Solar Engineering of Thermal Processes, 4th Edition

5.8 Effects of Surface Layers on Transmittance

If a film of low refractive index is deposited at an optical thickness of onto a transparent slab, radiation of wavelength reflected from the upper and lower surfaces of the film will have a phase difference of and will cancel. The reflectance will be decreased, and the transmittance will be increased relative to the uncoated material. This is the principal type of coating used on camera lenses, binoculars, and other expensive optical equipment.

Inexpensive and durable processes have been developed for treating glass to reduce its reflectance by the addition of films having a refractive index between that of air and the transparent medium (e.g., Thomsen, 1951). The solar reflectance of a single pane of untreated glass is approximately 8%. Surface treatment, by dipping glass in a silica-saturated fluosilic acid solution, can reduce the reflection losses to 2%, and a double-layer coating can, as shown by Mar et al. (1975), reduce reflection losses to less than 1%. Such an increase in solar transmittance can make a very significant improvement in the thermal performance of flat-plate collectors. Figure 5.8.1 shows typical monochromatic reflectance ...