A radiation detector window structure for use with a radiation detection system includes a frame, a plurality of upstanding spaced-apart ribs held in place by the frame, where the tops of the ribs terminate generally in common plane, and a thin film of material disposed on the tops of the ribs to span over the gaps therebetween for passing the radiation to be detected and for filtering at least some of the unwanted radiation. The tops of the ribs are smoothed and rounded to minimize a chance of piercing the film placed thereover. The ribs are spaced to provide sufficient support for the film so that the thickness of the film may be reduced to better transmit desired radiation.