An x-ray window including a support frame with a perimeter and an aperture. A plurality of ribs can extend across the aperture of the support frame and can be supported or carried by the support frame. Openings exist between ribs to allow transmission of x-rays through such openings with no attenuation of x-rays by the ribs. A film can be disposed over and span the ribs and openings. The ribs can have at least two different cross-sectional sizes including at least one larger sized rib with a cross-sectional area that is at least 5% larger than a cross-sectional area of at least one smaller sized rib.