Refractive indices and absorption coefficients of exemplary materials available for extrusion-based 3D printing technique. High density polyethylene (HDPE) is illustrated as a reference material but it cannot be processed using this technique. The group of biopolymers—FiberWood, FiberSilk, BioCREATE and BioWOOD, thermoplastic polyurethane (PolyFlex) and thermoplastic polyester elastomer (FiberFlex 40D) are characterized with largest absorption coefficient values—similar to polylactide (PLA). The butenediol vinyl alcohol co-polymer (BVOH) together with acrylonitrile styrene acrylate (ASA) have slightly smaller absorption coefficient value. The most transparent (not taking into account reference HDPE) is polypropylene (PP), then high impact polystyrene (HIPS) and nylon–poliamid 12 (PA12). The refractive index values for all illustrated materials are in the range between 1.5 and 1.65, which is very similar to the values of refractive index for glass for visible light.