Scattering, light efficiency

In short, particles to the left of the peak in Fig. 14.38a do not tend to shrink or grow. Because they are generally in the 0.1- to 1-jum size range which scatters light efficiently (see Chapter 9.A.4), these particles are known as haze particles or droplets. These often occur at relative humidities below 100%. 

Features Fine particle size tight particle size distribution jagged angular shape helps scatter light efficiently good optical chars. low oil absorp. high hydrophilicity acids ease of disp. low sp.gr. cost effective Properties 1.2 p avg. particle size fineness (Hegman) 2 sp.gr. 2 g/ml oil absorp. 60 g/100 g pH 11.2 (5% aq.) hardness (Mohs) 2-3 90% anhyd. solids... 

The first temi results in Rayleigh scattering which is at the same frequency as the exciting radiation. The second temi describes Raman scattering. There will be scattered light at (Vq - and (Vq -i- v ), that is at sum and difference frequencies of the excitation field and the vibrational frequency. Since a. x is about a factor of 10 smaller than a, it is necessary to have a very efficient method for dispersing the scattered light. 

Small air bubbles also scatter light because the refractive index of air is about 1.0, whereas the refractive index of most polymers is approximately 1.5. Air bubbles in films are sometimes usehil in increasing opacity but the efficiency in scattering light is much less than for mtile Ti02 because the difference in refractive index is much smaller. 

Compare the wavelengths of visible light with the range of pardde diameters which most efficiently scatter light. 

The advantage of Raman spectromicroscopy is that very small specimens can be studied while still allowing the determination of the second and fourth moments of the ODF. However, the expressions for the Raman intensities are more complex since the optical effects induced by the microscope objective have to be considered. Although the corrections may be small, they are not necessarily negligible [59]. This problem was first treated by Turrell [59-61] and later by Sourisseau and coworkers [5]. Turrell has mathematically quantified the depolarization of the incident electric field in the focal plane of the objective and the collection efficiency of the scattered light by high numerical aperture objectives. For brevity, only the main results of the calculations will be presented. Readers interested in more details are referred to book chapters and reviews of Turrell or Sourisseau [5,59,61]. The intensity in Raman spectromicroscopy is given by [59-61]... 

The accuracy with which a system can measure lifetimes depends on a number of different factors including calibration of the instrument, the number of detected photons and also the efficiency of the analysis routines. In addition, sources of background and scattered light should be eliminated. Emission filters should be chosen with great care to make sure that no scattered laser light reaches the detector. Detection of scattered excitation light results in a spurious fast component in the decay and complicates the interpretation of the data. The choice of emission filters is much more critical in FLIM than in conventional fluorescence intensity imaging methods. 

An efficient optical coupling to the WGMs is instrumental in order to harvest the full potential of the high-2 droplet resonators. In most reported experiments, the droplet resonators are probed by free-space excitation, where, e.g., a Gaussian laser beam excites resonator modes and scattered light or fluorescence is detected. This approach... 

Moreover, it is easy to show that, if the emission is observed without a polarizer, an excitation polarizer must be set at 0 = 35.3° (cos2 6 = 2/3). This arrangement is suitable when the fluorescence is detected through an optical filter (to reject scattering light) and not through a monochromator, because of the polarization dependence of the transmission efficiency of the latter. 

Particles are known to scatter light as well as absorb it and this produces the white or pale appearance of fine powders. The even smaller nano-sized particles, however, are transparent because the scattering efficiency is reduced. This effect has led to the use of nanoparticles in sunscreens and cosmetics. These will still absorb ultraviolet light but will scatter less visible light. 

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