The invention provides a continuous method for producing polarizing films that includes a step of monoaxially stretching a polyvinyl alcohol film having a width of at least 2 m in an aqueous boric acid solution, wherein the polyvinyl alcohol film is stretched on the condition that it satisfies the following formulae and :A\u22675 \u2003\u2003AB\u22670. A user interface may be provided for a user to indicate an approximate location of an artifact appearing in a digital image. A lubricant level sensor is adapted to detect the lubricant level in the lubricant tank upon a main switch of an engine being turned on. The rotor may further include a device for controlling the direction of the rotor axis based upon the wind direction such as a rudder.