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Strictly speaking, Fraunhofer diffraction is only a special case of Mie theory, which describes all diffraction and scattering phenomena comprehensively.The Royal Society of Chemistry and the Higher Education institutions involved in this project are not liable for the actions or activities of any reader or anyone else who uses the information in these resource pages or the associated materials. This kind of evaluation is done according to Mie Theory, named after the physicist Gustav Mie. These optical properties, primarily the refractive index, must be known for the evaluation of the particle size distribution.
However, this approach is not applicable for smaller and transparent particles, since here the optical properties of the particles also have an influence on the intensity distribution at the detectors. The advantage of the Fraunhofer approximation is that no other material properties of the sample need to be known. This distribution is described by the so-called Fraunhofer theory. Laser Diffraction and Static Light Scattering Analysis are often used interchangeably, although the term laser diffraction has become established in many industries and laboratories.ĭiffraction produces maxima and minima in the intensity distribution at characteristic angles. However, with a suitable design, an angular range of 0-163 ° can also be covered with the Fourier arrangement. Therefore, Microtrac laser diffraction analyzers use the Fourier arrangement. The advantage of the inverse Fourier method is that one can collect a wider angular range on a smaller detector array. All this generally leads to blurry diffraction patterns compared to the Fourier setup. With the inverse Fourier setup, the particle stream must be relatively narrow, and in addition, particles of the same size in the convergent beam have different diffraction angles relative to the optical axis. With Fourier optics, the particles are illuminated by a parallel beam, whereas with an inverse Fourier arrangement a convergent laser beam is used.įourier optics offer the advantage that the diffraction signal is always correctly detected regardless of the position of a particle in the laser beam, and equal diffraction conditions prevail at any point in the interrogated sample volume.
#Laser diffraction experiment discussion iso
Appropriate considerations must be made to properly account for irregular particle shape.Īccording to ISO 13320, measuring instruments for laser diffraction can be operated with either Fourier optics or reverse Fourier optics. An irregular particle shape leads to broader size distributions, since both the width and the length of the particles contribute to the overall scattering signal and are included in the result. Therefore, laser diffraction is a so-called ensemble measurement method.ĭuring evaluation, all signals are treated as if they were generated by ideal spherical particles. This is therefore an indirect measurement method since the size is not measured directly on the particle but is calculated via a secondary property (diffraction pattern).įurthermore, the recorded pattern is generated by particles of different sizes at the same time, so it is a superposition of the scattered light of many particles of different sizes. When evaluating the signal, it must be taken into account that a particle size does not correspond to a specific angle, but that each particle scatters light in all directions at different intensities. The evaluation of this signal is based on the principle that large particles preferentially scatter light at small angles whereas small particles have their scattered light maximum at large angles. In laser diffraction analysis, the scattered or diffracted light is recorded over the widest possible range of angles by means of a special laser and detector arrangement.
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