Figure 6 shows the transmittance of the metal mesh as a function of the wavelength for different cylinder diameters according to the FDTD simulations. The distance between the cylinders is 1875 nm.

The mean transmittance for wavelengths smaller than the distance between the cylinders decreases when the diameter of the cylinders increases. This is in accordance with the expectations from geometrical optics, which applies for wavelengths which are small compared to the distance between the cylinders. Figure 7 shows the transmittance of a grid of reflecting cylinders according to the laws of geometrical optics, calculated with the ray-tracing program ASAP [6].

It is apparent that the mean transmittance for wavelengths below the distance between the cylinders is higher according to the FDTD simulations than it is according to geometrical optics. This is due to diffraction by the cylinders, whose dimension is of the magnitude of the wavelength. This phenomenon is not accounted for by geometrical optics.

To derive the transmittance of the mesh for solar or heat radiation, the transmittance spectrum in Figure 6 is averaged over the corresponding spectral distributions. Figure 8 shows these properties calculated for cylinders with diameters of 250 nm, 500 nm and 1000 nm. These values of solar and heat transmittance can be used as guidelines to estimate the increase in solar transmittance and low-e properties of micro-structured coatings made from metals with finite conductivity.