The measurement principle of laser particle sizeThe measurement principle of laser particle size(Dandong Bettersize Instrument Ltd. Wangyongquan)Introduction At present, laser diffraction particle size tester has been applied widely, especially in abroad, it has been recognized conformably. The remarkable features are: high measurement precision, fast response speed, good repetitiveness, wide measurable particle diameter range and touchless measurement etc. The research and production of the kind of instruments in China is comparatively short. The need of the instruments is at least 100 every year in home market, but the lowest price in foreign countries is 50,000 $ every machine, so our country pays foreign exchange 5000,000 $ in purchasing the kind of instruments at least every year. In latest several years, we have developed multifold model laser particle size tester successfully, the main performance is alike with foreign same products. The measurement principle of laser diffraction particle size tester The operational principle of laser diffraction particle size tester we studied bases on Fraunhofer diffraction and Mie scattering theory. We know from physics optics deduction that the scattering of incidence light vs particles accord with classical Mie theory. Mie scattering theory is rigorous mathematic root of Maxwell electromagnetic wave equation group, while Fraunhofer diffraction is only a kind of approximation of Mie scattering theory. Fraunhofer diffraction is applicable to the situation that particle diameter is far more than the incidence wave length, and is assumed that the light source and receiving screen are all boundlessly far away the diffracting screen. Considering from the theory, Fraunhofer diffraction is relatively simpler in application. The basic device of laser diffraction particle size tester sees attached drawing 1. Low energy laser sends monochromic light of wavelength 0.635 um, and the light passes through space filtering and diffusing beam lens to filter miscellaneous light and form Max diameter 10mm parallel monochromic light beam. The light beam irradiates the particles in measurement area, and occurs light diffractive phenomenon. The intensity distribution of diffracted light follows to Fraunhofer diffraction theory. Fourier conversion lens at the back of measurement area is receiving lens (knowing lens range), scattered light forms far magnetic field diffractive graph on back focus surface. Multi-ring photoelectric detector on back focus surface of receiving lens can receive the energy of diffracted light and translate it into electric signal and output. The center hole of the detector measures the consistency of allowable sample volume. The diffractive graph of the particles is still and centralizes on light shaft range of the lens. So it does not matter that the particles are dynamic to pass through analyzing light beam, the diffractive graph is a constant to any lens distance. The lens conversion is optics, so it is very fast. According to Fraunhofer diffraction theory, when a spherical particle of diameter d is within measurement area, its light intensity distribution of any angle is:In the equation:f ：the focal length of receiving lens：the wavelength of incidence lightJ1 ：first order Bessel function：scattering angle When the diffracted light intensity distribution of the laser lays upon the No n ring of photoelectric detector (ring radius is from Sn to Sn+1, corresponding scattering angle is fromn ton+1), the light energy is:The equation (1) is substitute into I（）, then we get: J0 ：zero order Bessel function If there are N quantity particles of diameter d, the received light energy on No n light ring is N times（Nen）more than that of one particle. On the analogy of this, if there are Ni quantity particles of diameter di in the particles, total diffracted light energy in the particles is the sum of all particles diffracted light energy, that is, If we use W to represent dimension distribution, the relation between W and N is:In the equation： is particle density，above equation is substitute into equation（4）,we get：The equation (6) set up the corresponding relation between every ring diffracted light signal on photoelectric detector and particle diameter and distribution of measured particles. In particle calculating, there are 96 effective rings on photoelectric detector we use, so we divide the diameter into 96 sections, the geometric shape of photoelectric detector see attached drawing 2, radius data of every ring is as follows (unit: mm):Above formula shows inner radius is Sn and outer radius is Sn+1 of No n ring. Choose particle diameter section according to the following formula calculating: in the formula：f ：receiving lens of focal length 180mm：semiconductor laser of wavelength 0.635Above formula shows the section upper limit is Dn an