Method for thermal conditioning, within a thermal conditioning unit that is equipped with a row of adjacent emitters that each comprise a number of monochromatic electromagnetic-radiation sources, of a queue of preforms of containers having a head preform or a tail preform, with this method comprising the following operations: Periodically determining the position of the head preform, or, respectively, of the tail preform; As the queue of preforms moves along, gradually igniting the emitters that are located downstream and in the vicinity of the head preform or, respectively, gradually extinguishing the emitters that are located upstream and in the vicinity of the tail preform.