Multilayer dielectric reflectors are not feasible for this application. In addition, gold and copper coatings are soft and subject to mechanical damage. Rhodium and platinum both have inadequate reflectance over the entire spectral range but particularly below 1 micron. The reflectance of aluminum is intermediate between rhodium and gold for most of the spectral range, but has higher reflectance below 600 nm than all other metals except silver. Figure 2.9-2 shows the reflectance of aluminum over the spectral range at an angle of incidence of 20 for the s and p polarisations.
Silver has high reflectance over the entire spectral range.
Differences in reflectance between the s and p polarisations are also small over the spectral range and over the expected angles of incidence. Figure 2.9-3 shows the reflectance curves of silver over the spectral range at an angle of incidence of 20 for the s and p polarisations. The ripples near 1.4 microns and 1.9 microns are artifacts due to the spacing of the complex index data. Silver Pure silver is not suitable as a coating as it has poor adhesion to most substrates and tarnishes quickly if left unprotected. Coating designers have addressed these limitations and many coating facilities have developed durable protected silver coatings that do not tarnish and maintain high reflectance and low polarisation.