A window assembly (100,110,120,130) for irradiating infrared light (L) comprises a light guide (5) for infrared light (L), which is formed by a gap between a first transparent substrate (2), having an exterior surface and an interior surface, which faces the light guide (5), and a second transparent substrate (3) substantially parallel to the first transparent substrate (2) and having an exterior surface and an interior surface, which faces the light guide (5) and the interior surface of the first transparent substrate (3). A first and a second reflective layer (12,13), that are both substantially reflective for infrared light (L), extend over the interior surfaces of respectively the first and the second transparent substrate (2,3). The second reflective layer (13) is provided with an opening (21) through which at least part of the infrared light (L) exits the light guide (5). In one embodiment, the window assembly further comprises an infrared light source (1) for directing the infrared light (L) into the light guide (5). In this way the infrared light (L) leaves the light guide (5) in one main direction through the opening (21) of the second reflective layer (13) and through the second transparent substrate (3), thereby generating heat in one main direction only.