However, in the visible wavelengths region, transmission became less than 40% in air. Au has a P of 13.8 ×10 16 rad/s, which corresponds to the 136 nm wavelength. When the frequency of light becomes larger than the P , free electron cannot follow E-field of light and metal becomes endowed with dielectric-like property. The Au, Ag, Cu have similar P at 13.8, 14.0, 13.4×10 15 rad/s, respectively, and at the visible spectral range spectral(color) differences in and of those metals mainly comes from the damping constant differences. When the damping constant Γ became smaller (Au 1.07, Ag 0.322, Cu 1.45 ×10 14 1/s), the high reflectance region extends until the P . Therefore, reflectance of Au and Cu at visible wavelength starts decline into the red-to-near-IR wavelength range and it cause an increase in the visible transmission. It would be expected that for Cu, a high transmission region would extend until longer wavelengths than for Au. However, due to a chemical instability of Ag and Cu, it is difficult to fabricate non-oxided stable thin metal film of pure metal. Therefore, Au is the more promising material for the application of thin ∼10 nm thickness film for the required optical transmission. However Au has not enough transparency for some practical applications as a window. For the window application, it is imperative to have transmission beyond 70%.