3D simulation for scatter light distribution of optical surface defects

Surface defects evaluation system (SDES) works on the principle of microscopic scattering dark-field imaging (MS-DFI) and takes the criterion board as reference for calibration. Unfortunately, for criterion board with rectangular section scribed lines, image width of narrow lines doesn't follow the linear law, making it confusing to get real width. Besides, other criterion board except with rectangular section scribed lines in a flat plane is difficult to fabricate, which limits measurement accuracy and extensive use of SDES. In this paper, a 3D simulation model is established to simulate scatter light distribution induced by surface defects. The interactions between the incident light and surface defects in near field is calculated with the help of Finite-Difference Time-Domain (FDTD) method, a kind of Maxwell's solver. Skills as rotation and incoherent summation are applied to obtain results under illumination of unpolarized, broad-spectrum natural light sources in uniform annular layout. Finally, near to far field projections based on vector diffraction theory is carried out to get scatter light intensity distribution in far field. The data is also post-processed by scripts to describe imaging process simplified by a lens system so that it can be compared to experiment images. The 3D simulation model reveals MS-DFI process theoretically and may help to interpret image width of surface defects. The establishment of the 3D imaging model is an attempt to overcome the limits of the criterion board and is expected to provide reference for calibration for wider applications of SDES.