Tower Optical Corp. is offering optical flats made from BK7 or fused silica. They are used as a reference against which the flatness of an unknown surface can be determined and also as windows. The fused silica types are available in single- and double-sided versions and maintain a flatness of λ/10 or λ/20. Their diameters range from 25 to 200 mm with λ/10 flatness. The BK7 types offer flatness of λ/4 or better.