Title: Effect of pigments on laser beam transmission in diode laser transmission welding of poly(propylene) Abstract: Welding technologies are state of the art for joining polymer composites, with one of the two joining parts to be laser transmissive (LT) and the other to be laser absorptive (LA). Pigments are often added to LT to enhance the crystallinity of the polymer matrix. However, pigments lead to internal scattering of the laser beam, and the rate of transmission or the laser energy density decreases. Depending upon the type and amount of pigments added in the formulation of LT, the percentage of the laser beam transmitted, absorbed, or scattered differs. Laser welding performance depends upon the laser energy available for welding after considering the losses. In the present study, optical transmission of injection moulded isotactic polypropylene (iPP) samples was analysed with a varying dosage of organic pigment (neat PP, 2%, 3%, 4%, 5%, 6%, 8% and 10%) using LPKF TMG 3 transmission tester. The device uses a wavelength of 980 nm and simulates the optical radiation conditions of diode laser transmission welding (LTW). The percentage transmission varied with the sample thickness as well as with the dosage of pigment. The modified Bouguer-Lambert law described the transmission energy and apparent extinction coefficient. The model was validated with the experimental value of transmittances of the samples with a varying sample thickness of iPP. There was a decrease in the percentage of laser transmission with an increase in the pigment content of the samples. It was found that the apparent extinction coefficient is the function of the pigment levels. Keywords: polymer composites; pigments; optical transmission; polypropylene; laser transmission welding; extinction coefficient