Abstract: Our invention is a process for stabilizing a projection mask which is put in operation at an elevated temperature. The frame containing the mask foil is heated to a temperature which is higher than the temperature of the mask foil. The mask foil is thus kept under tension by controlling the temperature of the frame it is held in and distortions like the distortions which would otherwise occur in long time operation and as conditioned by the mask foil hanging through it are avoided. The effect of the expansion of the mask foil can be compensated in the image forming unit by correction of the image formation scale.