Abstract: Commercially available automatic iris lens assemblies typically respond to the peak-to-peak level of the video signal supplied from the imager, for controlling the amount of light imaged thereon. For preventing the dark current component of the video signal from undesirably reducing the dynamic control range of the iris control, the video signal is modified before it is applied to the automatic iris lens assembly. The modification comprises shifting the average value of the video signal to a given level approximating the average level of a normal video signal and then setting the levels of the blanking portions to the given level.

Abstract: Ventilator apparatus which includes rotary impeller or fan structure and an electric motor for rotation thereof. A shaft joins the rotor of the electric motor to the impeller or fan structure for rotation thereof. The rotor of the electric motor, in addition to being rotatable, is axially movable upon energization thereof. A shroud encompasses the impeller or fan structure. A closure member attached to the shaft is adapted to be closely positioned with respect to the end of the shroud to substantially close the shroud when the apparatus is not in ventilating operation. When the rotor of the electric motor is energized for operation of the impeller, the rotor, in addition to commencing rotation, moves axially and moves the closure member axially to spaced relationship from the shroud, to provide an opening between the shroud and the closure member for flow of air between the shroud and the closure member.