You need a laser flat window when wavefront preservation, wedge control, and surface finish matter more than simple protection. OPTOStokes specifies its large-aperture laser flat windows at better than λ/20 PV @ 632.8 nm, better than λ/200 RMS, better than 20/10 scratch-dig, 1 arcsec parallelism, and clear apertures from φ100 mm to φ620 mm. The available substrates are UV fused silica (Quartz) and K9, with high-power AR or metallic mirror coatings in Ag, Au, or Al. The page positions them for intracavity applications, beam steering, high-energy laser delivery, aerial surveillance, telescopic systems, and interferometry reference flats. OPTOStokes also offers custom coating stacks, including broadband AR up to 99.5% transmission, so this is a real engineering component, not just a cover plate. Contact their team when your build needs custom aperture, substrate, or coating schedules.