One way of increasing the transmittance of glass is AR treatment which results in an interference effect that is described by the Fresnel formulae.
Theoretical calculations using a thin layer with refraction index n = 1.22 on both sides of the glass with refraction index n = 1.5 yield a maximum decrease in the solar reflectance of 5.5 % or a corresponding increase of solar light transmittance.
Sunarc Technology A/S has developed a full scale industrial process for the production of glass that improves solar transmittance by about 5 % which is very close to the theoretical maximum.
Sunarc AR surface is a thin single surface, about 100 nm thick, produced by an etching process and consists of a porous silica skeleton as part of the glass itself. The surface has a very good proven stability and when samples were exposed to an outdoor environment for a period of 7 years they have kept their optical properties with little or no sign of degradation.
Since the Sunarc AR surface is part of the glass itself, it cannot be detached and is very resistant to mechanical, thermal and chemical degradation.
Transmittance and efficiency are usually characterized and measured at a 0 angle of incidence. In reality this will only be true for short periods of time. So this 0 angle of incident is not an appropriate angle of characterization for solar applications since an average angle of 30 to 50 degrees depending on the location, is probably more accurate.
Glass with a Sunarc AR surface has higher transmittance values at angles of incidence greater than the 5 % in direct light, which is shown in the enclosed transmittance curve.
See as pdf.