Installation for heating the bodies (1) of thermoplastic preforms (2) with a view to manufacturing containers by blow-molding or stretch-blow-molding, while the preforms (2) are being moved along with their bodies (1) following a predetermined path (T), this installation comprising at least one directional source (5) of electromagnetic radiation directed towards a location (E) on the path (T); the source (5) is inclined by an angle (α) of between about 60° and 10° and preferably less than about 45°, with respect to the tangent (6) to the path (T) at the location (E); thus, the electromagnetic radiation can pass through several preform bodies or parts of bodies without being appreciably reflected back towards the source (5).