Spatial coherence and diffraction effects in beam homogenizers for optical-based 3D printers

This work reports the effects of spatial coherence and diffraction on the image quality of an imaging beam homogenizer used in optical-based 3D printers, using digital light processing (DLP). A numerical wave-optics model is developed by combining the angular spectrum propagation method (ASM) with the elementary function method (EFM) to predict the output of the beam homogenizer used in optical-based 3D printers under various conditions. This allows studying the effects of spatial coherence and optical diffraction on the image quality of the imaging homogenizer, helping to optimize the illumination system. The performance of the beam homogenizer is investigated using a source with a variable size and viewing angle to modulate the degree of coherence. This model is simulated in a two-dimensional setting to reduce computational time and memory requirements. In addition, the impact of varying micro-lens array (MLA) parameters on the beam homogenizer image is studied. Also, the numerical model is used to compare imaging and non-imaging beam homogenizers, as well as regular and random microlens arrays. The results of this study demonstrate that diffraction effects and spatial coherence play a critical role in beam homogenizer performance, particularly with respect to achieving beam uniformity. Finally, the numerical model is compared with experimental results obtained under nearly the same lab setup parameters.