Light scattering extinction

A patent was granted in 1969 for a gravity. X-ray, sedimentation design. A predispersed sample is introduced into a temperature controlled measurement cell. Observations are taken of the incremental change in concentration or density, with respect to time, of the sample at a specific point within the cell. These observations are made by either X-ray or light scattering extinction techniques (Figure 5). 

Fig. 13.11 Illustration of the forward light scattering (extinction) (top) and light backscatter (reflection) bottom) principles applied in fiber optical probes. The figure is reproduced from Tayebi et al. [196] with permission from Taylor Francis Ltd...

The Rayleigh scattering extinction coefficient for particle-free air is 0.012 km for "green" light (y = 0.05 /rm) at sea level (4). This permits a visual range of —320 km. The particle-free, or Rayleigh scattering, case represents the best visibility possible with the current atmosphere on earth. 

Suspended particles are the most important factor in visibility reduction. In most instances, the visual quality of air is controlled by partide scattering and is characterized by the extinction coeffident The size of particles plays a crucial role in their interaction with light. Other factors are the refractive index and shape of the particles, although their effect is harder to measure and is less well understood. If we could establish these properties, we could calculate the amount of light scattering and absorption. Alternatively, the extinction coeffident associated with an aerosol can be measured directly. 

Extinction Coefficient a measure of the ability of particles or gases to absorb and scatter photons from a beam of light a number that is proportional to the number of photons removed from the sight path per unit length. See absorption. Extinction Cross Section the amount of light scattered and absorbed by a particle divided by its physical cross section. 

This fluorophore has an excitation maximum at 502 nm and an emission maximum at 510nm. The small Stoke s shift of only 8nm creates some difficulty in discrete excitation without contaminating the emission measurement with scattered or overlapping light. The extinction coefficient of the molecule in methanol is about 77,000M 1cm 1 at 502nm. 

The excitation maximum for BODIPY 493/503 C3 hydrazide occurs at 498 nm and its emission at 506 nm. Since this is an extremely small Stoke s shift, it may be difficult to avoid completely problems of excitation-light scattering interference in critical emission measurements unless sub-optimal excitation wavelengths are used. The molecule has an extinction coefficient in methanol of about 92,000M-1cm 1 at 493 nm. 

The extinction coefficient can be expressed as the sum of terms which account for light scattering and absorption by gases and suspended particles ( ) ... 

The quality of statistically inferred species extinction balances can be enhanced with proper aerosol sampling. Due to its important role in light scattering, only the fine aerosol should be sampled. A mass balance should account for all major fine particle species. Ideally the particle scattering coefficient should be measured directly at the location where aerosol is sampled by the filters. The importance of soot and other carbonaceous aerosol contributions to light extinction in arid regions should not be overlooked. 

Malm, W. C., J. V. Molenar, R. A. Eldred, and J. F. Sisler, "Examining the Relationship among Atmospheric Aerosols and Light Scattering and Extinction in the Grand Canyon Area, J. Geo-phys. Res., 101, 19251-19265 (1996). 

The full extinction Cext will be observed only if the detector subtends a sufficiently small solid angle. As the detector is moved closer to the particle, however, the observed extinction will decrease. We shall see when we consider specific examples that light scattered by particles much larger than the wavelength of the incident light tends to be concentrated around the forward direction. Therefore, the larger the particle, the more difficult it is to exclude scattered light from the detector. 

Measured extinction spectra for aqueous suspensions of polystyrene spheres—the light scatterer s old friend—are shown in Fig. 11.19. Water is transparent only between about 0.2 and 1.3 jam, which limits measurements to this interval. These curves were obtained with a Cary 14R spectrophotometer, a commonly available double-beam instrument which automatically adjusts for changing light intensity during a wavelength scan and plots a continuous, high-resolution curve of optical density. To reproduce the fine structure faithfully, the curves were traced exactly as they were plotted by the instru-... 

Absorption resonances resulting from excitation of surface modes are accompanied by scattering resonances at approximately the same frequencies this was pointed out following (12.26). In most experiments transmission is measured to determine extinction, which is nearly equal to absorption for sufficiently small particles. However, surface mode resonances have been observed in spectra of light scattered at 90° by very small particles of silver, copper, and gold produced by nucleation of vapor in an inert gas stream (Eversole and Broida, 1977). The scattering resonance peak was at 3670 A, near the expected position of the Frohlich mode, for the smallest silver particles. Although peak positions were predictable, differences in widths and shapes of the bands were concluded to be the result of nonsphericity. 

Hodkinson, J. R., 1963. Light scattering and extinction by irregular particles larger than the wavelength, in Electromagnetic Scattering, M. Kerker (Ed.), Macmillan, New York, pp. 87-100. 

Hodkinson, J. R., and I. Greenleaves, 1963. Computations of light-scattering and extinction by spheres according to diffraction and geometrical optics and some comparisons with the Mie theory, J. Opt. Soc. Am., 53, 577-588. 

---