- A Boolean ring is absolutely flat.
- A flat ring made of rubber or plastic, used as a gasket.
- At present, goggles myopic lens into the surface of the mirror and asperity absolutely flat mirror the two major types.
- Metal random packings include conjugate ring，VSP ring．Intaloxsaddles, HQM flat ring, Cascade ring and Pall ring.
- Absolutely flat mirror than an ordinary mirror goggles slightly some of the Lowland, which has the advantage of goggles in the water that did not arise as a result of differences in refractive effect.