A method and device for heating at least one blank which is used to produce a container, such as a bottle, from a thermoplastic material, particularly PET, by means of blow molding or stretch blow molding. The blank is disposed between two opposing lateral walls which are positioned at a distance from one another. One of the walls supports several radiation sources which are disposed one above the other opposite the body of the blank. The space between the two lateral walls is closed at least partially beyond and close to the base of the blank. At least part of the radiation emitted beyond the base of the blank is reflected in the direction of the body of the blank. In this way, losses due to lost radiation are limited.