A T-Stop is the term used for the true value of an f-stop, rather than a theoretically calculated f-stop . The T-Stop is the real amount light transmitted by a lens, measured on an optical bench (Camera crew)
Take Down refers to the method used in film & video production, of damping down the light on an object via use of a dimmer, nets, scrims or simply by wasting light. (Grip/Lighting)