Abstract: In a discharge arc lamp, an arc tube comprises an arc tube body and a main electrode at each end inside of the arc tube body, between which in the operating condition of the lamp a discharge takes place. At each end of the arc tube is an end closure comprising an end plug joined to respective each end and a cap member, the end plug and the cap member having facing surfaces. A seal provided by sealing means to seal the cap member to the end plug includes an annular seal portion between the facing surfaces of the end plug and the cap member. One or both of the facing surfaces is so shaped that the annular seal portion has a predetermined and uniform thickness t.