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Applications of Highly Reflective Silver Coatings for Car Headlamp Reflectors Torsten Schmauder, Stephan Küper Leybold Optics GmbH, Alzenau Keywords: Headlamps, Reflectivity, Device Efficiency,… 1 Introduction Modern car headlamps pose high requirements to their device efficiency. LED light sources still fall short in luminous flux compared to HID lights and they often have one or more optical elements to shape the beam. As a consequence, the reflectivity of the (reflective) optical elements becomes one of the key factors for the overall performance and efficiency of the device. Further, conventional systems with halogen or HID light sources test the thermal limits of the materials used in their design. Here, reflectivity is a key critical factor for the thermal design of the components. Finally, with energy conservation becoming more and more important, poor device efficiency becomes unacceptable for environmental reasons. Still, most of today’s head lamp reflectors are coated conventionally with Aluminum and deliver a reflectivity between 85 and 90 % on a manufacturing quality level. Silver, on the contrary, offers a reflectivity up to 97 % and can therefore contribute to the improvement of the device efficiency. One of the major challenges for applying Silver as the reflective material is its corrosion stability. Even though Silver is a noble metal and Aluminum is chemically very reactive, Aluminum can be passivated and protected to excellent quality levels by mature and established manufacturing processes. Finally, the cost of manufacturing Silver coated parts is important, as Silver targets come with the price tag of a noble metal. However, a complete cost per piece calculation reveals that the costs of Silver coated parts may be overestimated if viewed in relation with the total cost of a headlamp and with the performance increase achieved. The paper discusses the advantages and challenges of reflectors which are vacuum coated with Silver instead of Aluminum. 2 Optical Properties of Silver and Aluminum 2.1 Comparison of Spectra Silver and Aluminum exhibit quite different reflection spectra. Silver rises to a level of > 98 % for all wavelengths above 600 nm. In the visible range from 400 to 700 nm the integral reflectivity is 96.9 %. Aluminum, on the other hand has a (theoretical) integral reflectivity of 92.5 % in the visible range and rises to app. 97.5 % in the infrared. In day to day production, however, the reflectivity of Aluminum mirrors is frequently lower (sometimes below 85 % in the visible range) due to the application of a corrosion protective siloxane top coat. The calculations below are based on the ideal reflectivities of Aluminum plus Siloxane top coat and Silver. The differences between Silver and Aluminum coated reflectors calculated in this paper therefore represent a conservative estimation for the achievable improvements. 100 99 98 Reflectivity [%] 97 96 95 94 93 92 Ag 91 90 200 Al + Siloxan TC 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 Wavelength [nm] Figure 1: Optical reflection spectra of mirrors coated with Aluminum with Siloxane top coat and Silver respectively. Silver has a significantly higher reflectivity in the visible and near IR. 2.2 Device Efficiency The device efficiency of car lights describes the percentage of the light emitted by the source which actually leaves the device in the intended way and is available for illumination. It is determined exclusively by the optical design of the device and can be calculated from the optical data and reflection spectra of the individual elements. Losses are due to absorption on reflective optical elements (mirrors) and due to reflection on refractive elements (lenses, transparent covers). For the illumination performance we consider the visible range from 400 to 700 nm for the calculations. In case of a single reflective optical element the integral reflectivity is at the same time the device efficiency. In case of 2 reflective elements (like in example 1) the device efficiency is the product of the two integral reflectivities of both reflective elements. In all head lights there is also a front lens, mostly made of Polycarbonate, which finally reflects another 10 % of the light into the lamp body. As an example, in a head light device like in figure 1, about 14.3 % of the initial light are lost only due to the reflectors, if they are coated with Aluminum. After the front lens little more than 75 % of the initial light is available for road illumination. If the mirrors were coated with Silver, the reflective losses could be reduced to 6.2 %, which would lift the overall efficiency after the front lens to 84.4 %. In total, the light output from the Silver coated device will be 11 % higher than from the Aluminum coated device. The reflection on the front lens could also be reduced drastically by interference anti reflex coatings, which however are not topic of this paper. Al Mirror 2 PC Front Lens 7,4 % 85,7 % 75,7 % 93,6 % Al Mirror 1 100 % 7,4 % 10 % Figure 2: Optical Losses in a head light utilizing two Aluminum reflectors for beam shaping. Due to the reflectors alone, 14.3 % of the light is lost. At the PC front lens additional 10 % are reflected within the device. Only 75.7 % of the light from the light source leave the device in the intended way and are available for illumination . 2.3 Heat Management All light that is lost due to absorption is converted to heat in the system. For the heat management, the range between 350 and 2500 nm needs to be considered and covers most of the light that is emitted by a halogen or HID lamp. The heat load onto a reflector is due to radiation and thermal convection. The part that is due to radiation can be influenced by the reflective properties of the optical elements. Because the absolute percentage of absorption is small (and still the reflector materials are often stressed to their thermal limits), the relative difference between Aluminum and Silver coated mirrors is large. The integral reflectivity of Silver over the full range from 350 to 2500 nm is twice as high as for Aluminum. Therefore, as an example, in a headlight device like in figure 1, the heat load due to radiation on each mirror could be cut in half, if the mirrors were coated with Silver instead of Aluminum. 3 Corrosion Protection 3.1 Silver vs. Aluminum Aluminum reflectors are generally coated with a siloxane based top coat which is deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD). This mature technology provides an excellent protection against the current corrosion tests (damp heat, sodium hydroxide, salt spray, climatic change etc.). The first thing that comes to mind, if the protection of Silver layers is addressed, is to simply copy the PECVD technology from Aluminum to Silver. The results of this approach, however, are disappointing. Silver parts that are protected with a siloxane based top coat do not reach anywhere near the performance of Aluminum coated parts in accelerated climatic tests. 3.2 New Approaches Therefore, new approaches have to be found to provide corrosion protection for Silver. In the literature several ways have been explored, a large part of the work being performed for solar concentrating mirrors. Progress was made with several µm thick oxide layers and underlying layers from various metals including Copper, Chromium and Nickel. Especially for the automotive industry however, the costs of the layer stack always have to be kept in mind. In this study coating stacks with underlying and covering metal layers like Copper, Aluminum or Chromium have been tested besides the initial approach with PECVD siloxane top coats. The different samples were evaluated by damp heat, damp UV and climatic change tests. It could be shown that thin barrier layers on top of the reflectors in combination with underlying protective and adhesion layers can boost the climatic performance of Silver mirrors to promising levels. The testing methods for Aluminum mirrors are as mature as the Aluminum technology itself. It may be useful to reevaluate, if the same tests that are used for Aluminum mirrors are also adequate to test Silver mirrors. Besides new coating processes also new testing methods may be necessary to accompany the development of better optical devices. Figure 3: Progress in corrosion protection of Silver mirrors. From left to right: Silver with siloxane PECVD topcoat, Silver with Al base coat plus PECVD top coat, Silver with copper base coat plus PECVD top coat , Silver with metal top coat and Silver with metal base- and top coat. All samples were submitted to 1 week climate change test (65 °C , 85 % r.H., -40 °C) 4 Cost Comparison 4.1 Silver vs. Aluminum The automotive industry is very cost sensitive. Silver as a representative of the noble metals clearly meets severe prejudices against the extra cost of a Silver vs. an Aluminum mirror. Since the reflective layers are very thin however, many reflectors can be made from an (admittedly expensive) Silver target. Further, the remainder of the target as well as the material deposited on the shields can be recycled. A true cost of ownership calculation shows that a reflector sized 20 X 20 cm2 can be coated with Aluminum for app. 17 € Cents, whereas the same part with Silver is coated for app. 65 € Cents. The additional coating costs need to be put into perspective with a) the costs that can be saved because of the better optical performance - lower power light sources - lower thermal grade reflector material - lower energy consumption during use and/or b) the costs that would have to be spent to reach the higher performance in terms of lighting. Compared to the overall costs of an advanced automotive headlight the extra costs of Silver coating are small. 5 Conclusions Coating of reflectors in car lights with Silver offers the advantage of increased device efficiency and lower thermal load. Considering the fact that LED headlights still fall short of their Xenon or Halogen competitors, every possible percent of light that LEDs produce should be delivered to the road instead of being converted to heat inside the head light. Further, expensive materials that are necessary just to tolerate the heat load produced by the light sources, could be avoided. In a headlamp device with two reflective elements for beam shaping Silver coated reflector surfaces increase the overall device efficiency by 11 % (or more, if the Aluminum coating is of poor quality). At the same time the heat load to the reflectors due to absorption is reduced by 50 %. Silver competes with Aluminum as a cheap, mature coating material. In order to utilize Silver in the automotive industry, traditional testing and performance evaluation may have to be reevaluated against today´s requirements. As a result further (co-)development of the manufacturing methods may be necessary until a mature technology like Aluminum coating will be achieved. However, the benefits in performance will be well worth the efforts. References: 1. C.E. Kennedy, K. Terwillger. “Optical durability of candidate solar reflectors”, Journal of Solar Energy Engineering., Vol. 127, p. 262, 2005 2. Jesse D. Wolfe, Norman L. Thomas, U.S. Patent #6,078,425, “Durable Silver coating for mirrors“, June 20, 2000 3. J. Wolfe, D. Sanders, “Optical & Environmental Performance of Durable Silver Mirror Coatings Fabricated at LLNL“, Mirror technology days 2004, Huntsville, AL, 17-19 Aug 2004