Research On Measurement Accuracy Of Particle Sizing By Laser Diffraction Method

The method of characterizing particles by measuring the light that they scatter is called laser diffraction method and there are a variety of commercialized instruments based on this method at home and abroad.These instruments are widely used in particle size characterization.However,there are still some shortcomings in the basis theory and implementation scheme,which ultimately limit the measurement accuracy of certain particles.In this paper,to improve the measurement accuracy of laser diffraction method,a research on the anomalous change of Airy disk(ACAD),the measurement of the refractive index of particles and the detection of large and back scattering angles is carried out,the main work and innovation are as follows: 1.The anomalous change of Airy disk is studied in detail.The general existence of ACAD for transparent particles is revealed.And,the physical mechanism of ACAD is interpreted by using the geometric optical approximation(GOA).The analytical formulae between the anomalous size range and the refractive index are derived and the laws of ACAD are summarized.2.The mechanism of the indetermination of particle sizing in the anomalous size ranges is interpreted by using the singular-value decomposition method.And the existence of this indetermination is validated through inversion simulations.This indetermination is caused by the signatures of the Mie kernel and cannot be avoided by applying smoothness criteria in the inversion algorithm.3.The incorrect refractive index of particles to be measured will lead to the incorrect measurement results by laser diffraction method.The particle size distribution can be estimated by using the diffracted light in the near-forward angle.And then by using the estimated particle size distribution and the large angle scattered energy,the refractive index of particles can be evaluated.The particle size distribution is inverted again by using the Mie kernel of the evaluated refractive index.Numerical simulations and experiments show that(1): the lower limit of this method is about 6 ?m;(2)the evaluated value is correct for non-absorbing particles;(3)the evaluated imaginary part of absorbing particles is correct.4.To satisfy the measurement requirement of submicron particles,a laser particle size measurement based on an annular sample cell is proposed which overcomes the total reflection of the tranditional sample cell.The device is constructed,and the measurement results show a good agreement with the reference value and a good resolution for mixed standard particles.