Optical domes are lenses with two concentric spherical surfaces. Sometimes they are being referred to as bent plan parallel plates. Consequently it is customary to define parallelism between two surfaces as the maximum thickness variation between the two surfaces. 

In perfect domes the convex radius is equal to the sum of the dome thickness and concave radius. Slight departures from that rule are sometimes desired. Optical domes are used in IR missiles, for underwater photography as part of protective case and as a part of sun tracking solar collectors.