What is the test limit of laser particle size analyzer?

The particle size measurement principle of laser particle size analyzer is the theory of MIE scattering. The MIE scattering theory describes precisely by mathematical language the spherical particles of d particle size with refractive index of n and absorptibility of m. Under the irradiation of monochromatic light with wavelength of λ, the scattering light intensity varies with the scattering Angle θ as a spatial distribution function. This function is also called the scattering spectrum. It can be concluded that the larger the particle is, the stronger the forward scattering is and the weaker the backward scattering is. With the decrease of particle size, the forward scattering is weakened and then the scattering becomes stronger. Chart 2 is a schematic diagram of scattering spectra of large, medium and small particles with wavelength as scale. Laser particle size analyzer is to test the scattered light intensity distribution of particles (in polar coordinates) through the array of photodetectors set at different scattering angles, so as to determine the particle size. This scattering spectrum has the characteristics of stable distribution of specific particles in space, so the instrument based on this principle is called static laser particle size analyzer.

When the particle size is small enough (about 1/10 of the wavelength), the distribution of light intensity becomes two overlapping circles. When the particles are very small, the forward and backward circles are perfectly symmetric (then called Rayleigh scattering). Therefore, when the scattering intensity distribution of dm particles is similar to that of very small particles, so that they cannot be distinguished by the photodetector array and subsequent signal processing circuit, it is considered that dm is the measurement limit of laser particle size meter.

This limit is also related to the laser wavelength. Studies show that the limit of the laser particle size analyzer for red light 635mm wavelength is 50nm, while that for blue light 405nm wavelength is 30nm.

In theory, the static laser particle size analyzer needs at least two conditions to resolve nano-scale particles: (1) a photodetector array to measure backscattering; (2) Shorter wavelength lasers are needed. In the range of visible light, 30 ~ 50nm is the lower limit of static laser particle size analyzer.