Laser diffraction Fraunhofer approximation

Particle Size Laser Refractometiy is based upon Mie scattering of particles in a liquid medium. Up until about 1985, the power of computers supplied with laser diffraction instruments was not sufficient to utilize the rigorous solution for homogeneous spherical particles formulated by Gustave Mie in 1908. Laser particle instrument manufacturers therefore used approximations conceived by Fraunhofer. 

To monitor nanoparticle swelling in salt environment, we employed laser diffraction Mastersizer Micro Particle Analyzer MAF5000 (Malvern) with a dynamic range of 0.3 to 300 pm. This instrument utilizes Mie scattering algorithm with the Fraunhofer approximation. 

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. 

Figure 7-11 Calculated response curves for the intensity of diffracted light based on Fraunhofer approximation for different minimum collection angles (maximum collection angle 4.76°, laser wave length 514.5 ran)...

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