Enhanced Schlieren System for In-Situ Observation of Dynamic Light-Resin Interactions in Projection-based Stereolithography Process

Digital maskless lithography is growing in popularity due to its unique ability to fabricate high-resolution parts at a fast speed without the need for physical masks. Though the theoretical foundation for photopolymerization exists, it is difficult to observe the voxel growth process in situ. This can be attributed to the low refractive index difference between cured and uncured resin, the microscopic size of the parts, and the rapid rate of photopolymerization after crossing the threshold. Therefore, a system that can address these issues is highly desired. Schlieren optics is a tool that makes the minute changes in the refractive indices visible. This paper proposes a modified schlieren-based observation system with confocal magnifying optics that create a virtual screen at the focal plane of the camera. The proposed technique visualizes the light deflection by the changing density induced refractive index gradient, and the use of focusing optics enables flexible positioning of the virtual screen and optical magnification. Single-shot binary images with a different number of pixels were used for fabricating voxels. Different factors affecting the voxel shape like chemical composition, energy input is studied. The observed results are compared against simulations based on Beer-Lambert's law, photopolymerization curve, and Gaussian beam propagation theory. The physical experimental results demonstrated the effectiveness of the proposed observation system. Application of this system in fabrication of microlenses and its advantages over theoretical model-based profile predictions are briefly discussed.