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What are the differences between light extinction and laser diffraction technique?


In an optical particle counter, particles are closely arranged one by one in a straight line in one dimension in the sample flow. In an ideal situation, particles will enter the illuminated area individually, and one pulse signal corresponds to one particle whose signal height will be applied for particle size calculation. In a laser diffraction particle size analyzer, particles are circulated in the dispersion system and irradiated by the laser beam, leading to a signal diagram that is generated through the overlapping of numerous single signals. In the optical particle counter, only the scattered signal at a certain angle will be received. In contrast, photodetectors are arranged in large quantities in specific locations in the laser diffraction particle size analyzer. 


For a single particle, most of the scattered signals generated can be received, theoretically, resulting in a signal diagram of a single particle. When numerous particles are illuminated by a laser beam simultaneously, the signal diagram displayed is integrated by innumerable signals of the single particles. Under these circumstances, the particle size calculation cannot be achieved using a calibration curve. Instead, the inverse algorithm is a solution, in which the particle size distribution can be acquired by comparing the theoretical light intensity distribution based on assumption with the measured light intensity distribution. When the residual is within the acceptable range, the assumption can be considered the measurement result.





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