Hi-Ends are made with polished optically-flat glass to ensure there are no funny refractions. They are made in all the normal sizes but with different ring thicknesses. We keep a range of 18mm in 1.4mm thickness in stock, in even sizes from 3.6mm to 4.2mm. Other sizes are available but we may have to order them. You will probably be very impressed with how clear the sight picture is.