According Christies London: The composition, with Christ in Judgment at the apex of two arcs, often described as rainbows, which lead to the Virgin and St John on either side, goes back at least to Stefan Lochner in Cologne (his Last Judgement of circa 1435 is still there, in the Wallraf-Richartz-Museum), and remained a convention in the Cologne School: for example in the panel by the Master of St Severin from the late 15th Century in the same museum.  The figure of St Michael weighing souls in the balance occurs in Rogier van der Weyden's Last Judgement painted for the Hospice de Beaune in circa 1443-5, and was widely adopted in The Netherlands and the Rhine valley thereafter.