Newport Thin Film Laboratory
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Newport Thin Film Laboratory
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NTFL UV cold mirrors are extremely effective at increasing the amount of UV energy at the irradiation zone while simultaneously significantly reducing the visible and infrared energy. The result is lower temperatures at the irradiation zone, allowing the processing of temperature sensitive substrates. UV cold mirrors only work on radiant energy from the lamp that is reflected from the cold mirror. The direct radiation from the lamp is not affected by the cold mirror. To reduce the direct visible and infrared radiation, system designers also employ hot mirrors. Hot mirrors act on both direct and reflected radiation, but since they are less efficient than cold mirrors, cold mirrors are almost always used for systems that require relatively low temperatures at the irradiation zone. There are two basic system configurations for UV curing, and the system configuration dictates the type of cold mirror required. The most common system is shown schematically in Figure 1a. It consists of an elliptical reflector in line with the work, with the UV lamp at the first focal point and the irradiation zone at the second focal point. The most common reflector material is aluminum, but transmitting reflectors are also used. The transmitting reflectors are typically made of fused silica, quartz, Vycor, or Borosilicate depending on the total heat load on the reflector and system manufacturer preferences.
WE APOLOGIZE THAT THIS SECTION IS STILL UNDER CONSTRUCTION
Systems that utilize aluminum reflectors require the use of an optical absorber to remove visible and infrared energy. The absorber is incorporated into the UV cold mirror coating at the time of manufacture. The thermal contact and physical properties of the absorber and the ceramic layers that make up the cold mirror are critical to the manufacture of a durable, high quality cold mirror. Poorly manufactured cold mirrors not only reduce the UV output of a system, but typically fail catastrophically. For transmitting reflectors, no absorber is used since the visible and infrared energy is transmitted through the reflector. Another configuration consists of an aluminum elliptical reflector orthogonal to the work, with the UV lamp at the first focal point and the irradiation zone at the second focal point (Figure 1b). The UV radiation is reflected by a second mirror (UV cold mirror) at 45° to the irradiation zone. The second (turning) mirror must be transmitting and is typically fused silica. Vycor or Borosilicate have been used in low power systems, depending on the UV frequencies desired. However, fused silica is usually necessary because of the high thermal load on the turning mirror. For these systems, the cold mirror must be optimized for use at 45° angle of incidence. The cold mirror does not utilize an absorber, since the visible and infrared radiation is transmitted through the mirror. |
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