The Si1102/Si1120 should be placed as close to the window as possible in the design. If the Si1102/Si1120 are deeply recessed into the product enclosure, it will require a convex lens with a focal length equal to the distance from the window to the detector to collect light in the axial direction. The larger the hole, the better the light collection. Please keep in mind that lenses are not recommended for IR slider applications.