In laser processes, the absorption factor of laser Nd:YAG by metals plays a very important role. In order to model laser welding, we need to know its evolution during the process. The theoretical calculation does not enable the prediction of the absorption factor in the case of a keyhole mode. It is difficult to predict the effect of plasma and recoil pressure on the shape of the keyhole. In this paper, an integrating sphere is used to determine the absorption factor during the laser process, which is carried out on two types of magnesium alloys (WE43 and RZ5) and an aluminum alloy. We obtain the evolution in time of the absorption factor according to different steps of the evolution of the keyhole. (c) 2006 Elsevier B.V. All rights reserved.