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.
#REFLECTIVE LASER DIFFRACTION IMAGE 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. D 1 or D 99).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. The beginning and end of the distribution are commonly defined by D 10 and D 90, although other D values can be used to define the cumulative distribution as well (e.g. D 50 defines the point where 50 % of the particles are smaller and 50 % bigger than that certain diameter. In either direction, the cumulative curve always ranges from 0 % to 100 %, with the middle point D 50 being the most commonly reported result of particle sizing by laser diffraction. This is done either from the smallest to the biggest diameter (called the "undersize curve") or in the opposite direction (called the "oversize curve"). To get this distribution, values for all previous classes are added to the next. For this reason, usually the cumulative distribution is analyzed. spikey, flat, etc.), so peak values are rather unreliable. However, there might be more peaks or the peak might be weakly defined (e.g. The D mode value defines the position of the highest peak. The basic particle size distribution might have one or more peaks for size classes, which indicate the most common particle sizes. The sample de-agglomerates (breaks down into smaller sized particles) as particles collide with each other or with the wall of the dispersion unit.Ī typical result of a laser diffraction measurement is shown in Figure 11. In dry mode the powder is put into motion either by compressed air or by gravity, creating a dry flow which is positioned in front of the laser beam. The liquid dispersion unit is usually equipped with a mechanical stirrer with adjustable speed and with a sonicator with adjustable duration and power. The sample keeps circulating until the measurement is done. In liquid mode the particles are dispersed in a liquid and pumped into a glass measurement cell which is placed in front of the laser. it should be measured in liquid mode if the final product is a liquid dispersion and in dry mode if the final product is a powder.
Usually a sample should be analyzed in a state relevant to its application, i.e. This means that each particle should be visible as a single particle in front of the laser, moving through either liquid medium or air. In order to get a clear diffraction, it is necessary to have a proper dispersion of the sample.
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