A viewing panel unit is disclosed, comprising: a first panel comprising one or more light transparent regions that are substantially transparent to visible light and one or more light hindering regions which substantially hinder transmission of visible light; a second panel comprising one or more light transparent regions that are substantially transparent to visible light and one or more light hindering regions which substantially hinder transmission of visible light; and an actuator (6) for moving the second panel relative to the first panel in a plane parallel to the plane of the first panel, wherein the actuator comprises: a cam (10) configured such that rotational movement of the cam causes linear movement of said second panel relative to said first panel; a shaft (8) extending through the cam; and an engaging member (15), wherein: the engaging member provides a non-rotatable connection between the shaft and the cam such that rotation of the shaft in use causes a corresponding rotation of said cam; the engaging member comprises an engaging portion that extends radially outwards from the shaft; and the engaging portion is axially encapsulated within the cam.