summary
From three aspects: experimental data, qualitative analysis of physics and geometric optics, and rigorous calculations of Mie theory, it has been proven that even particles with scattering light fields much larger than the wavelength of light have significant errors between their actual light energy distribution and the results given by diffraction theory. This is manifested in the fact that the actual scattered light energy at larger scattering angles is much greater than the theoretical diffraction light energy. According to the calculation results of diffraction theory, this error is equivalent to the distribution of light energy generated by particles of about 1 µ m. If the particles have absorption of light, the error will be significantly reduced.

Keywords: diffraction, MIE, scattering, light energy distribution

Introduction
In the field of laser scattering particle measurement, it is generally believed that the light scattering of larger particles (generally referring to particles with a diameter greater than 2 µ m) can be described by the diffraction of the projection disk of the particle. However, in years of practical work on laser particle measurement, the author found that even particles with a diameter of approximately 100-200 µ m had significant differences between the distribution calculated by diffraction theory and the measured light energy distribution. This article first compares a set of measured results with diffraction theory results, points out their differences, and then analyzes this phenomenon from several aspects such as physical optics, geometric optics, and Mie theory. It is concluded that using diffraction theory to calculate the scattered light energy distribution of large particles generally has significant errors, and this error can lead to errors in particle size analysis of the measured sample.