Fabrication/application of single-layer AR coating is limited by the availability of proper material. For example, the RI of glass and transparent plastic substrates are nS~ 1.5, therefore RI of the thin film coating, with quarter-wavelength thickness, must require n ~1.22. Unfortunately, materials with RI~ 1.22 are rare. As depicted, the materials with low RI, such as magnesium fluoride (MgF2, n = 1.38), can be selected for AR coating to reduce the reflectance from 4.26% (uncoated glass) down to 1.5%. Nevertheless, it is still impractical to achieve exactly zero reflectance from a single-layer AR coating.