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 .