An assimilation lamp device (1) for stimulating plant and crop growth comprises a central lamp unit (10), comprising a body (14) and a plurality of LEDs (12) mounted to an under surface of the body ( 14 ), wherein the body ( 14) is made of a thermally well conducting material, for instance aluminum, and acts as a heat sink for the heat generated by the LEDs. The body (14) is provided with cooling fins or lamellae (41). The assimilation lamp device further comprises air stream generating means (42) such as a fan for generating a downward air stream (43) in heat exchanging contact with the cooling fins or lamellae, so that heat is removed from the respective cooling block and used to increase the temperature of said downward air stream (43). The air stream generating means ( 42) such as a fan is controllable independently from the lamp operation.
