Optical domes are lenses with two concentric spherical surfaces. Sometimes they are referred to as bent plane parallel plates. Consequently it is usual to define parallelism between two surfaces as the maximum thickness variation between the two surfaces. In perfect domes, the convex radius equals the sum of the dome thickness and concave radius. Optical domes are used for underwater photography, as a part of a protective case. Also they are as a part of sun tracking used in solar collectors.