In order for laboratory weathering to simulate accurately the effects of real-world exposure, it is essential that the output spectrum of the light source closely match the solar spectrum. Historically, absorptive glass filters have been the solution for eliminating the unwanted UV and IR portions of the xenon arc lamp spectrum. IR absorption filters, however, are notoriously unstable in the critical UV, and significant changes in UV transmission challenge efforts to achieve repeatability and reproducibility. Thin-film coatings are inherently more stable, and can also be designed to block the IR spikes in the xenon lamp output more precisely. In the case of the Atlas Ci4000 Weather-Ometer, however, the filters are in the form of long concentric cylindrical tubes that surround the lamp.
Water is flowed in through the inner tube and back out through the outer tube to cool the lamp.
Traditional evaporative coating technology cannot meet these demands for a number of reasons. First, it is difficult to uniformly coat the entire surface of highly curved parts using evaporation methods. More importantly, this type of film is relatively porous, and a porous film has poor resistance to heat and mechanical wear, and also tends to absorb water, causing a shift in its spectral characteristics. Sputtering is an alternative coating method that provides denser films and also enables uniform deposition on highly curved parts. Sputtering is conducted in a chamber filled with an inert gas at low pressure.