Fraunhofer diffraction pattern particle size measurements

D < 500 pm, Fraunhofer Diffraction Pattern Analysis (FDPA) can be employed in measuring particle size distributions (4,5). For the particles in the intermediate range, 0.7 pm < D < 10 pm, Mie theory of scattering holds and Turbidity Spectra (TS) can furnish information about particle sizes (6). 

In the early days of particle size measurement, the advantage of this relatively simple theory was that it usually describes the scattering patterns of transparent particles of a few micrometers in size in liquid media better than Fraunhofer theory. Similar to the Fraunhofer theory, the anomalous diffraction theory requires no exact knowledge of the refractive index, but it should not be used for opaque particles. 

Fraunhofer rules do not include the influence of refraction, reflection, polarization and other optical effects. Early Iziser particle analyzers used Fraunhofer approximations because the computers of that time could not handle the storage cuid memory requirements of the Mie method. For example, it has been found that the Fraunhofer-based instrumentation cannot be used to measure the particle size of a suspension of lactose (R.I. = 1.533) in iso-octane (R.I. = 1.391) because the relative refractive index is 1.10, i.e.- 1.533/1.391. This is due to the fact that diffraction of light passing through the particles is nearly the same as that passing around the particles, creating a combined interference pattern which is not indicative of the true... 

The newer la.ser diffraction instrument allows measurement for particle sizes ranging from 0.1 pm to 8 mm (7). Most of the laser diffraction instruments in the pharmaceutical industry use the optical model based on several theories, either Fraunhofer, (near-) forward light scattering, low-angle laser light scattering, Mie, Fraunhofer approximation, or anomalous diffraction. These laser diffraction instruments assume that the particles measured are spherical. Hence, the instrument will convert the scattering pattern into an equivalent volume diameter. A typical laser diffraction instrument consists of a laser, a sample presentation system, and a series of detectors. 

A number of expensive but very powerful analysers available on the market are based on Fraunhofer diffraction. An assembly of particles in a liquid or in a gas are illuminated by an expanded laser beam and the particle size distribution is derived from the measurement of the spatial distribution of the diffraction patterns on a flat detector behind the sample. 

Fraunhofer diffraction (FD), PCS, and high angle intensity measurements share many common attributes as techniques for particle sizing even though PCS does not depend on interpreting an intensity pattern as the other techniques do. 

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