The installation (10) is adapted for the heat treatment of objects, such as plastic preforms (17), and comprises a reflective device exhibiting a plurality of elongated and opened IR-reflective cavities stacked one onto the other according to a stacking axis and arranged to lodge elongated IR lamps (16) within, where the aperture of each cavity faces generally a main axis parallel to the stacking axis along which the object would be placed. The reflective device (20) further comprises protrusions separating the cavities one to the other and extending generally transversal/transverse to the stacking axis, the reflective device being made as at least one integral block of a heat-conductive material. The cavities may each comprise a curved bottom portion and two opposite side surfaces provided with respective longitudinal breaks of slope at a junction with the curved bottom.