The general purpose of these designs are described in “3D Printed UV-vis Cuvette Adapter for Low Cost and Versatile Spectroscopic Experiments”, which is published in the open access journal ACS Omega. Link to publication:
This particular part is meant to allow reproducible analysis of films on a microscope slide with a film in the shape of a triangle. This adapter can be used in most standard UV-Vis spectrometers. We recommend printing this design with a black filament to reduce the amount of light scattering.
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