Back-scattered light

If we view the thin plate from the left where it is illmninated with intensity, L, what we see is the back-scattered light, or light diffusion from the surface. If the plate is a perfect diffuser, then we will see an exact amount of L scattered back along the same plane as L as a diffuse component. Note that we are not speaking of reflection (which is ein entirely different mechcinism where the... 

Figure 7.3 shows the two-beam photon-force measurement system using a coaxial illumination photon force measurement system. Two microparticles dispersed in a liquid are optically trapped by two focused near-infrared beams ( 1 pm spot size) of a CW Nd YAG laser under an optical microscope (1064 nm, 1.2 MWcm , lOOX oil-immersion objective, NA = 1.4). The particles are positioned sufficiently far from the surface of a glass slide in order to neglect the interaction between the particles and the substrate. Green and red beams from a green LD laser (532 nm, 21 kWcm ) and a He-Ne laser (632.8 nm, 21 kW cm ) are introduced coaxially into the microscope and slightly focused onto each microparticle as an illumination light (the irradiated area was about 3 pm in diameter). The sizes of the illumination areas for the green and red beams are almost the same as the diameter of the microparticles (see Figure 7.4). The back scattered light from the surface of each microparticle is...

It was apparent that when the two immiscible fluids were stirred droplets of chloroform formed in the aqueous phase. It was hypothesised that the response of the phototrode was dominated by light scattered back from the droplets without reaching the mirror and that, as the droplet size decreases, the intensity of the back scattered light increases. This was confirmed in tests by increasing the rate of stirring and so decreasing the droplet size. 

A further difference is that the value of the transmittance minima of the titration curves is higher than that found for the water and chloroform system. This is believed to be due to the fact that in a titration the equivalence point will not correspond to an integer number of hyamine aliquots and so, even if the contents of the reaction vessel were allowed to equilibrate fully, there would still be a small amount of uncomplexed surfactant present which will be sufficient to decrease the droplet size and so increase the back-scattered light. 

The absorption fraction of a particle is related to the volume of the particle. Thus, the larger the volume of a particle, the more of the incident light is absorbed. In contrast, reflectance is related to the particles surface area, being in turn dependent on material porosity. The absorption/remission function relates to the fraction of absorbed light, the fraction of remitted (or back scattered) light, and the fraction of light transmitted by a representative layer... 

Figure 16. Effect of particle aggregation on the back-scattered light intensity profile of food emulsion, containing 20 wt% emulsified fat, at 5 °C (green light).

An interesting modification of this technique is the fibre-optic dynamic anemometer (FODA)143. A length of fibre-optic cable carries the laser beam to the interior of the dispersion. Back-scattered light, with its Doppler frequency shift, is returned to the detector along with reflected light and, again, the resulting beat frequency pattern is analysed. Since only a very small volume around... 

Figure 19.20. In situ pressure dissociation-association kinetics of casein-micelles after a pressure jump monitored by the average back-scattered light intensity (a) dissociation upon pressure inerease (b) reassociation upon pressure release (Gebhardt et al. 2006).

Shimadzu Said 2001 covers the size range from 0.03 to 700 pm using a 76 element sensor to capture the forward scattered light while side and back-scattered light is captured by three other sensors. 

Leeds and Northrup Microtrac Ultrafme Particle Analyzer (VPA) uses the controlled reference method, using a sapphire tipped waveguide that collects back-scattered light within 100 pm of probe tip, to cover the size range 0.003 to 6.5 pm. 

For particle sizing, many varieties of cascade imp-actors perform well, although care must be taken to avoid errors introduced by sampling (such as collection in sampling lines). The laser/Doppler-type particle-size device can be used to measure aerodynamic size and low concentrations with a rapid readout. In a system described by Cook, a powerful pulsed laser using temporal analysis of back-scattered light can be used to measure the spatial distribution of particles. 

By confining the liquid flow in a glass capillary to a diameter of only 1 jum, a fully illuminated effective probing volume of only about 1 fl could be formed, allowing one to count essentially every molecule in the flow [45], The capillary was embedded in immersion oil to achieve a high NA and to keep the level of back-scattered light low. A time-dependent fluorescence... 

Type I probe has a diameter larger than that of the particle, the output signals are all generated by back scattered light from the particles, and the integrated values of the output signals can be correlated with the concentration of particles by any calibration method, from which the instantaneous concentration can be obtained by output signals analysis. These probes have been widely used in concentration measurement of particles. 

Experimentally, we obtained the following form of the Mueller matrix for transmitted and back scattered light for English oak leaves [4] ... 

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