A method and an apparatus are provided for taking heat images of heat sealed seams. Such seams are made in heat sealing stations and in particular when making covers or lids, respectively, for containers, being provided with a tear-off foil that closes the container up to the first use of its contents. For ascertaining the air-tightness of the heat sealed seam during manufacturing a heat image of the seam is taken while the seam is still hot. By evaluating the image it can be detected whether the temperature of the seam is within a range that assures a correct seam or has exceeded an upper or lower limit for the temperature which is indicative of a defective heat sealed seam. Defective objects can then be discarded based on the evaluation of the heat image. This test of the correctness of the heat sealing is much faster than a pressure differential test testing the actual air tightness of the seam and can thus be used in high speed manufacturing apparatus for covers or lids, respectively.