Anisotropic optical artifacts in autofluorescence confocal imaging of FDM-Printed PLA: The impact of raster orientation on directional scattering and signal yield

Abstract

This study investigates the optical metrology failure of Fused Deposition Modeling (FDM) structures scanned via autofluorescence confocal microscopy. By analyzing 0°, 45°, and 90° raster orientations using 2D topographical profiles, 2D Fast Fourier Transforms (FFT), and 3D areal surface roughness metrics (S_a, S_q), we establish the direct impact of print geometry on signal fidelity. Results demonstrate that 0° and 90° orientations act as macroscopic reflection diffraction gratings, causing catastrophic geometrical shadowing and directional glare. This anisotropic Fraunhofer scattering artificially inflates apparent optical roughness by over 300% (p < 0.001) and introduces extreme statistical variance, rendering the metrology data clinically unusable. Conversely, the 45° diagonal orientation effectively bypasses these artifacts, preserving the macroscopic spatial periodicity and yielding a mathematically stable metrological baseline. Ultimately, configuring slicing software to a 45° raster fill is critical to circumventing systemic optical scattering and ensuring the high-fidelity topographical digitalization of FDM biomodels.