A structure (10) for simulating a thermal image generated by a real world object, the structure (10) comprising a body (22), (24), (26) and (28) having at least one cavity (106), and at least one inlet opening into the cavity (106) for receiving a flow of fluid medium into the cavity (106). The fluid medium having a temperature that differs from ambient. The cavity (106) having internal configurations defining flow paths for the fluid medium to cause temperature variations on an external surface (26) of the body to simulate the thermal image of the real world object.