transmitted intensity measurement

When the intensity of the incident beam is low, the nonlinear effects are negligible, and the transmitted intensity (measured by a detector) is linear with the input intensity (see Fig. 10). After the intensity reaches a critical power, P, self-focusing occurs. A more focused beam induces other nonlinear effects which levels-off the transmitted power (see Figure 11). Therefore, for powers greater than P, the transmitted intensity does not depend linearly on the input intensity. 

Determining Concentration by Turbidimetry In turbidimetry the measured transmittance, T, is the ratio of the transmitted intensity of the source radiation, fy, to the intensity of source radiation transmitted by a blank, Iq. 

Direct photography of drops in done with the use of fiber optic probes using either direct or reflected light. StiU or video pictures can be obtained for detailed analysis. The light transmittance method uses three components a light source to provide a uniform collimated beam, a sensitive light detector, and an electronic circuit to measure the amplified output of the detector. The ratio of incident light intensity to transmitted intensity is related to interfacial area per unit volume. 

Equipment has been developed that records measured values in steps of 2.5 mV through a digital converter connected to it, analyzes them and transmits them to a hard disk. The measured values can be fed to a central computer and further analyzed and plotted. This system can be used for intensive measurements and also for monitoring measurements, and offers the following possibilities listing... 

Measuring EXAFS spectra In general, transmission EXAFS can be used, provided that the concentration of the element to be investigated is sufficiently high. The sample is placed between two ionization chambers, the signals of which are proportional to the incident intensity Iq and the transmitted intensity through the sample 7f The transmission of the sample is dependent on the thickness of the sample X and on the absorption coefficient, //, in a Beer-Lambert relationship ... 

If the pulse from laser 1 is ultrafast, the bond-energizing step occurs in a veiy short time, about 10 fs (lfs = 10 S). As the bond stretches through the specific length at which it absorbs photons from laser 2, the molecule can absorb a photon from the second laser beam. This absorption causes the transmitted intensity of laser 2 to fall rapidly as the bond stretches. When the bond breaks, photons from laser 2 are no longer absorbed and the transmitted intensity returns to its original value. By measuring the time it takes for this to occur, chemists have determined that it takes about 200 fs for a chemical bond to break. 

X-rays are absorbed whenever they travel through matter. As a result, the total transmitted intensity, It, measured after passing the absorber is only a fraction, It/lo, of the incident intensity, Iq. For amorphous or polycrystalline material the incremental absorption within a layer of thickness dl is constant, and by integration along the complete light path i through the sample the absorption law... 

If the optically active medium is not transparent at the wavelength of the incident radiation, the transmitted intensity may be further reduced by an absorptive contribution to the index of refraction. Because of preferential absorption of either the left or the right circularly polarized component, the emerging beam would no longer be the sum of equal amplitudes and trace out an ellipse with ellipticity tp = (kt — kr). Practical details of the measurement and chemical applications of optical activity are discussed by Charney[34],... 

When a molecule absorbs light at a certain frequency, it means that a specific bond is stretching, bending, or vibrating. The frequency where each bond absorbs light energy to stretch, bend, or vibrate is very specific. The absorption results in a decrease in intensity of the light transmitted, as measured by the sensor. This is how an infrared... 

Detectors for quantitative measurement of X-ray absorption spectra must measure the flux (photons s of the X-ray beam. Ionization chambers consisting of X-ray transparent windows on each end of a chamber holding an inert gas work well as transmission detectors for concentrated samples. For transmission detectors, ln(/o//) is proportional to the absorption coefficient of the absorbing atom, p (/o = incident X-ray photon intensity, /= transmitted intensity), according to Beer s Law ... 

For quantitative purposes in AAS, a magnitude called transmittance (T) which relates, for a given wavelength, the intensity (measured by the detector) of the light source (Zq) and the intensity not absorbed which has passed through the atomiser or transmitted light (7) is used ... 

The initial single beam dispersive spectrometers that did not, at the time, produce digitised spectra (this would have allowed for baseline correction) were soon replaced by double beam spectrometers. This more complex arrangement can directly yield the spectrum corrected for background absorption. The use of two distinct but similar optical paths, one as a reference and the other for measurement, allows the alternate measurement of the transmitted intensity ratios at each wavelength. 

To obtain a sample spectrum equivalent to the one obtained on a double beam spectrometer, two spectra of transmitted intensities are recorded the first without sample (absorption background), and the second with sample. The conventional %Tspectrum can be obtained from the two preceding measurements (Fig. 10.12). 

Some spectrophotometers allow the measurement of absorbance over a dynamic range of 4 to 6 decades. However, elevated values of absorbance are less reliable because they correspond to very weak transmitted intensities (///0 = 10-6 for A = 6). For most instruments, there are three independent causes of error that can affect transmittance (Fig. 11.23) ... 

Diverse instruments employing the principle of this experiment are used to measure mercury, a toxic and volatile element, present in many work areas. A device has been designed as a colorimeter dedicated for this single element. The source is a mercury vapour lamp and the cell is a transparent tube filled with the atmosphere to be monitored. If mercury vapours are present in the optical path, absorption of radiation emitted by the lamp will occur and this will lead to a decrease in the transmitted light intensity measured by the instrument. 

Figure 14.12 —Schematic of an instrument showing deuterium lamp background correction. Perkin Elmer, model 3300 with a Littrow-type monochromator. This double beam assembly includes a deuterium lamp whose continuum spectrum is superimposed, with the aid of semitransparent mirrors, on the lines emitted by the hollow cathode lamp. One beam path goes through the flame while the other is a reference path. The instrument measures the ratio of transmitted intensities from both beams. The correction domain is limited to the spectral range of the deuterium lamp, which is from 200-350 nm. (Reproduced by permission of Perkin Elmer.)...

An ultrafast time-resolved near- and mid-IR absorption spectrometer was designed to achieve high sensitivity, ultrafast time resolution, and broad tunability in the near- and mid-IR regions (see Fig. 2). The details of this spectrometer are described elsewhere (9). Briefly, MbCO was photolyzed with a linearly polarized laser pulse, whose polarization direction was controlled electronically by a liquid crystal polarization rotator. The photolyzed sample was probed with an optically delayed, linearly polarized IR pulse whose transmitted intensity was spectrally resolved with a monochromator and detected with either a Si photodiode (near-IR RilO cm-1 bandpass) or a liquid nitrogen-cooled InSb photodetector (mid-IR 3 cm-1 bandpass). To measure the sample transmission, this signal was divided by a corresponding signal from a reference IR pulse... 

With absorbance differences on the order of only 1 part in 10 for CD activity, the ratio of transmitted intensities for the left and right circularly polarized beams (Il/Ir) is essentially one because the errors in Ae would be very large if II and Ir are measured directly. 

The conventional XAS experiment involves the direct measurement of the incident and transmitted beam intensity using ionization chambers. The first chamber contains a weakly absorbing gas, which permits <70% of the incident radiation to fall on the sample, and the second ionization chamber contains a mixture of inert gases that will absorb virtually all of the transmitted intensity. The measured absorption coefficient comprises that due to the matrix (/um) and that due to the atom of interest (/u,a). The application of transmission method is ultimately limited by the incident number of photons and the ratio of fiM to fi. In cases where = 1, it is difficult to use the transmis-... 

Organic molecular crystals not only show an increase of the transmitted intensity with the incident light intensity, but also show a small increase of the transmitted intensity with the polarization of natural crystal faces. Only tiny TPA cross sections were measured for the case of perpendicular alignment of the TP transition dipole moments to natural faces or the light polarization perpendicular to these transition dipole moments. They can even vanish because polarization of light has a strong impact on TPA [597],... 

It should be emphasized that the beam entering the counter is never physically monochromatic, as it is when a crystal monochromator is used. Radiation with a great many wavelengths enters the counter when either filter is in place, but every wavelength transmitted by one filter has the same intensity as that transmitted by the other filter, except those wavelengths lying in the pass band, and these are transmitted quite unequally by the two filters. Therefore, when the intensity measured with one filter is subtracted from that measured with the other filter, the difference is zero for every wavelength except those in the pass band. 

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