It is a calibrated neutral density filter, not a generic clear window. OPTOStokes lists the ND90 at 350–1100 nm, 90% ± 3% transmittance, OD 0.046, 1.1 mm thickness, and 60/40 surface quality. The page also states why that matters: it gives controlled 10% attenuation for multi-sensor calibration, protective windows, ghosting reduction, and industrial vision. If you need only slight light reduction without disturbing the spectrum, this part makes more sense than a plain window. OPTOStokes also offers OEM formats beyond 1.1 mm, plus CNC sizing, edge blackening, specialized AR coatings, and large-inventory support for fast R&D and production turnaround.