This model represents the cross section of a spherical Fresnel lens fused with the cross section of a conventional spherical plano-convex lens of equivalent power.
About Fresnel Lens:
"A Fresnel lens is a type of compact lens originally developed by French physicist Augustin-Jean Fresnel for lighthouses.
The design allows the construction of lenses of large aperture and short focal length without the mass and volume of material that would be required by a lens of conventional design. A Fresnel lens can be made much thinner than a comparable conventional lens, in some cases taking the form of a flat sheet. A Fresnel lens can capture more oblique light from a light source, thus allowing the light from a lighthouse equipped with one to be visible over greater distances."