Abstract: Selection criteria for dyes that predicts efficiency and performance for plastics welding. A first stage quantitative calculation screens dyes that possess an absorption band that can be matched to a laser that is suitable for plastic welding. It also identifies absorption troughs in the visible spectrum and establishes relationships between the relative optical densities of the absorption band and trough. A second stage quantitative calculation screens dyes for their contribution to the transparency of the substrate. By combining these two stages, the usefulness of a candidate dye can be quickly, easily and inexpensively determined.