A method for thermally processing a sheet of a thermographic material provides good flatness and dimensional stability together with a high optical homogeneity. The method incorporates the steps of supplying a sheet of thermographic material m (1) to a thermal processor (10) having a processing chamber (12), heating the processing chamber to a predetermined processing temperature, and transporting the sheet of thermographic material through the processing chamber in a sinuous way (4). This transporting is carried out by a first drivable belt (21), a second drivable belt (22) and backing means (27).