A resonant optical cavity is disclosed that has a first partially reflective input end face and a second partially reflective end spaced a predetermined distance apart. Disclosed is a device for use with a stent deployed on a catheter balloon. The user defined window, comprising a closed polygon, is converted on a frame-by-frame basis to a matte that is keyed with image processing such as color correction values. Such a structure may be particularly applied to the electronic modules on board aircraft.