A load port interface for determining the presence or absence of wafers in a cassette, i.e. a wafer mapper, is integrated with a port closure assembly in a load port interface separating a process environment from an operator environment. A cover plate and a door are arranged in an L-shape, with both members being able to seal the port in a bulkhead of the load port interface. In one configuration, the cover plate seals the port, while door panel members rest horizontally on the operator side of the bulkhead. In this position, a wafer stack, W, inside of a cassette may be placed on the inside door panel, with the top of the wafer stack, W, being open. As the door is raised from the horizontal position to a vertical position, the wafer stack, W, is rotated through an opening in the bulkhead so that the wafer stack, W, is now on the process side. The wafer mapper features a moveable trolley connected to, or within, the door. The trolley can move parallel to the wafer stack, W, under control of a stepper motor and examine each row of the stack, counting wafers as light is reflected or scattered from the edges of the wafers. Each wafer may be optically probed through a window in a cover plate and characteristic signals may be plotted to provide subsequent wafer manufacturing operations with wafer locations. A movable air knife on a side of the window opposite the trolley is magnetically coupled to the trolley and is moved together with the trolley in order to clear the window of liquid droplets from the process environment. When the load port interface positions the cassette and wafer stack, W, in the process environment, a fine spray of liquid droplets may be directed about the wafer stack, W.